The shared and separate roles of aposematic (warning) coloration and the co-evolution hypothesis in defending autumn leaves

The potential anti-herbivory functions of colorful (red and yellow) autumn leaves received considerable attention in the last decade. The most studied and discussed is the co-evolutionary hypothesis, according to which autumn coloration signals the quality of defense to insects that migrate to the trees in autumn. In addition to classic aposematism (repellency due to signaling unpalatability, non profitability of consumption, or danger for whatever reasons) that operates immediately, this hypothesis also proposes that the reduced fitness of the insects is in their next generation hatching in the spring from eggs laid on the trees in autumn. Supporters of the co-evolutionary hypothesis either posited that this hypothesis differs from visual aposematism or ignored the issue of aposematism. Interestingly, other authors that cited their papers considered the co-evolutionary hypothesis as visual aposematism. Recently, the overlap between the co-evolutionary hypothesis and visual aposematism was finally recognized, with the exception of yellow autumn leaves not signaling defense to aphids, which are known to be attracted to yellow leaves. However, the detailed relationships between these two hypotheses have not been discussed yet. Here I propose that the co-evolutionary hypothesis generally equals visual aposematism in red and yellow autumn leaves towards all herbivores except for yellow not operating with aphids. The co-evolutionary signaling extends beyond classic aposematism because it may operate later and not only immediately. The possibility that for yellow autumn leaves the co-evolutionary hypothesis may also operate via olfactory aposematism should not be dismissed.Key words: aposematic, autumn coloration, co-evolution, defense, evolution, herbivory, treesColorful (red and yellow) autumn leaves dominate large areas of America, Asia and Europe, expressed by thousands of tree, shrub and climber species.^1–5 In the last decade, this phenomenon received considerable scientific attention. For a long time it was a common belief that this coloration is the by-product of the cessation of masking by chlorophylls that degrade in autumn. However, two key theoretical and experimental developments stimulated the recent wave of study of autumn leaf coloration. The first was the recognition that anthocyanins are synthesized de novo in red autumn leaves,^1,2 and the second was the formulation of the anti-herbivory co-evolutionary hypothesis.^6–8The updated version of the co-evolutionary hypothesis⁹ posits that red autumn coloration signals to all types of insects (including aphids) that migrate to the trees in autumn about their chemical defense, lower nutritional quality or imminent leaf fall, or any other characteristic that would induce a lower fitness in the insects. In addition, yellow leaves signals the same to all herbivores except aphids. A special aspect of the co-evolutionary hypothesis is that the reduced fitness of the insects is not only immediate, reducing insect feeding in autumn, but also related to the reduced development of the next generation that hatches in the following spring from eggs laid on the trees in the autumn.⁹ Originally, the co-evolutionary hypothesis addressed both red and yellow autumn leaves.^6–8 However, with the later understanding that yellow leaves usually attract rather than repel aphids,^9–13 the co-evolutionary hypothesis was later restricted to red leaves when aphids are concerned.⁹In addition to other various potential anti-herbivory roles,^14,15 red autumn leaf coloration has several potential physiological functions, such as protection from photoinhibition and photo oxidation, and other physiological functions have been proposed but not agreed upon.^{1,2,9,16–21}     

Cell Migration from Baby to Mother

Fetal cells migrate into the mother during pregnancy. Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades. Microchimeric fetal cells are found in various maternal tissues and organs including blood, bone marrow, skin and liver. In mice, fetal cells have also been found in the brain. The fetal cells also appear to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants. Further understanding of the ability of fetal cells to cross both the placental and blood-brain barriers, to migrate into diverse tissues, and to differentiate into multiple cell types may also advance strategies for intravenous transplantation of stem cells for cytotherapeutic repair. Here we discuss hypotheses for how fetal cells cross the placental and blood-brain barriers and the persistence and distribution of fetal cells in the mother.Key Words: fetomaternal microchimerism, stem cells, progenitor cells, placental barrier, blood-brain barrier, adhesion, migrationMicrochimerism is the presence of a small population of genetically distinct and separately derived cells within an individual. This commonly occurs following transfusion or transplantation.^1–3 Microchimerism can also occur between mother and fetus. Small numbers of cells traffic across the placenta during pregnancy. This exchange occurs both from the fetus to the mother (fetomaternal)^4–7 and from the mother to the fetus.^8–10 Similar exchange may also occur between monochorionic twins in utero.^11–13 There is increasing evidence that fetomaternal microchimerism persists lifelong in many child-bearing women.^7,14 The significance of fetomaternal microchimerism remains unclear. It could be that fetomaternal microchimerism is an epiphenomenon of pregnancy. Alternatively, it could be a mechanism by which the fetus ensures maternal fitness in order to enhance its own chances of survival. In either case, the occurrence of pregnancy-acquired microchimerism in women may have implications for graft survival and autoimmunity. More detailed understanding of the biology of microchimeric fetal cells may also advance progress towards cytotherapeutic repair via intravenous transplantation of stem or progenitor cells.Trophoblasts were the first zygote-derived cell type found to cross into the mother. In 1893, Schmorl reported the appearance of trophoblasts in the maternal pulmonary vasculature.¹⁵ Later, trophoblasts were also observed in the maternal circulation.^16–20 Subsequently various other fetal cell types derived from fetal blood were also found in the maternal circulation.^21,22 These fetal cell types included lymphocytes,²³ erythroblasts or nucleated red blood cells,^24,25 haematopoietic progenitors^7,26,27 and putative mesenchymal progenitors.^14,28 While it has been suggested that small numbers of fetal cells traffic across the placenta in every human pregnancy,^29–31 trophoblast release does not appear to occur in all pregnancies.³² Likewise, in mice, fetal cells have also been reported in maternal blood.^33,34 In the mouse, fetomaternal transfer also appears to occur during all pregnancies.³⁵     

RALFs: Peptide regulators of plant growth

Peptide signaling regulates a variety of developmental processes and environmental responses in plants.^1–6 For example, the peptide systemin induces the systemic defense response in tomato⁷ and defensins are small cysteine-rich proteins that are involved in the innate immune system of plants.^8,9 The CLAVATA3 peptide regulates meristem size¹⁰ and the SCR peptide is the pollen self-incompatibility recognition factor in the Brassicaceae.^11,12 LURE peptides produced by synergid cells attract pollen tubes to the embryo sac.⁹ RALFs are a recently discovered family of plant peptides that play a role in plant cell growth.Key words: peptide, growth factor, alkalinization     

Is crypsis a common defensive strategy in plants?: Speculation on signal deception in the New Zealand flora

