BROTHER OF FT AND TFL1 (BFT), a member of the FT/TFL1 family,shows distinct pattern of expression during the vegetative growth of Arabidopsis

Transition to the flowering stage is precisely controlled by a few classes of regulatory molecules. BROTHER OF FT AND TFL1 (BFT) is a member of FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family, an important class of flower development regulators with unidentified biochemical function. BFT has a TFL1-like activity and plays a role in axillary inflorescence development. To elucidate the expression pattern of BFT, we analyzed the subcellular localization and conditional expression of BFT in this study. We generated 35S::BFT:GFP plants to investigate the subcellular localization of BFT protein. 35S::BFT:GFP plants showed late flowering, similarly as did 35S::BFT plants. BFT:GFP fusion protein was localized in the nucleus and the plasma membrane, which was different from the localization pattern of FT and TFL1. BFT expression was induced by abiotic stress conditions. ABA, drought, and osmotic stress treatments induced BFT expression, whereas cold, salt, and heat stress conditions did not, suggesting that BFT plays a role in regulating flowering time and inflorescence structure under drought conditions. The induction pattern of BFT was different from those of other FT/TFL1 family genes. Our studies indicated that BFT showed a distinct expression pattern from its homologous genes during the vegetative growth in Arabidopsis.Key words: flowering time, flowering locus T, terminal flower 1, brother of FT and TFL1, abiotic stress, subcellullar localizationThe FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family is a small gene family whose members play a pivotal role in flower development in Arabidopsis. The family includes FT, TFL1, TWIN SISTER OF FT (TSF), Arabidopsis thaliana CENTRORADIALIS homologue (ATC), MOTHER OF FT AND TFL1 (MFT) and BROTHER OF FT AND TFL1 (BFT).^{3,5,6,9,15,17} FT is a floral promoter that integrates signal inputs from various pathways that regulate flowering time in Arabidopsis.^5,6 TFL1 plays an antagonistic role to that of FT, functioning as a floral inhibitor. Unlike FT, TFL1 also plays an important role in controlling plant architecture by regulating the expression of LEAFY (LFY) and APETALA1 (AP1), two important floral meristem identity genes in the shoot apical meristem (SAM).^3,7 TSF regulates flowering by a mechanism similar to that of FT, although a lesion in TSF does not have an apparent effect on the determination of flowering time. MFT has a weak FT-like activity.¹⁷ ATC acts as a floral repressor, and its role is similar to that of TFL1.⁹ Finally, BFT has a TFL1-like activity, in spite of its amino acid homology to FT,^2,4,16 and functions redundantly with TFL1 in inflorescence meristem development in Arabidopsis.¹⁶ Although genetic studies elucidated an intricate role of the FT/TFL1 family genes, not much is known about the expression pattern of the remaining members except FT and TFL1. Here, we report that BFT expression showed a distinct pattern from its homologous genes during the vegetative phase. BFT:GFP fusion protein was detected in the nucleus and the plasma membrane. BFT expression was induced by abiotic stress conditions, distinct from other FT/TFL1 family genes, raising the possibility that BFT plays a role in regulating flowering time and inflorescence structure under drought conditions.     

