Impaired growth in transgenic plants over-expressing an expansin isoform

Expansins are cell wall proteins characterised by their ability to stimulate wall loosening during cell expansion. The expression of some expansin isoforms is clearly correlated with growth and the external application of expansins can stimulate cell expansion in vivo in several systems. We report here the expression of a heterologous expansin coding sequence in transgenic tomato plants (Lycopersicon esculentum Mill.) under the control of a constitutive promoter. In some transgenic lines with high levels of expansin activity extractable from cell walls, we observed alterations of growth: mature plants were stunted, with shorter leaves and internodes, and dark-grown seedlings had shorter and wider hypocotyls than their wild-type counterparts. Examination of hypocotyl sections revealed similar differences at the cellular level: cortical and epidermal cells were shorter and wider than those from wild-type seedlings. The observed stimulation of radial expansion did not compensate for the decreased elongation, and overall growth was reduced in the transgenics. As this observation can seem paradoxical given the known effect of expansins on isolated cell walls, we examined the mechanical behaviour of transgenic tissue. We measured a decrease in hypocotyl elongation in response to acidic pH in the transformants. This result may account for the alterations in cell expansion, and could itself be explained by a reduced susceptibility of transgenic cell walls to expansin action.     

Gibberellins control Arabidopsis hypocotyl growth via regulation of cellular elongation

The gibberellins (GAs) are endogenous regulators of plant growth. Experiments are described here that test the hypothesis that GA regulates hypocotyl growth by altering the extent of hypocotyl cell elongation. These experiments use GA-deficient and altered GA-response mutants of Arabidopsis thaliana (L.) Heyhn. It is shown that GA regulates elongation, in both light- and dark-grown hypocotyls, by influencing the rate and final extent of cellular elongation. However, light- and dark-grown hypocotyls exhibit markedly different GA dose-response relationships. The length of dark-grown hypocotyls is relatively unaffected by exogenous GA, whilst light-grown hypocotyl length is significantly increased by exogenous GA. Further analysis suggests that GA control of hypocotyl length is close to saturation in dark-grown hypocotyls, but not in light grown hypocotyls. The results show that a large range of possible hypocotyl lengths is achieved via dose-dependent GA-regulated alterations in the degree of elongation of individual hypocotyl cells.Key words: Arabidopsis, cell elongation, gibberellin (GA), GA mutants, hypocotyl.      

Genome-Wide Analysis of the Expansin Gene Superfamily Reveals Grapevine-Specific Structural and Functional Characteristics

Background

Expansins are proteins that loosen plant cell walls in a pH-dependent manner, probably by increasing the relative movement among polymers thus causing irreversible expansion. The expansin superfamily (EXP) comprises four distinct families: expansin A (EXPA), expansin B (EXPB), expansin-like A (EXLA) and expansin-like B (EXLB). There is experimental evidence that EXPA and EXPB proteins are required for cell expansion and developmental processes involving cell wall modification, whereas the exact functions of EXLA and EXLB remain unclear. The complete grapevine (Vitis vinifera) genome sequence has allowed the characterization of many gene families, but an exhaustive genome-wide analysis of expansin gene expression has not been attempted thus far.

Methodology/Principal Findings

We identified 29 EXP superfamily genes in the grapevine genome, representing all four EXP families. Members of the same EXP family shared the same exon–intron structure, and phylogenetic analysis confirmed a closer relationship between EXP genes from woody species, i.e. grapevine and poplar (Populus trichocarpa), compared to those from Arabidopsis thaliana and rice (Oryza sativa). We also identified grapevine-specific duplication events involving the EXLB family. Global gene expression analysis confirmed a strong correlation among EXP genes expressed in mature and green/vegetative samples, respectively, as reported for other gene families in the recently-published grapevine gene expression atlas. We also observed the specific co-expression of EXLB genes in woody organs, and the involvement of certain grapevine EXP genes in berry development and post-harvest withering.

Conclusion

Our comprehensive analysis of the grapevine EXP superfamily confirmed and extended current knowledge about the structural and functional characteristics of this gene family, and also identified properties that are currently unique to grapevine expansin genes. Our data provide a model for the functional characterization of grapevine gene families by combining phylogenetic analysis with global gene expression profiling.     