Color is a common feature of animal defense. Herbivorous insects are often colored in shades of green similar to their preferred food plants, making them difficult for predators to locate. Other insects advertise their presence with bright colors after they sequester enough toxins from their food plants to make them unpalatable. Some insects even switch between cryptic and aposomatic coloration during development.¹ Although common in animals, quantitative evidence for color-based defense in plants is rare. After all, the primary function of plant leaves is to absorb light for photosynthesis, rather than reflect light in ways that alter their appearance to herbivores. However, recent research is beginning to challenge the notion that color-based defence is restricted to animals.Key words: aposomatic colouration, cryptic colouration, herbivory, moa, plant defenseTemperate deciduous forests provide what is arguably the most extraordinary display of color in nature. Prior to leaf-fall in autumn, the leaves of many deciduous tree species in Asia, Europe and North America turn red, leading to brilliantly colored landscapes. Once thought to be a by-product of chlorophyll re-absorption prior to leaf abscission, autumn flushes in red leaf colors are now known to result from the active synthesis of red-colored pigments.^2,3 Although the exact reason for the production of red-colored pigments prior to leaf-fall is unknown, it has recently been hypothesized to be a form of defense.⁴ Aphids are common phloem-feeding herbivores in deciduous forests, which disperse from the forest floor into tree crowns in autumn, and the synthesis of red pigments could signal the timing of leaf fall and the reduction in the supply of photosynthate.^5,6 Although there are also physiological explanations,⁷ red leaf colors could be a reliable signal of unpalatably to herbivores.⁸Lev-Yadun and Holopainen⁹ recently showed that there are fewer red-colored deciduous tree species in Europe than in North America, and they speculate that historical processes are the cause. During the advance and retreat of glaciers in the Pleistocene, the European Alps would have hindered the movement of plants and their herbivores in response to long-term climate change. Mountain ranges in North America run perpendicular to the equator, which would facilitate these migrations. Therefore, if red leaves are signals to herbivores, geographic differences in leaf pigmentation may be ‘anachronistic’.¹⁰ In other words, they may result from historical coevolutionary dynamics between plants and their herbivores, rather than present day selection pressures alone.Despite these important insights, our understanding of color-based defense in plants is in its infancy and progress hinges on quantitative tests in other parts of the globe. Previous work is also restricted largely to aposomatic, or warning colors.¹¹ Given that cryptic coloration is widespread in animals, it might also be common in plants. Yet we are far from determining if this is true.Here, I discuss several New Zealand plant species that seem to be colored in ways that would make them difficult for herbivores to locate. I suggest that these plants are anachronisms; their unusual appearance is the result of selection from flightless browsing birds called moa, which went extinct following the arrival of humans in New Zealand 750 years ago. I also discuss the difficulties associated with testing for cypsis in plants and finish by outlining a methodological approach to test for color-based defense in plants when the putative herbivores are either unknown or extinct.     

Why have chloroplasts developed a unique motility system?

Organelle movement in plants is dependent on actin filaments with most of the organelles being transported along the actin cables by class XI myosins. Although chloroplast movement is also actin filament-dependent, a potential role of myosin motors in this process is poorly understood. Interestingly, chloroplasts can move in any direction and change the direction within short time periods, suggesting that chloroplasts use the newly formed actin filaments rather than preexisting actin cables. Furthermore, the data on myosin gene knockouts and knockdowns in Arabidopsis and tobacco do not support myosins'' XI role in chloroplast movement. Our recent studies revealed that chloroplast movement and positioning are mediated by the short actin filaments localized at chloroplast periphery (cp-actin filaments) rather than cytoplasmic actin cables. The accumulation of cp-actin filaments depends on kinesin-like proteins, KAC1 and KAC2, as well as on a chloroplast outer membrane protein CHUP1. We propose that plants evolved a myosin XI-independent mechanism of the actin-based chloroplast movement that is distinct from the mechanism used by other organelles.Key words: actin, Arabidopsis, blue light, kinesin, myosin, organelle movement, phototropinOrganelle movement and positioning are pivotal aspects of the intracellular dynamics in most eukaryotes. Although plants are sessile organisms, their organelles are quickly repositioned in response to fluctuating environmental conditions and certain endogenous signals. By and large, plant organelle movements and positioning are dependent on actin filaments, although microtubules play certain accessory roles in organelle dynamics.^1,2 Actin inhibitors effectively retard the movements of mitochondria,^3–6 peroxisomes,^5,7–11 Golgi stacks,^12,13 endoplasmic reticulum (ER),^14,15 and nuclei.^16–18 These organelles are co-aligned and associated with actin filaments.^{5,7,8,10–12,15,18} Recent progress in this field started to reveal the molecular motility system responsible for the organelle transport in plants.¹⁹Chloroplast movement is among the most fascinating models of organelle movement in plants because it is precisely controlled by ambient light conditions.^20,21 Weak light induces chloroplast accumulation response so that chloroplasts can capture photosynthetic light efficiently (Fig. 1A). Strong light induces chloroplast avoidance response to escape from photodamage (Fig. 1B).²² The blue light-induced chloroplast movement is mediated by the blue light receptor phototropin (phot). In some cryptogam plants, the red light-induced chloroplast movement is regulated by a chimeric phytochrome/phototropin photoreceptor neochrome.^23–25 In a model plant Arabidopsis, phot1 and phot2 function redundantly to regulate the accumulation response,²⁶ whereas phot2 alone is essential for the avoidance response.^27,28 Several additional factors regulating chloroplast movement were identified by analyses of Arabidopsis mutants deficient in chloroplast photorelocation.^29–32 In particular, identification of CHUP1 (chloroplast unusual positioning 1) revealed the connection between chloroplasts and actin filaments at the molecular level.²⁹ CHUP1 is a chloroplast outer membrane protein capable of interacting with F-actin, G-actin and profilin in vitro.^29,33,34 The chup1 mutant plants are defective in both the chloroplast movement and chloroplast anchorage to the plasma membrane,^22,29,33 suggesting that CHUP1 plays an important role in linking chloroplasts to the plasma membrane through the actin filaments. However, how chloroplasts move using the actin filaments and whether chloroplast movement utilizes the actin-based motility system similar to other organelle movements remained to be determined.Open in a separate windowFigure 1Schematic distribution patterns of chloroplasts in a palisade cell under different light conditions, weak (A) and strong (B) lights. Shown as a side view of mid-part of the cell and a top view with three different levels (i.e., top, middle and bottom of the cell). The cell was irradiated from the leaf surface shown as arrows. Weak light induces chloroplast accumulation response (A) and strong light induces the avoidance response (B).Here, we review the recent findings pointing to existence of a novel actin-based mechanisms for chloroplast movement and discuss the differences between the mechanism responsible for movement of chloroplasts and other organelles.     