Flowering regulation by tissue specific functions of photoreceptors

Flowering is one of the most important steps in a plant life cycle. Plants utilize light as an informational source to determine the timing of flowering. In Arabidopsis, phytochrome A (phyA), phyB and cryptochrome2 (cry2) are major photoreceptors that regulate flowering. These photoreceptors perceive light stimuli by leaves for the regulation of flowering. A leaf is an organ consisting of different tissues such as epidermis, mesophyll and vascular bundles. In the present study, we examined in which tissue the light signals are perceived and how those signals are integrated within a leaf to regulate flowering. For this purpose, we established transgenic Arabidopsis lines that expressed a phyB-green fluorescent protein (GFP) fusion protein or a cry2-GFP fusion protein in organ/tissue-specific manners. Consequently, phyB was shown to perceive light stimuli in mesophyll. By contrast, cry2 functioned only in vascular bundles. We further confirmed that both phyB-GFP and cry2-GFP regulated flowering by altering the expression of a key flowering gene, FT, in vascular bundles. In summary, perception sites for different spectra of light are spatially separated within a leaf and the signals are integrated through the inter-tissue communication.Key words: photoreceptor, light, flowering, phytochrome, cryptochrome, inter-tissue signalThe timing of flowering is strictly regulated by environmental conditions such as light. Two aspects of light, spectral nature and photoperiod, dramatically affect flowering. In Arabidopsis, phyB and phyA/cry2 are the major photoreceptors mediating these responses. Although photoreceptors are expressed in almost all organs,¹ partial irradiation and grafting analyses have demonstrated that plants perceive light signals only in leaves.^2–4 However, roles for different tissues in a leaf remained unknown due to a lack of a proper method. To answer the question, we established Arabidopsis transgenic lines that expressed phyB-GFP or cry2-GFP on the respective mutant backgrounds. The resultant transgenic lines were examined for their flowering phenotype. Consequently, we found that phyB-GFP in mesophyll but not in other tissues regulated flowering.⁵ By contrast, cry2-GFP functioned only in vascular bundles.⁶A strong genetic interaction between phyB and cry2 in the regulation of flowering is known.^7,8 Cry2 regulates the flowering by suppressing the inhibitory effect of phyB on flowering. Hence, cry2 function is observed only in the presence of phyB. Conversely, the effect of phyB is exaggerated in the cry2 mutant, because phyB is not counteracted by cry2 in its absence. Here, we tested how phyB and cry2 in different tissues regulated flowering in the absence of the other photoreceptor. For this purpose, we took a physiological approach. Phenotype of the phyB-GFP lines was examined under monochromatic red light, in which phyB but not cyr2 is activated. As expected, phyB-GFP in mesophyll but not in vascular bundles strongly affected the flowering in this condition (Fig. 1A). We also tested the cry2-GFP function when phyB was not activated. Namely, plants were placed under blue light supplemented with strong far-red light. As expected, cry2-GFP failed to affect the flowering even under this condition regardless of where it was expressed (Fig. 1B).Open in a separate windowFigure 1FT expression under phyB or cry2 inactive conditions. Total RNA was extracted from the seedlings grown under long-day condition for 10 days and subjected to qRT-PCR for FT expression analysis. Data were normalized to the level of FT mRNA in (A) of the wild type, which was set to 1 arbitrary unit (a.u.). Mean ± SE (n = 4). WT, wild type. (A) Long-day red light, (16L 8D; 10 µmol m^-2 s^-1). WT, wild type; phyB, phyB mutant; Bpro, PHYB promoter-PHYB-GFP; PBT56, phyB-GFP in mesophyll; PBT239, phyB-GFP in vascular bundles.⁵ (B) Long-day blue and far-red light (16L 8D; blue light, 3 µmol m^-2 s^-1; far-red light, 10 µmol m^-2 s^-1). WT, wild type; cry2, cry2 mutant; pCRY-C2G, CRY2 promoter-CRY2-GFP; pCAB-C2G, CAB3 promoter-CRY2-GFP; pSUC-C2G, SUC2 promoter-CRY2-GFP.⁶Photoreceptors regulate flowering by altering the expression of a key flowering regulator, FT.^9,10 Interestingly, the FT gene is expressed specifically in vascular bundles.¹¹ Indeed, mesophyll phyB-GFP controlled the expression of FT in vascular bundles. Hence, there must be a mechanism by which the light signal is transduced from mesophyll to vascular bundles to regulate the FT expression in vascular bundles. It should be noted here that FT is not the sole factor involved in the light regulation of flowering. Factors such as CO, SPA, COP1 and PFT1 are known to link the photoreceptors and FT.^12–14 These factors most likely function in leaves. However, their function sites at the tissue level remain totally unknown except for CO. The biological clock is another class of machinery that is tightly related to the light signal transduction pathway.¹⁵ Unfortunately, function sites of the clock components for the regulation of flowering remain unclear. The future work should reveal those sites. Such analyses should finally provide a complete picture illustrating a network of the inter-tissue signaling for the regulation of flowering.The present work urges us to indentify the molecule that mediates the inter-tissue signaling between mesophyll and vascular bundles. Potential candidates include phytohormones, microRNA¹⁶ and peptides.¹⁷ Among phytohormones, gibberellin promotes flowering.¹⁸ However, gibberellin is probably not the answer because gibberellin does not alter the FT expression directly. Except gibberellin, no exogenously added phythromone dramatically affects flowering in Arabidopsis. It is known that microRNA such as miR172, miR159 and miR156 are involved in the regulation of flowering time.¹⁹ However, those microRNA''s neither regulate the FT expression nor are regulated by light. Since most of microRNA''s has not been intensively studied yet, it remains possible that one of them may mediate the above inter-tissue signal. Another potential candidate is a peptide. Although not much is known about peptide hormones in plants yet, peptides such as PSK,²⁰ xylogen²¹ and CLE²² have been shown to regulate cell growth and differentiation. Although none of peptides is known to regulate flowering in plants at present, a future work may reveal a novel peptide that mediates the inter-tissue signals for flowering.     