Light-Regulated Hypocotyl Elongation Involves Proteasome-Dependent Degradation of the Microtubule Regulatory Protein WDL3 in Arabidopsis

Light significantly inhibits hypocotyl cell elongation, and dark-grown seedlings exhibit elongated, etiolated hypocotyls. Microtubule regulatory proteins function as positive or negative regulators that mediate hypocotyl cell elongation by altering microtubule organization. However, it remains unclear how plants coordinate these regulators to promote hypocotyl growth in darkness and inhibit growth in the light. Here, we demonstrate that WAVE-DAMPENED 2–LIKE3 (WDL3), a microtubule regulatory protein of the WVD2/WDL family from Arabidopsis thaliana, functions in hypocotyl cell elongation and is regulated by a ubiquitin-26S proteasome–dependent pathway in response to light. WDL3 RNA interference Arabidopsis seedlings grown in the light had much longer hypocotyls than controls. Moreover, WDL3 overexpression resulted in overall shortening of hypocotyl cells and stabilization of cortical microtubules in the light. Cortical microtubule reorganization occurred slowly in cells from WDL3 RNA interference transgenic lines but was accelerated in cells from WDL3-overexpressing seedlings subjected to light treatment. More importantly, WDL3 protein was abundant in the light but was degraded through the 26S proteasome pathway in the dark. Overexpression of WDL3 inhibited etiolated hypocotyl growth in regulatory particle non-ATPase subunit-1a mutant (rpn1a-4) plants but not in wild-type seedlings. Therefore, a ubiquitin-26S proteasome–dependent mechanism regulates the levels of WDL3 in response to light to modulate hypocotyl cell elongation.     

Composite Cucurbita pepo plants with transgenic roots as a tool to study root development

Background and Aims

In most plant species, initiation of lateral root primordia occurs above the elongation zone. However, in cucurbits and some other species, lateral root primordia initiation and development takes place in the apical meristem of the parental root. Composite transgenic plants obtained by Agrobacterium rhizogenes-mediated transformation are known as a suitable model to study root development. The aim of the present study was to establish this transformation technique for squash.

Methods

The auxin-responsive promoter DR5 was cloned into the binary vectors pKGW-RR-MGW and pMDC162-GFP. Incorporation of 5-ethynyl-2′-deoxyuridine (EdU) was used to evaluate the presence of DNA-synthesizing cells in the hypocotyl of squash seedlings to find out whether they were suitable for infection. Two A. rhizogenes strains, R1000 and MSU440, were used. Roots containing the respective constructs were selected based on DsRED1 or green fluorescent protein (GFP) fluorescence, and DR5::Egfp-gusA or DR5::gusA insertion, respectively, was verified by PCR. Distribution of the response to auxin was visualized by GFP fluorescence or β-glucuronidase (GUS) activity staining and confirmed by immunolocalization of GFP and GUS proteins, respectively.

Key Results

Based on the distribution of EdU-labelled cells, it was determined that 6-day-old squash seedlings were suited for inoculation by A. rhizogenes since their root pericycle and the adjacent layers contain enough proliferating cells. Agrobacterium rhizogenes R1000 proved to be the most virulent strain on squash seedlings. Squash roots containing the respective constructs did not exhibit the hairy root phenotype and were morphologically and structurally similar to wild-type roots.

Conclusions

The auxin response pattern in the root apex of squash resembled that in arabidopsis roots. Composite squash plants obtained by A. rhizogenes-mediated transformation are a good tool for the investigation of root apical meristem development and root branching.     

Ectomycorrhizas of two species of <Emphasis Type="Italic">Tuber</Emphasis> (clade Puberulum) in the Mexican subtropical cloud forest

Expansins are non-enzymatic cell wall proteins that mediate plant growth by catalyzing loosening of cell walls without lysing the wall polymers. Advances in the field of bioinformatics have facilitated the prediction of the members of expansin gene family across several model plants. Expansins constitutes into four sub-families; α-expansin, β-expansin, expansin-like A and expansin-like B. Biological functions of expansin gene family include diverse aspects of plant growth and development, shoot and root elongation, leaf morphogenesis, flower and fruit development, embryogenesis, pollen tube growth, stress tolerance, etc. Recent studies have demonstrated the role of expansins in plant-symbiotic interactions. The present review reveals the factors that govern plant-arbuscular mycorrhizal fungi (AMF) and legume-rhizobia symbioses; and the genes that participate in these diverse symbiont interactions. Further, we focus on the expression profiles and the functions of expansins during plant-AMF and legume-rhizobia interactions. The key roles of expansin proteins during AMF invasion, arbuscule formation, rhizobial infection and nodule organogenesis were uncovered during symbioses. This review summarizes discoveries that support the key and versatile roles of various expansin members in the plant-mycorrhizal and legume-rhizobial symbioses.     