Hypoxia: A novel function for VIN3

VERNALIZATION INSENSITIVE 3 (VIN3) encodes a PHD domain chromatin remodelling protein that is induced in response to cold and is required for the establishment of the vernalization response in Arabidopsis thaliana.¹ Vernalization is the acquisition of the competence to flower after exposure to prolonged low temperatures, which in Arabidopsis is associated with the epigenetic repression of the floral repressor FLOWERING LOCUS C (FLC).^2,3 During vernalization VIN3 binds to the chromatin of the FLC locus,¹ and interacts with conserved components of Polycomb-group Repressive Complex 2 (PRC2).^4,5 This complex catalyses the tri-methylation of histone H3 lysine 27 (H3K27me3),^4,6,7 a repressive chromatin mark that increases at the FLC locus as a result of vernalization.^4,7–10 In our recent paper¹¹ we found that VIN3 is also induced by hypoxic conditions, and as is the case with low temperatures, induction occurs in a quantitative manner. Our experiments indicated that VIN3 is required for the survival of Arabidopsis seedlings exposed to low oxygen conditions. We suggested that the function of VIN3 during low oxygen conditions is likely to involve the mediation of chromatin modifications at certain loci that help the survival of Arabidopsis in response to prolonged hypoxia. Here we discuss the implications of our observations and hypotheses in terms of epigenetic mechanisms controlling gene regulation in response to hypoxia.Key words: arabidopsis, VIN3, FLC, hypoxia, vernalization, chromatin remodelling, survival     

Plant defensins: Defense,development and application

Plant defensins are small, highly stable, cysteine-rich peptides that constitute a part of the innate immune system primarily directed against fungal pathogens. Biological activities reported for plant defensins include antifungal activity, antibacterial activity, proteinase inhibitory activity and insect amylase inhibitory activity. Plant defensins have been shown to inhibit infectious diseases of humans and to induce apoptosis in a human pathogen. Transgenic plants overexpressing defensins are strongly resistant to fungal pathogens. Based on recent studies, some plant defensins are not merely toxic to microbes but also have roles in regulating plant growth and development.Key words: defensin, antifungal, antimicrobial peptide, development, innate immunityDefensins are diverse members of a large family of cationic host defence peptides (HDP), widely distributed throughout the plant and animal kingdoms.^1–3 Defensins and defensin-like peptides are functionally diverse, disrupting microbial membranes and acting as ligands for cellular recognition and signaling.⁴ In the early 1990s, the first members of the family of plant defensins were isolated from wheat and barley grains.^5,6 Those proteins were originally called γ-thionins because their size (∼5 kDa, 45 to 54 amino acids) and cysteine content (typically 4, 6 or 8 cysteine residues) were found to be similar to the thionins.⁷ Subsequent “γ-thionins” homologous proteins were indentified and cDNAs were cloned from various monocot or dicot seeds.⁸ Terras and his colleagues⁹ isolated two antifungal peptides, Rs-AFP1 and Rs-AFP2, noticed that the plant peptides'' structural and functional properties resemble those of insect and mammalian defensins, and therefore termed the family of peptides “plant defensins” in 1995. Sequences of more than 80 different plant defensin genes from different plant species were analyzed.¹⁰ A query of the UniProt database (www.uniprot.org/) currently reveals publications of 371 plant defensins available for review. The Arabidopsis genome alone contains more than 300 defensin-like (DEFL) peptides, 78% of which have a cysteine-stabilized α-helix β-sheet (CSαβ) motif common to plant and invertebrate defensins.¹¹ In addition, over 1,000 DEFL genes have been identified from plant EST projects.¹²Unlike the insect and mammalian defensins, which are mainly active against bacteria,^2,3,10,13 plant defensins, with a few exceptions, do not have antibacterial activity.¹⁴ Most plant defensins are involved in defense against a broad range of fungi.^2,3,10,15 They are not only active against phytopathogenic fungi (such as Fusarium culmorum and Botrytis cinerea), but also against baker''s yeast and human pathogenic fungi (such as Candida albicans).² Plant defensins have also been shown to inhibit the growth of roots and root hairs in Arabidopsis thaliana¹⁶ and alter growth of various tomato organs which can assume multiple functions related to defense and development.⁴     

Human non-CG methylation: Are human stem cells plant-like?