Flowering time regulation by the SUMO E3 ligase SIZ1

Flowering is a developmental process, which is influenced by chemical and environmental stimuli. Recently, our research established that the Arabidopsis SUMO E3 ligase, AtSIZ1, is a negative regulator of transition to flowering through mechanisms that reduce salicylic acid (SA) accumulation and involve SUMO modification of FLOWERING LOCUS D (FLD). FLD is an autonomous pathway determinant that represses the expression of FLOWERING LOCUS C (FLC), a floral repressor. This addendum postulates mechanisms by which SIZ1-mediated SUMO conjugation regulates SA accumulation and FLD activity.Key words: SIZ1, SA, flowering, SUMO, FLD, FLCSUMO conjugation and deconjugation are post-translational processes implicated in plant defense against pathogens, abscisic acid (ABA) and phosphate (Pi) starvation signaling, development, and drought and temperature stress tolerance, albeit only a few of the modified proteins have been identified.^1–8 The Arabidopsis AtSIZ1 locus encodes a SUMO E3 ligase that regulates floral transition and leaf development.^8,9 siz1 plants accumulate substantial levels of SA, which is the primary cause for dwarfism and early short-day flowering exhibited by these plants.^1,9 How SA promotes transition to flowering is not yet known but apparently, it is through a mechanism that is independent of the known floral signaling pathways.^9,10 Exogenous SA reduces expression of AGAMOUS-like 15 (AGL15), a floral repressor that functions redundantly with AGL18.^11,12 A possible mechanism by which SA promotes transition to flowering may be by repressing expression of AGL15 and AGL18 (Fig. 1).Open in a separate windowFigure 1Model of how SUMO conjugation and deconjugation regulate plant development in Arabidopsis. SIZ1 and Avr proteins regulate biosynthesis and accumulation of SA, a plant stress hormone that is involved in plant innate immunity, leaf development and regulation of flowering time. SA promotes transition to flowering may through AGL15/AGL18 dependent and independent pathways. FLC expression is activated by FRIGIDA but repressed by the autonomous pathway gene FLD, and SIZ1-mediated sumoylation of FLD represses its activity. Lines with arrows indicate upregulation (activation), and those with bars identify downregulation (repression).siz1 mutations also cause constitutive induction of pathogenesis-related protein genes leading to enhanced resistance against biotrophic pathogens.¹ Several bacterial type III effector proteins, such as YopJ, XopD and AvrXv4, have SUMO isopeptidase activity.^13–15 PopP2, a member of YopJ/AvrRxv bacterial type III effector protein family, physically interacts with the TIR-NBS-LRR type R protein RRS1, and possibly stabilizes the RRS1 protein.¹⁶ Phytopathogen effector and plant R protein interactions lead to increased SA biosynthesis and accumulation, which in turn activates expression of pathogenesis-related proteins that facilitate plant defense.¹⁷ SIZ1 may participate in SUMO conjugation of plant R proteins to regulate Avr and R protein interactions leading to SA accumulation, which, in turn, affects phenotypes such as diseases resistance, dwarfism and flowering time (Fig. 1).Our recent work revealed also that AtSIZ1 facilitates FLC expression, negatively regulating flowering.⁹ AtSIZ1 promotes FLC expression by repressing FLD activity.⁹ Site-specific mutations that prevent SUMO1/2 conjugation to FLD result in enhanced activity of the protein to represses FLC expression, which is associated with reduced acetylation of histone 4 (H4) in FLC chromatin.⁹ FLD, an Arabidopsis ortholog of Lysine-Specific Demethylase 1 (LSD1), is a floral activator that downregulates methylation of H3K4 in FLC chromatin and represses FLC expression.^18,19 Interestingly, bacteria expressing recombinant FLD protein did not demethylate H3K4me2, inferring that the demethylase activity requires additional co-factors as are necessary for LSD1.^18,20 Together, these results suggest that SIZ1-mediated SUMO modification of FLD may affect interactions between FLD and co-factors, which is necessary for FLC chromatin modification.Despite our results that implicate SA in flowering time control, how SIZ1 regulates SA accumulation and the identity of the effectors involved remain to be discovered. In addition, it remains to be determined if SIZ1 is involved in other mechanisms that modulate FLD activity and FLC expression, or the function of other autonomous pathway determinants.     