Tuning of Pectin Methylesterification: PECTIN METHYLESTERASE INHIBITOR 7 MODULATES THE PROCESSIVE ACTIVITY OF CO-EXPRESSED PECTIN METHYLESTERASE 3 IN A pH-DEPENDENT MANNER*

Pectin methylesterases (PMEs) catalyze the demethylesterification of homogalacturonan domains of pectin in plant cell walls and are regulated by endogenous pectin methylesterase inhibitors (PMEIs). In Arabidopsis dark-grown hypocotyls, one PME (AtPME3) and one PMEI (AtPMEI7) were identified as potential interacting proteins. Using RT-quantitative PCR analysis and gene promoter::GUS fusions, we first showed that AtPME3 and AtPMEI7 genes had overlapping patterns of expression in etiolated hypocotyls. The two proteins were identified in hypocotyl cell wall extracts by proteomics. To investigate the potential interaction between AtPME3 and AtPMEI7, both proteins were expressed in a heterologous system and purified by affinity chromatography. The activity of recombinant AtPME3 was characterized on homogalacturonans (HGs) with distinct degrees/patterns of methylesterification. AtPME3 showed the highest activity at pH 7.5 on HG substrates with a degree of methylesterification between 60 and 80% and a random distribution of methyl esters. On the best HG substrate, AtPME3 generates long non-methylesterified stretches and leaves short highly methylesterified zones, indicating that it acts as a processive enzyme. The recombinant AtPMEI7 and AtPME3 interaction reduces the level of demethylesterification of the HG substrate but does not inhibit the processivity of the enzyme. These data suggest that the AtPME3·AtPMEI7 complex is not covalently linked and could, depending on the pH, be alternately formed and dissociated. Docking analysis indicated that the inhibition of AtPME3 could occur via the interaction of AtPMEI7 with a PME ligand-binding cleft structure. All of these data indicate that AtPME3 and AtPMEI7 could be partners involved in the fine tuning of HG methylesterification during plant development.     

Effect of Gibberellic Acid and Metabolic Inhibitors of DNA and RNA Synthesis on Hypocotyl Elongation and Cell Wall Loosening in Dark-Grown Lettuce Seedlings

The effect of inhibitors of nucleic acid metabolism on cellwall loosening in hypocotyls of dark-grown lettuce seedlingswas studied in relation to the gibberellic acid (GA) actionthat stimulates hypocotyl elongation. 5-Fluorodeoxyuridine (FUDR),trifluorothymine deoxyribose, mitomycin C. aminopterin, 5-fluorouraciland cordycepin produced mechanical rigidity of the cell wallof the hypocotyl when they inhibited elongation in the presenceor absence of added GA. The effect of FUDR on the mechanicalproperty of the cell wall and on hypocotyl elongation was nullifiedby thymidine, but not by uridine. ¹Present address: Department of Tuberculosis, National Instituteof Health, Shinagawa-ku, Tokyo 141, Japan. (Received September 2, 1980; Accepted January 5, 1981)     

Change in XET activities,cell wall extensibility and hypocotyl elongation of soybean seedlings at low water potential