Non-CG methylation is well characterized in plants where it appears to play a role in gene silencing and genomic imprinting. Although strong evidence for the presence of non-CG methylation in mammals has been available for some time, both its origin and function remain elusive. In this review we discuss available evidence on non-CG methylation in mammals in light of evidence suggesting that the human stem cell methylome contains significant levels of methylation outside the CG site.Key words: non-CG methylation, stem cells, Dnmt1, Dnmt3a, human methylomeIn plant cells non-CG sites are methylated de novo by Chromomethylase 3, DRM1 and DRM2. Chromomethylase 3, along with DRM1 and DRM2 combine in the maintenance of methylation at symmetric CpHpG as well as asymmetric DNA sites where they appear to prevent reactivation of transposons.¹ DRM1 and DRM2 modify DNA de novo primarily at asymmetric CpH and CpHpH sequences targeted by siRNA.²Much less information is available on non-CG methylation in mammals. In fact, studies on mammalian non-CG methylation form a tiny fraction of those on CG methylation, even though data for cytosine methylation in other dinucleotides, CA, CT and CC, have been available since the late 1980s.³ Strong evidence for non-CG methylation was found by examining either exogenous DNA sequences, such as plasmid and viral integrants in mouse and human cell lines,^4,5 or transposons and repetitive sequences such as the human L1 retrotransposon⁶ in a human embryonic fibroblast cell line. In the latter study, non-CG methylation observed in L1 was found to be consistent with the capacity of Dnmt1 to methylate slippage intermediates de novo.⁶Non-CG methylation has also been reported at origins of replication^7,8 and a region of the human myogenic gene Myf3.⁹ The Myf3 gene is silenced in non-muscle cell lines but it is not methylated at CGs. Instead, it carries several methylated cytosines within the sequence CCTGG. Gene-specific non-CG methylation was also reported in a study of lymphoma and myeloma cell lines not expressing many B lineage-specific genes.¹⁰ The study focused on one specific gene, B29 and found heavy CG promoter methylation of that gene in most cell lines not expressing it. However, in two other cell lines where the gene was silenced, cytosine methylation was found almost exclusively at CCWGG sites. The authors provided evidence suggesting that CCWGG methylation was sufficient for silencing the B29 promoter and that methylated probes based on B29 sequences had unique gel shift patterns compared to non-methylated but otherwise identical sequences.¹⁰ The latter finding suggests that the presence of the non-CG methylation causes changes in the proteins able to bind the promoter, which could be mechanistically related to the silencing seen with this alternate methylation.Non-CG methylation is rarely seen in DNA isolated from cancer patients. However, the p16 promoter region was reported to contain both CG and non-CG methylation in breast tumor specimens but lacked methylation at these sites in normal breast tissue obtained at mammoplasty.¹¹ Moreover, CWG methylation at the CCWGG sites in the calcitonin gene is not found in normal or leukemic lymphocyte DNA obtained from patients.¹² Further, in DNA obtained from breast cancer patients, MspI sites that are refractory to digestion by MspI and thus candidates for CHG methylation were found to carry CpG methylation.¹³ Their resistance to MspI restriction was found to be caused by an unusual secondary structure in the DNA spanning the MspI site that prevents restriction.¹³ This latter observation suggests caution in interpreting EcoRII/BstNI or EcoRII/BstOI restriction differences as due to CWG methylation, since in contrast to the 37°C incubation temperature required for full EcoRII activity, BstNI and BstOI require incubation at 60°C for full activity where many secondary structures are unstable.The recent report by Lister et al.¹⁴ confirmed a much earlier report by Ramsahoye et al.¹⁵ suggesting that non-CG methylation is prevalent in mammalian stem cell lines. Nearest neighbor analysis was used to detect non-CG methylation in the earlier study on the mouse embryonic stem (ES) cell line,¹⁵ thus global methylation patterning was assessed. Lister et al.¹⁴ extend these findings to human stem cell lines at single-base resolution with whole-genome bisulfite sequencing. They report¹⁴ that the methylome of the human H1 stem cell line and the methylome of the induced pluripotent IMR90 (iPS) cell line are stippled with non-CG methylation while that of the human IMR90 fetal fibroblast cell line is not. While the results of the two studies are complementary, the human methylome study addresses locus specific non-CG methylation. Based on that data,¹⁴ one must conclude that non-CG methylation is not carefully maintained at a given site in the human H1 cell line. The average non-CG site is picked up as methylated in about 25% of the reads whereas the average CG methylation site is picked up in 92% of the reads. Moreover, non-CG methylation is not generally present on both strands and is concentrated in the body of actively transcribed genes.¹⁴Even so, the consistent finding that non-CG methylation appears to be confined to stem cell lines,^14,15 raises the possibility that cancer stem cells¹⁶ carry non-CG methylation while their nonstem progeny in the tumor carry only CG methylation. Given the expected paucity of cancer stem cells in a tumor cell population, it is unlikely that bisulfite sequencing would detect non-CG methylation in DNA isolated from tumor cells since the stem cell population is expected to be only a very minor component of tumor DNA. Published sequences obtained by bisulfite sequencing generally report only CG methylation, and to the best of our knowledge bisulfite sequenced tumor DNA specimens have not reported non-CG methylation. On the other hand, when sequences from cell lines have been reported, bisulfite-mediated genomic sequencing⁸ or ligation mediated PCR¹⁷ methylcytosine signals outside the CG site have been observed. In a more recent study plasmid DNAs carrying the Bcl2-major breakpoint cluster¹⁸ or human breast cancer DNA¹³ treated with bisulfite under non-denaturing conditions, cytosines outside the CG side were only partially converted on only one strand¹⁸ or at a symmetrical CWG site.¹³ In the breast cancer DNA study the apparent CWG methylation was not detected when the DNA was fully denatured before bisulfite treatment.¹³In both stem cell studies, non-CG methylation was attributed to the Dnmt3a,^14,15 a DNA methyltransferase with similarities to the plant DRM methyltransferase family¹⁹ and having the capacity to methylate non-CG sites when expressed in Drosophila melanogaster.¹⁵ DRM proteins however, possess a unique permuted domain structure found exclusively in plants¹⁹ and the associated RNA-directed non-CG DNA methylation has not been reproducibly observed in mammals despite considerable published^20–23 and unpublished efforts in that area. Moreover, reports where methylation was studied often infer methylation changes from 5AzaC reactivation studies²⁴ or find that CG methylation seen in plants but not non-CG methylation is detected.^21,22,25,26 In this regard, it is of interest that the level of non-CG methylation reported in stem cells corresponds to background non-CG methylation observed in vitro with human DNA methyltransferase I,²⁷ and is consistent with the recent report that cultured stem cells are epigenetically unstable.²⁸The function of non-CG methylation remains elusive. A role in gene expression has not been ruled out, as the studies above on Myf3 and B29 suggest.^9,10 However, transgene expression of the bacterial methyltransferase M.EcoRII in a human cell line (HK293), did not affect the CG methylation state at the APC and SerpinB5 genes²⁹ even though the promoters were symmetrically de novo methylated at ^mCWGs within each CCWGG sequence in each promoter. This demonstrated that CG and non-CG methylation are not mutually exclusive as had been suggested by earlier reports.^9,10 That observation is now extended to the human stem cell line methylome where CG and non-CG methylation co-exist.¹⁴ Gene expression at the APC locus was likewise unaffected by transgene expression of M.EcoRII. In those experiments genome wide methylation of the CCWGG site was detected by restriction analysis and bisulfite sequencing,²⁹ however stem cell characteristics were not studied.Many alternative functions can be envisioned for non-CG methylation, but the existing data now constrains them to functions that involve low levels of methylation that are primarily asymmetric. Moreover, inheritance of such methylation patterns requires low fidelity methylation. If methylation were maintained with high fidelity at particular CHG sites one would expect that the spontaneous deamination of 5-methylcytosine would diminish the number of such sites, so as to confine the remaining sites to those positions performing an essential function, as is seen in CG methylation.^30–33 However, depletion of CWG sites is not observed in the human genome.³⁴ Since CWG sites account for only about 50% of the non-CG methylation observed in the stem cell methylome¹⁴ where methylated non-CG sites carry only about 25% methylation, the probability of deamination would be about 13% of that for CWG sites that are subject to maintenance methylation in the germ line. Since mutational depletion of methylated cytosines has to have its primary effect on the germ line, if the maintenance of non-CG methylation were more accurate and more widespread, one would have had to argue that stem cells in the human germ lines lack CWG methylation. As it is the data suggests that whatever function non-CG methylation may have in stem cells, it does not involve accurate somatic inheritance in the germ line.The extensive detail on non-CG methylation in the H1 methylome¹⁴ raises interesting questions about the nature of this form of methylation in human cell lines. A key finding in this report is the contrast between the presence of non-CG methylation in the H1 stem cell line and its absence in the IMR90 human fetal lung fibroblast cell line.¹⁴ This suggests that it may have a role in the origin and maintenance of the pluripotent lineage.¹⁴By analogy with the well known methylated DNA binding proteins specific for CG methylation,³⁵ methylated DNA binding proteins that selectively bind sites of non-CG methylation are expected to exist in stem cells. Currently the only protein reported to have this binding specificity is human Dnmt1.^36–38 While Dnmt1 has been proposed to function stoichiometrically³⁹ and could serve a non-CG binding role in stem cells, this possibility and the possibility that other stem-cell specific non-CG binding proteins might exist remain to be been explored.Finally, the nature of the non-CG methylation patterns in human stem cell lines present potentially difficult technical problems in methylation analysis. First, based on the data in the H1 stem cell methylome,⁴⁰ a standard MS-qPCR for non-CG methylation would be impractical because non-CG sites are infrequent, rarely clustered and are generally characterized by partial asymmetric methylation. This means that a PCR primer that senses the 3 adjacent methylation sites usually recommended for MS-qPCR primer design^41,42 cannot be reliably found. For example in the region near Oct4 (Chr6:31,246,431), a potential MS-qPCR site exists with a suboptimal set of two adjacent CHG sites both methylated on the + strand at Chr6:31,252,225 and 31,252,237.^14,40 However these sites were methylated only in 13/45 and 30/52 reads. Thus the probability that they would both be methylated on the same strand is about 17%. Moreover, reverse primer locations containing non-CG methylation sites are generally too far away for practical bisulfite mediated PCR. Considering the losses associated with bisulfite mediated PCR⁴³ the likelihood that such an MS-qPCR system would detect non-CG methylation in the H1 cell line or stem cells present in a cancer stem cell niche^44,45 is very low.The second difficulty is that methods based on the specificity of MeCP2 and similar methylated DNA binding proteins for enriching methylated DNA (e.g., MIRA,⁴⁶ COMPARE-MS⁴⁷) will discard sequences containing non-CG methylation since they require cooperative binding afforded by runs of adjacent methylated CG sites for DNA capture. This latter property of the methylated cytosine capture techniques makes it also unlikely that methods based on 5-methylcytosine antibodies (e.g., meDIP⁴⁸) will capture non-CG methylation patterns accurately since the stem cell methylome shows that adjacent methylated non-CG sites are rare in comparison to methylated CG sites.¹⁴In summary, whether or not mammalian stem cells in general or human stem cells in particular possess functional plant-like methylation patterns is likely to continue to be an interesting and challenging question. At this point we can conclude that the non-CG patterns reported in human cells appear to differ significantly from the non-CG patterns seen in plants, suggesting that they do not have a common origin or function.     