Loss of LORELEI function in the pistil delays initiation but does not affect embryo development in Arabidopsis thaliana

Double fertilization, uniquely observed in plants, requires successful sperm cell delivery by the pollen tube to the female gametophyte, followed by migration, recognition and fusion of the two sperm cells with two female gametic cells. The female gametophyte not only regulates these steps but also controls the subsequent initiation of seed development. Previously, we reported that loss of LORELEI, which encodes a putative glycosylphosphatidylinositol (GPI)-anchored protein, in the female reproductive tissues causes a delay in initiation of seed development. From these studies, however, it was unclear if embryos derived from fertilization of lre-5 gametophytes continued to lag behind wild-type during seed development. Additionally, it was not determined if the delay in initiation of seed development had any lingering effects during seed germination. Finally, it was not known if loss of LORELEI function affects seedling development given that LORELEI is expressed in eight-day-old seedlings. Here, we showed that despite a delay in initiation, lre-5/lre-5 embryos recover, becoming equivalent to the developing wild-type embryos beginning at 72 hours after pollination. Additionally, lre-5/lre-5 seed germination, and seedling and root development are indistinguishable from wild-type indicating that loss of LORELEI is tolerated, at least under standard growth conditions, in vegetative tissues.Key words: LORELEI, glycosylphosphatidylinositol (GPI)-anchored protein, embryogenesis, DD45, seed germination, primary and lateral root growth, seedling developmentDouble fertilization is unique to flowering plants. Upon female gametophyte reception of a pollen tube, the egg and central cells of the female gametophyte fuse with the two released sperm cells to form zygote and endosperm, respectively and initiate seed development.¹ The female gametophyte controls seed development by (1) repressing premature central cell or egg cell proliferation until double fertilization is completed,^1–3 (2) supplying factors that mediate early stages of embryo and endosperm development^1,4,5 and (3) regulating imprinting of genes required for seed development.^1,6The molecular mechanisms underlying female gametophyte control of early seed development are poorly understood. Although much progress has been made in identifying genes and mechanisms by which the female gametophyte represses premature central cell or egg cell proliferation until double fertilization is completed and regulates imprinting of genes required for seed development,^1,6 only a handful of female gametophyte-expressed genes that affect early embryo^7,8 and endosperm⁹ development after fertilization have been characterized. This is particularly important given that a large number of female gametophyte-expressed genes likely regulate early seed development.⁵We recently reported on a mutant screen for plants with reduced fertility and identification of a mutant that contained a large number of undeveloped ovules and very few viable seeds.¹⁰ TAIL-PCR revealed that this mutant is a new allele of LORELEI(LRE) [At4g26466].^10,11 Four lre alleles have been reported;¹¹ so, this mutant was designated lre-5.¹⁰ The Arabidopsis LORELEI protein contains 165 amino acids and possesses sequence features indicative of a glycosylphosphatidylinositol (GPI)-anchor containing cell surface protein. GPI-anchors serve as an alternative to transmembrane domains for anchoring proteins in cell membranes and GPI-anchored proteins participate in many functions including cell-cell signaling.¹²     

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.     