Strigolactones as mediators of plant growth responses to environmental conditions

Strigolactones (SLs) have been recently identified as a new group of plant hormones or their derivatives thereof, shown to play a role in plant development. Evolutionary forces have driven the development of mechanisms in plants that allow adaptive adjustments to a variety of different habitats by employing plasticity in shoot and root growth and development. The ability of SLs to regulate both shoot and root development suggests a role in the plant''s response to its growth environment. To play this role, SL pathways need to be responsive to plant growth conditions, and affect plant growth toward increased adaptive adjustment. Here, the effects of SLs on shoot and root development are presented, and possible feedback loops between SLs and two environmental cues, light and nutrient status, are discussed; these might suggest a role for SLs in plants'' adaptive adjustment to growth conditions.Key words: strigolactones, light, nutrient status, root, shoot, branching, lateral roots, root hairsStrigolactones (SLs) are carotenoid-derived terpenoid lactones suggested to stem from the carotenoid pathway¹ via the activity of various oxygenases.^2,3 SLs production has been demonstrated in both monocotyledons and eudicotyledons (reviewed in ref. ⁴), suggesting their presence in many plant species.⁵ SLs are synthesized mainly in the roots and in some parts of the stem and then move towards the shoot apex (reviewed ref. ⁷).^6,8,9SLs were first characterized more than 40 years ago as germination stimulants of the parasitic plants Striga and Orobanche and later, as stimulants of arbuscular mycorrhiza hyphal branching as well (reviewed in ref. ^{4, 10–13}). Recently, SLs or derivatives thereof, have been identified as a new group of plant hormones, shown to play a role in inhibition of shoot branching,^2,3,8,9 thereby affecting shoot architecture; more recently they have also been shown to affect root growth by affecting auxin efflux.¹⁴Plants have developed mechanisms that allow adaptive adjustments to a variety of different habitats by employing plasticity in their growth and development.¹⁵ Shoot architecture is affected by environmental cues, such as light quality and quantity and nutrient status.^16–19 Root-system architecture and development are affected by environmental conditions such as nutrient availability (reviewed in ref. ^{20, 21}). At the same time, plant hormones are known to be involved in the regulation of plant growth, development and architecture (reviewed in ref. ^22–24) and to be mediators of the effects of environmental cues on plant development; one classic example is auxin''s role in the plant''s shade-avoidance response (reviewed in ref. ²⁵).The ability of SLs to regulate shoot and root development suggests that these phytohormones also have a role in the plant''s growth response to its environment. To play this putative role, SL pathways need to be responsive to plant growth conditions, and affect plant growth toward enhancing its adaptive adjustment. The present review examines the SLs'' possible role in adaptive adjustment of the plant''s response to growth conditions, by discussing their effect on plant development and the possible associations and feedback loops between SLs and two environmental cues: light and nutrient status.     

Tuba and N-WASP function cooperatively to position the central lumen during epithelial cyst morphogenesis

The process of epithelial lumenogenesis requires coordination of a network of signaling machinery communicated to each cell through subsequent cell divisions. Formation of a single hollow lumen has previously been shown to require Tuba, a Cdc42 GEF, for Cdc42 activation and correct spindle orientation. Using a Caco-2 model of lumenogenesis, we show that knockdown (KD) of the actin regulator N-WASP, causes a multilumen phenotype similar to Tuba KD. Defects in lumenogenesis in Tuba KD and N-WASP KD cells are observed at the two-cell stage with inappropriate marking of the pre-apical patch (PAP )—the precursor to lumen formation. Strikingly, both Tuba and N-WASP depend on each other for localization to the PAP. We conclude that N-WASP functions cooperatively with Tuba to facilitate lumenogenesis and this requires the polyproline region of N-WASP.Key words: lumen, N-WASP, tuba, E-cadherin, pre-apical patchMany epithelial tissues are organized as hollow tubes whose open lumina connect the body with its external environment.^1,2 These tubes consist of a monolayer of polarized cells that envelope the central lumen. Lumen formation is thus a key process in epithelial morphogenesis that depends upon cell polarity to establish three cell surface domains: a basal surface adherent to the extracellular matrix, a lateral surface between cells, and an apical surface that is exposed to the luminal fluids. Of note, the apical membrane is biochemically and morphologically distinct from the baso-lateral surfaces and effectively defines the luminal surface.^3,4For a lumen to form, cells must first mark the site at which apical membrane is to be inserted, something that is achieved at the first cell division.⁵ Targeted trafficking of apical membrane constituents defines a pre-apical patch (PAP), the precursor to the definitive lumen.⁵ Such insertion of apical membrane must presumably be coordinated with the assembly of apical junctions to segregate nascent apical from lateral membrane domains.² Subsequent cell divisions direct apical membrane and protein constituents to this point of initial apical membrane placement.⁶ Coordinated luminal positioning enables the initial formation of a single hollow lumen that subsequently expands through polarized fluid secretion to separate apical membranes, such as occurs in the embryonic gastrointestinal tract,⁷ or by apoptosis or autophagy of the central cells as is observed in mammary gland development.^8,9 Failure to establish initial luminal positioning causes defective lumenogenesis, often resulting in multiple, morphologically abnormal lumina.^5,6Crucial to lumenal morphogenesis is then the mechanism(s) that mark the site where the PAP will form. Cdc42 signaling is increasingly implicated in this process,^2,10 with downstream consequences that include control of mitotic spindle orientation,⁵ which itself influences PAP placement⁵ and potentially regulation of cell-cell junctions. Like other Rho family GTPases, the subcellular location of Cdc42 signaling is determined by the action of upstream proteins, notably guanine nucleotide exchange factors (GEFs).^11,12 Of these, Tuba, a Cdc42-specific GEF,¹³ has emerged as a regulator of lumenal morphogenesis that controls PAP placement through mitotic spindle orientation.¹⁰Tuba is also a scaffolding protein¹³ capable of linking the actin assembly machinery with trafficking pathways. Not only is Tuba required for Cdc42 activation to direct spindle orientation,⁵ it also has the potential to interact with phosphoinositides that define the PAP.¹⁴ Additionally, Tuba binds directly to the actin regulator N-WASP, a key molecule in the organization of actin and itself a Cdc42 effector.¹⁵ Further, Tuba and N-WASP cooperate in various forms of actin-driven cellular motility, such as vesicle propulsion and cell invasive behavior.¹⁶ Interestingly, in epithelial cells N-WASP is also found at cadherin-based cell-cell junctions.¹⁷ In fact it has been proposed that N-WASP functions downstream of Tuba in the maintenance of epithelial junctional homeostasis as N-WASP overexpression was capable of rescuing a Tuba KD phenotype.¹⁸ Therefore, Tuba has the potential to play a central role in coordinating the molecular complexes required for productive polarization of epithelial cells and placement of the PAP during lumenogenesis. However, whether other protein interactions contribute to the morphogenetic impact of Tuba remain to be assessed.Three-dimensional cell culture systems are being utilized to identify critical components in lumen formation. In particular, Madin-Darby canine kidney cells (MDCK) and Caco-2 gastrointestinal cells are commonly used to study cyst and/or tubule formation. MDCK cells undergo both cyst and tubule growth, apoptosis being primarily responsible for the final step in lumen formation,¹⁹ while Caco-2 cells primarily utilize fluid influx to expand cysts.⁵ Cyst culture systems replicate aspects of in vivo organogenesis²⁰ providing tangible, powerful models to analyze and dissect the coordinated cellular mechanisms and processes that occur during epithelial morphogenesis.In this study we examined the relationship between Tuba and N-WASP in early epithelial lumenogenesis using Caco-2 three dimensional cyst cultures. Both Tuba and N-WASP RNAi cell lines result in mature cysts with multiple lumina, and at the two-cell stage, formed multiple PAPs. Interestingly, N-WASP KD perturbed Tuba localization at the PAP, however, N-WASP localization to the PAP was not affected to the same extent by Tuba KD. Taken together, these results suggest a complex interrelationship between Tuba and N-WASP for the coordinated formation of a single hollow lumen.     

Matrix metalloproteinase (MMP) and TGFβ1-stimulated cell migration in skin and cornea wound healing

Cell migration during wound healing is a complex process that involves the expression of a number of growth factors and cytokines. One of these factors, transforming growth factor-beta (TGFβ) controls many aspects of normal and pathological cell behavior. It induces migration of keratinocytes in wounded skin and of epithelial cells in damaged cornea. Furthermore, this TGFβ-induced cell migration is correlated with the production of components of the extracellular matrix (ECM) proteins and expression of integrins and matrix metalloproteinases (MMPs). MMP digests ECMs and integrins during cell migration, but the mechanisms regulating their expression and the consequences of their induction remain unclear. It has been suggested that MMP-14 activates cellular signaling processes involved in the expression of MMPs and other molecules associated with cell migration. Because of the manifold effects of MMP-14, it is important to understand the roles of MMP-14 not only the cleavage of ECM but also in the activation of signaling pathways.Key words: wound healing, migration, matrix metalloproteinase, transforming growth factor, skin, corneaWound healing is a well-ordered but complex process involving many cellular activities including inflammation, growth factor or cytokine secretion, cell migration and proliferation. Migration of skin keratinocytes and corneal epithelial cells requires the coordinated expression of various growth factors such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), transforming growth factor (TGF), keratinocyte growth factor (KGF), hepatocyte growth factor (HGF), insulin-like growth factor (IGF), epidermal growth factor (EGF), small GTPases, and macrophage stimulating protein (reviewed in refs. ¹ and ²). The epithelial cells in turn regulate the expression of matrix metalloproteinases (MMPs), extracellular matrix (ECM) proteins and integrins during cell migration.^1,3,4 TGF-β is a well-known cytokine involved in processes such as cell growth inhibition, embryogenesis, morphogenesis, tumorigenesis, differentiation, wound healing, senescence and apoptosis (reviewed in refs. ⁵ and ⁶). It is also one of the most important cytokines responsible for promoting the migration of skin keratinocytes and corneal epithelial cells.^3,6,7TGFβ has two quite different effects on skin keratinocytes: it suppresses their multiplication and promotes their migration. The TGFβ-induced cell growth inhibition is usually mediated by Smad signaling, which upregulates expression of the cell cycle inhibitor p21^WAF1/Cip1 or p12^CDK2-AP1 in HaCaT skin keratinocyte cells and human primary foreskin keratinocytes.^8,9 Keratinocyte migration in wounded skin is associated with strong expression of TGFβ and MMPs,¹ and TGFβ stimulates the migration of manually scratched wounded HaCaT cells.¹⁰ TGFβ also induces cell migration and inhibits proliferation of injured corneal epithelial cells, whereas it stimulates proliferation of normal corneal epithelial cells via effects on the MAPK family and Smad signaling.^2,7 Indeed, skin keratinocytes and corneal epithelial cells display the same two physiological responses to TGFβ during wound healing; cell migration and growth inhibition. However as mentioned above, TGFβ has a different effect on normal cells. For example, it induces the epithelial to mesenchymal transition (EMT) of normal mammary cells and lens epithelial cells.^11,12 It also promotes the differentiation of corneal epithelial cells, and induces the fibrosis of various tissues.^2,6The MMPs are a family of structurally related zinc-dependent endopeptidases that are secreted into the extracellular environment.¹³ Members of the MMP family have been classified into gelatinases, stromelysins, collagenases and membrane type-MMPs (MT-MMPs) depending on their substrate specificity and structural properties. Like TGFβ, MMPs influence normal physiological processes including wound healing, tissue remodeling, angiogenesis and embryonic development, as well as pathological conditions such as rheumatoid arthritis, atherosclerosis and tumor invasion.^13,14The expression patterns of MMPs during skin and cornea wound healing are well studied. In rats, MMP-2, -3, -9, -11, -13 and -14 are expressed,¹⁵ and in mice, MMP-1, -2, -3, -9, -10 and -14 are expressed during skin wound healing.¹ MMP-1, -3, -7 and -12 are increased in corneal epithelial cells during Wnt 7a-induced rat cornea wound healing.¹⁶ Wound repair after excimer laser keratectomy is characterized by increased expression of MMP-1, -2, -3 and -9 in the rabbit cornea, and MMP-2, -9 in the rat cornea.^17,18 The expression of MMP-2 and -9 during skin keratinocyte and corneal epithelial cell migration has been the most thoroughly investigated, and it has been shown that their expression generally depends on the activity of MMP-14. MMP-14 (MT1-MMP) is constitutively anchored to the cell membrane; it activates other MMPs such as MMP-2, and also cleaves various types of ECM molecules including collagens, laminins, fibronectin as well as its ligands, the integrins.¹³ The latent forms of some cytokines are also cleaved and activated by MMP-14.¹⁹ Overexpression of MMP-14 protein was found to stimulate HT1080 human fibrosarcoma cell migration.²⁰ In contrast, the attenuation of MMP-14 expression using siRNA method decreased fibroblast invasiveness,²¹ angiogenesis of human microvascular endothelial cells,²² and human skin keratinocyte migration.¹⁰ The latter effect was shown to result from lowering MMP-9 expression. Other studies have shown that EGF has a critical role in MMP-9 expression during keratinocyte tumorigenesis and migration.^23,24 On the other hand, TGFβ modulates MMP-9 production through the Ras/MAPK pathway in transformed mouse keratinocytes and NFκB induces cell migration by binding to the MMP-9 promoter in human skin primary cultures.^25,26 Enhanced levels of pro-MMP-9 and active MMP-9 have also been noted in scratched corneal epithelia of diabetic rats.²⁷There is evidence that MMP-14 activates a number of intracellular signaling pathways including the MAPK family pathway, focal adhesion kinase (FAK), Src family, Rac and CD44, during cell migration and tumor invasion.^19,20,28 In COS-7 cells, ERK activation is stimulated by overexpression of MMP-14 and is essential for cell migration.²⁹ These observations all indicate that MMP-14 plays an important role in cell migration, not only by regulating the activity or expression of downstream MMPs but also by processing and activating migration-associated molecules such as integrins, ECMs and a variety of intracellular signaling pathays.³⁰Cell migration during wound healing is a remarkably complex phenomenon. TGFβ is just one small component of the overall process of wound healing and yet it triggers a multitude of reactions needed for cell migration. It is important to know what kinds of molecules are expressed when cell migration is initiated, but it is equally important to investigate the roles of these molecules and how their expression is regulated. Despite the availability of some information about how MMPs and signaling molecules can influence each other, much remains to be discovered in this area. It will be especially important to clarify how MMP-14 influences other signaling pathways since its role in cell migration is not restricted to digesting ECM molecules but also includes direct or indirect activation of cellular signaling pathways.     