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     

Complete Genome Sequence of Croceibacter atlanticus HTCC2559T

Here we announce the complete genome sequence of Croceibacter atlanticus HTCC2559^T, which was isolated by high-throughput dilution-to-extinction culturing from the Bermuda Atlantic Time Series station in the Western Sargasso Sea. Strain HTCC2559^T contained genes for carotenoid biosynthesis, flavonoid biosynthesis, and several macromolecule-degrading enzymes. The genome confirmed physiological observations of cultivated Croceibacter atlanticus strain HTCC2559^T, which identified it as an obligate chemoheterotroph.The phylum Bacteroidetes comprises 6 to ∼30% of total bacterial communities in the ocean by fluorescence in situ hybridization (8-10). Most marine Bacteroidetes are in the family Flavobacteriaceae, most of which are aerobic respiratory heterotrophs that form a well-defined clade by 16S rRNA phylogenetic analyses (4). The members of this family are well known for degrading macromolecules, including chitin, DNA, cellulose, starch, and pectin (17), suggesting their environmental roles as detritus decomposers in the ocean (6). Marine Polaribacter and Dokdonia species in the Flavobacteriaceae have also shown to have photoheterotrophic metabolism mediated by proteorhodopsins (11, 12).Several strains of the family Flavobacteriaceae were isolated from the Sargasso Sea and Oregon coast, using high-throughput culturing approaches (7). Croceibacter atlanticus HTCC2559^T was cultivated from seawater collected at a depth of 250 m from the Sargasso Sea and was identified as a new genus in the family Flavobacteriaceae based on its 16S rRNA gene sequence similarities (6). Strain HTCC2559^T met the minimal standards for genera of the family Flavobacteriaceae (3) on the basis of phenotypic characteristics (6).Here we report the complete genome sequence of Croceibacter atlanticus HTCC2559^T. The genome sequencing was initiated by the J. Craig Venter Institute as a part of the Moore Foundation Microbial Genome Sequencing Project and completed in the current announcement. Gaps among contigs were closed by Genotech Co., Ltd. (Daejeon, Korea), using direct sequencing of combinatorial PCR products (16). The HTCC2559^T genome was analyzed with a genome annotation system based on GenDB (14) at Oregon State University and with the NCBI Prokaryotic Genomes Automatic Annotation Pipeline (15, 16).The HTCC2559^T genome is 2,952,962 bp long, with 33.9 mol% G+C content, and there was no evidence of plasmids. The number of protein-coding genes was 2,715; there were two copies of the 16S-23S-5S rRNA operon and 36 tRNA genes. The HTCC2559^T genome contained genes for a complete tricarboxylic acid cycle, glycolysis, and a pentose phosphate pathway. The genome also contained sets of genes for metabolic enzymes involved in carotenoid biosynthesis and also a serine/glycine hydroxymethyltransferase, which is often associated with the assimilatory serine cycle (13). The potential for HTCC2559^T to use bacterial type III polyketide synthase (PKS) needs to be confirmed because this organism had a naringenin-chalcone synthase (CHS) or chalcone synthase (EC 2.3.1.74), a key enzyme in flavonoid biosynthesis. CHS initiates the addition of three molecules of malonyl coenzyme A (malonyl-CoA) to a starter CoA ester (e.g., 4-coumaroyl-CoA) (1) and takes part in a few bacterial type III polyketide synthase systems (1, 2, 5, 18).The complete genome sequence confirmed that strain HTCC2559^T is an obligate chemoheterotroph because no genes for phototrophy were found. As expected from physiological characteristics (6), the HTCC2559^T genome contained a set of genes coding for enzymes required to degrade high-molecular-weight compounds, including peptidases, metallo-/serine proteases, pectinase, alginate lyases, and α-amylase.     

Timing the switch to phototrophic growth: A possible role of GUN1

In young Arabidopsis seedlings, retrograde signaling from plastids regulates the expression of photosynthesis-associated nuclear genes in response to the developmental and functional state of the chloroplasts. The chloroplast-located PPR protein GUN1 is required for signalling following disruption of plastid protein synthesis early in seedling development before full photosynthetic competence has been achieved. Recently we showed that sucrose repression and the correct temporal expression of LHCB1, encoding a light-harvesting chlorophyll protein associated with photosystem II, are perturbed in gun1 mutant seedlings.¹ Additionally, we demonstrated that in gun1 seedlings anthocyanin accumulation and the expression of the “early” anthocyanin-biosynthesis genes is perturbed. Early seedling development, predominantly at the stage of hypocotyl elongation and cotyledon expansion, is also affected in gun1 seedlings in response to sucrose, ABA and disruption of plastid protein synthesis by lincomycin. These findings indicate a central role for GUN1 in plastid, sucrose and ABA signalling in early seedling development.Key words: ABA, ABI4, anthocyanin, chloroplast, GUN1, retrograde signalling, sucroseArabidopsis seedlings develop in response to light and other environmental cues. In young seedlings, development is fuelled by mobilization of lipid reserves until chloroplast biogenesis is complete and the seedlings can make the transition to phototrophic growth. The majority of proteins with functions related to photosynthesis are encoded by the nuclear genome, and their expression is coordinated with the expression of genes in the chloroplast genome. In developing seedlings, retrograde signaling from chloroplasts to the nucleus regulates the expression of these nuclear genes and is dependent on the developmental and functional status of the chloroplast. Two classes of gun (genomes uncoupled) mutants defective in retrograde signalling have been identified in Arabidopsis: the first, which comprises gun2–gun5, involves mutations in genes encoding components of tetrapyrrole biosynthesis.^2,3 The other comprises gun1, which has mutations in a nuclear gene encoding a plastid-located pentatricopeptide repeat (PPR) protein with an SMR (small MutS-related) domain near the C-terminus.^4,5 PPR proteins are known to have roles in RNA processing⁶ and the SMR domain of GUN1 has been shown to bind DNA,⁴ but the specific functions of these domains in GUN1 are not yet established. However, GUN1 has been shown to be involved in plastid gene expression-dependent,⁷ redox,⁴ ABA1,⁴ and sucrose signaling,^1,4,8 as well as light quality and intensity sensing pathways.^9–11 In addition, GUN1 has been shown to influence anthocyanin biosynthesis, hypocotyl extension and cotyledon expansion.^1,11