Neutralization of Clostridium difficile toxin A using antibody combinations

The pathogenicity of Clostridium difficile (C. difficile) is mediated by the release of two toxins, A and B. Both toxins contain large clusters of repeats known as cell wall binding (CWB) domains responsible for binding epithelial cell surfaces. Several murine monoclonal antibodies were generated against the CWB domain of toxin A and screened for their ability to neutralize the toxin individually and in combination. Three antibodies capable of neutralizing toxin A all recognized multiple sites on toxin A, suggesting that the extent of surface coverage may contribute to neutralization. Combination of two noncompeting antibodies, denoted 3358 and 3359, enhanced toxin A neutralization over saturating levels of single antibodies. Antibody 3358 increased the level of detectable CWB domain on the surface of cells, while 3359 inhibited CWB domain cell surface association. These results suggest that antibody combinations that cover a broader epitope space on the CWB repeat domains of toxin A (and potentially toxin B) and utilize multiple mechanisms to reduce toxin internalization may provide enhanced protection against C. difficile-associated diarrhea.Key words: Clostridium difficile, toxin neutralization, therapeutic antibody, cell wall binding domains, repeat proteins, CROPs, mAb combinationThe most common cause of nosocomial antibiotic-associated diarrhea is the gram-positive, spore-forming anaerobic bacillus Clostridium difficile (C. difficile). Infection can be asymptomatic or lead to acute diarrhea, colitis, and in severe instances, pseudomembranous colitis and toxic megacolon.^1,2The pathological effects of C. difficile have long been linked to two secreted toxins, A and B.^3,4 Some strains, particularly the virulent and antibiotic-resistant strain 027 with toxinotype III, also produce a binary toxin whose significance in the pathogenicity and severity of disease is still unclear.⁵ Early studies including in vitro cell-killing assays and ex vivo models indicated that toxin A is more toxigenic than toxin B; however, recent gene manipulation studies and the emergence of virulent C. difficile strains that do not express significant levels of toxin A (termed “A⁻ B⁺”) suggest a critical role for toxin B in pathogenicity.^6,7Toxins A and B are large multidomain proteins with high homology to one another. The N-terminal region of both toxins enzymatically glucosylates small GTP binding proteins including Rho, Rac and CDC42,^8,9 leading to altered actin expression and the disruption of cytoskeletal integrity.^9,10 The C-terminal region of both toxins is composed of 20 to 30 residue repeats known as the clostridial repetitive oligopeptides (CROPs) or cell wall binding (CWB) domains due to their homology to the repeats of Streptococcus pneumoniae LytA,^11–14 and is responsible for cell surface recognition and endocytosis.^12,15–17C. difficile-associated diarrhea is often, but not always, induced by antibiotic clearance of the normal intestinal flora followed by mucosal C. difficile colonization resulting from preexisting antibiotic resistant C. difficile or concomitant exposure to C. difficile spores, particularly in hospitals. Treatments for C. difficile include administration of metronidazole or vancomycin.^2,18 These agents are effective; however, approximately 20% of patients relapse. Resistance of C. difficile to these antibiotics is also an emerging issue^19,20 and various non-antibiotic treatments are under investigation.^20–25Because hospital patients who contract C. difficile and remain asymptomatic have generally mounted strong antibody responses to the toxins,^26,27 active or passive immunization approaches are considered hopeful avenues of treatment for the disease. Toxins A and B have been the primary targets for immunization approaches.^20,28–33 Polyclonal antibodies against toxins A and B, particularly those that recognize the CWB domains, have been shown to effectively neutralize the toxins and inhibit morbidity in rodent infection models.³¹ Monoclonal antibodies (mAbs) against the CWB domains of the toxins have also demonstrated neutralizing capabilities; however, their activity in cell-based assays is significantly weaker than that observed for polyclonal antibody mixtures.^33–36We investigated the possibility of creating a cocktail of two or more neutralizing mAbs that target the CWB domain of toxin A with the goal of synthetically re-creating the superior neutralization properties of polyclonal antibody mixtures. Using the entire CWB domain of toxin A, antibodies were raised in rodents and screened for their ability to neutralize toxin A in a cell-based assay. Two mAbs, 3358 and 3359, that (1) both independently demonstrated marginal neutralization behavior and (2) did not cross-block one another from binding toxin A were identified. We report here that 3358 and 3359 use differing mechanisms to modify CWB-domain association with CHO cell surfaces and combine favorably to reduce toxin A-mediated cell lysis.     

Novel protein-protein interaction family proteins involved in chloroplast movement response

To optimize photosynthetic activity, chloroplasts change their intracellular location in response to ambient light conditions; chloroplasts move toward low intensity light to maximize light capture and away from high intensity light to avoid photodamage. Although several proteins have been reported to be involved in chloroplast photorelocation movement response, any physical interaction among them was not found so far. We recently found a physical interaction between two plant-specific coiled-coil proteins, WEB1 (Weak Chloroplast Movement under Blue Light 1) and PMI2 (Plastid Movement Impaired 2), that were indentified to regulate chloroplast movement velocity. Since the both coiled-coil regions of WEB1 and PMI2 were classified into an uncharacterized protein family having DUF827 (DUF: Domain of Unknown Function) domain, it was the first report that DUF827 proteins could mediate protein-protein interaction. In this mini-review article, we discuss regarding molecular function of WEB1 and PMI2, and also define a novel protein family composed of WEB1, PMI2 and WEB1/PMI2-like proteins for protein-protein interaction in land plants.Key words: Arabidopsis, blue light, chloroplast velocity, coiled-coil region, organelle movement, phototropin, protein-protein interactionIntracellular locations of chloroplasts change in response to different light conditions to capture sunlight efficiently for energy production through photosynthesis. Chloroplasts move toward weak light to maximize light capture (the accumulation response),^1,2 and away from strong light to reduce photodamage (the avoidance response).³ In higher plants such as Arabidopsis thaliana, the responses are induced by blue light-dependent manner.^1,2 Recently, chloroplast actin (cp-actin) filaments were found to be involved in chloroplast photorelocation movement and positioning.^4,5 The cp-actin filaments are localized at the interface between the chloroplast and the plasma membrane to anchor the chloroplast to the plasma membrane, and are relocalized to the leading edge of chloroplasts before and during the movement.^4,5 The difference of cp-actin filament amounts between the front and the rear halves of chloroplasts determines the chloroplast movement velocity; as the difference increases, chloroplast velocity also increases.^4,5Several proteins have been reported to be involved in chloroplast movement. The blue light receptors, phototropin 1 (phot1) and phot2, mediate the accumulation response,⁶ and phot2 solely mediates the avoidance response.^7,8 Chloroplast Unusual Positioning 1 (CHUP1), Kinesin-like Protein for Actin-Based Chloroplast Movement 1 (KAC1) and KAC2 are involved in the cp-actin filament formation.^4,9–11 Other proteins with unknown molecular function involved in the chloroplast movement responses have also been reported. They are J-domain Protein Required for Chloroplast Accumulation Response 1 (JAC1),^12,13 Plastid Movement Impaired 1 (PMI1),¹⁴ a long coiled-coil protein Plastid Movement Impaired 2 (PMI2), a PMI2-homologous protein PMI15,¹⁵ and THRUMIN1.¹⁶Recently, we characterized two plant-specific coiled-coil proteins, Weak Chloroplast Movement under Blue Light 1 (WEB1) and PMI2, which regulate the velocity of chloroplast photorelocation movement.¹⁷ In this mini-review article, we discuss about molecular function of WEB1 and PMI2 in chloroplast photorelocation movement, and also define the WEB1/PMI2-related (WPR) protein family as a new protein family for protein-protein interaction.     

Protein interactors of acyl-CoA-binding protein ACBP2 mediate cadmium tolerance in Arabidopsis

In our recent paper in the Plant Journal, we reported that Arabidopsis thaliana lysophospholipase 2 (lysoPL2) binds acyl-CoA-binding protein 2 (ACBP2) to mediate cadmium [Cd(II)] tolerance in transgenic Arabidopsis. ACBP2 contains ankyrin repeats that have been previously shown to mediate protein-protein interactions with an ethylene-responsive element binding protein (AtEBP) and a farnesylated protein 6 (AtFP6). Transgenic Arabidopsis ACBP2-overexpressors, lysoPL2-overexpressors and AtFP6-overexpressors all display enhanced Cd(II) tolerance, in comparison to wild type, suggesting that ACBP2 and its protein partners work together to mediate Cd(II) tolerance. Given that recombinant ACBP2 and AtFP6 can independently bind Cd(II) in vitro, they may be able to participate in Cd(II) translocation. The binding of recombinant ACBP2 to [¹⁴C]linoleoyl-CoA and [¹⁴C]linolenoyl-CoA implies its role in phospholipid repair. In conclusion, ACBP2 can mediate tolerance to Cd(II)-induced oxidative stress by interacting with two protein partners, AtFP6 and lysoPL2. Observations that ACBP2 also binds lysophosphatidylcholine (lysoPC) in vitro and that recombinant lysoPL2 degrades lysoPC, further confirm an interactive role for ACBP2 and lysoPL2 in overcoming Cd(II)-induced stress.Key words: acyl-CoA-binding protein, cadmium, hydrogen peroxide, lysophospholipase, oxidative stressAcyl-CoA-binding proteins (ACBP1 to ACBP6) are encoded by a multigene family in Arabidopsis thaliana.¹ These ACBP proteins are well studied in Arabidopsis in comparison to other organisms,^1–4 and are located in various subcellular compartments.¹ Plasma membranelocalized ACBP1 and ACBP2 contain ankyrin repeats that have been shown to function in protein-protein interactions.^5,6 ACBP1 and ACBP2 which share 76.9% amino acid identity also confer tolerance in transgenic Arabidopsis to lead [Pb(II)] and Cd(II), respectively.^1,5,7 Since recombinant ACBP1 and ACBP2 bind linolenoyl-CoA and linoleoyl-CoA in vitro, they may possibly be involved in phospholipid repair in response to heavy metal stress at the plasma membrane.^5,7 In contrast, ACBP3 is an extracellularly-localized protein⁸ while ACBP4, ACBP5 and ACBP6 are localized to cytosol.^9,10 ACBP1 and ACBP6 have recently been shown to be involved in freezing stress.^9,11 ACBP4 and ACBP5 bind oleoyl-CoA ester and their mRNA expressions are lightregulated.^12,13 Besides acyl-CoA esters, some ACBPs also bind phospholipids.^9,11,13 To investigate the biological function of ACBP2, we have proceeded to establish its interactors at the ankyrin repeats, including AtFP6,⁵ AtEBP⁶ and now lysoPL2 in the Plant Journal paper. While the significance in the interaction of ACBP2 with AtEBP awaits further investigations, some parallels can be drawn between those of ACBP2 with AtFP6 and with lysoPL2.