In dark-grown soybean (Glycine max [L.] Merr.) seedlings, exposing the roots to water-deficient vermiculite (_w=–0.36 MPa) inhibited hypocotyl (stem) elongation. The inhibition was associated with decreased extensibility of the cell walls in the elongation zone. A detailed spatial analysis showed xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity on the basis of unit cell wall dry weight was decreased in the elongation region after seedlings were transplanted to low _w. The decrease in XET activity was at least partially due to an accumulation of cell wall mass. Since cell number was only slightly altered, wall mass had increased per cell and probably led to increased wall thickness and decreased cell wall extensibility. Alternatively, an increase in cell wall mass may represent a mechanism for regulating enzyme activity in cell walls, XET in this case, and therefore cell wall extensibility. Hypocotyl elongation was partially recovered after seedlings were grown in low-_w vermiculate for about 80 h. The partial recovery of hypocotyl elongation was associated with a partial recovery of cell wall extensibility and an enhancement of XET activity in the hypocotyl elongation zone. Our results indicate XTH proteins may play an important role in regulating cell wall extensibility and thus cell elongation in soybean hypocotyls. Our results also showed an imperfect correlation of spatial elongation and XET activity along the hypocotyls. Other potential functions of XTH and their regulation in soybean hypocotyl growth are discussed.     

Influence of cobalt on soybean hypocotyl growth and its ethylene evolution

Development of dark-grown “Clark” soybean (Glycine max [L.] Merr.) seedlings is abnormal at 25 C but normal at 20 and 30 C. At 25 C, hypocotyls swell and fail to elongate normally; lateral root formation and seedling ethylene evolution are enhanced.

Co²⁺ promoted hypocotyl elongation of etiolated “Clark” soybean seedlings by 28% when grown at 25 C. The same growth-promoting concentration reduced hypocotyl thickness and primary root elongation by 28 and 43%, respectively. Co²⁺ inhibited ethylene production both of intact seedlings and of apical 1-centimeter hypocotyl segments with attached epicotyls and cotyledons by 65 and 60%, respectively. These results suggest that Co²⁺ exerts its effects on the hypocotyl growth by inhibiting ethylene production, and also confirm our previous conclusion that abnormal ethylene production at 25 C is responsible for the inhibition of hypocotyl elongation and for its swelling.

     

Structure–Function Relationships of Four Stereoisomers of a Brassinolide Mimetic on Hypocotyl and Root Elongation of the Brassinosteroid-Deficient <Emphasis Type="Italic">det2-1</Emphasis> Mutant of <Emphasis Type="Italic">Arabidopsis</Emphasis>

Brassinosteroids (BRs) are a group of plant hormones and the bioactive BR, brassinolide (BL), is causally implicated in promoting cell elongation and cell proliferation. In Arabidopsis, the biosynthesis of BL is essential for hypocotyl etiolation in the dark, and application of bioactive BRs can promote both hypocotyl and root elongation, although high concentrations of applied BRs result in inhibition of root elongation. A non-steroidal structure consisting of four stereoisomers was designed to contain subunits bearing key functional groups mimicking those of BL. The bioactivity of each of these individual stereoisomers was tested using the Arabidopsis thaliana det2-1 mutant line, which is deficient in BL, and thus does not etiolate in the dark. Application of BL at each of 0.1, 1.0, and 10.0 µM promotes hypocotyl elongation in dark-grown det2-1 plants while simultaneously inhibiting elongation of their primary root. In contrast, the mimetic structures, when applied to dark-grown det2-1 plants, promote hypocotyl elongation without negatively affecting primary root elongation. In fact, two of the mimetic structures, applied at a 10 µM concentration, significantly promoted both hypocotyl and root elongation. Correlation of this contrasting behavior with the configurations of the hydroxylated stereocenters of the mimetics is described. This is the first example of a non-steroidal BL mimetic where the biological activities of individual stereoisomers were tested and compared.     

Antagonistic relationship between AtRALF1and brassinosteroid regulates cellexpansion-related genes

Rapid alkalinization factor (RALF) is a peptide signal that plays a role in plant cell expansion. We have recently proposed that AtRALF1 negatively regulates root cell elongation and lateral root formation by opposing the effects of brassinosteroid (BR). We reported 6 AtRALF1-inducible cell wall-related genes and 2 P450 monooxygenase -encoding genes involved in the BR biosynthetic pathway. The AtRALF1-inducible genes implicated in cell wall remodeling were not downregulated by brassinolide (BL) treatment alone; their induction was only compromised following simultaneous treatment with AtRALF1 and BL. We further examined the cell wall-remodeling gene EXPANSIN A5 (AtEXPA5), which is upregulated by BL and has been shown to positively affect root cell elongation. Herein, we report that AtEXPA5 expression is downregulated by AtRALF1 in a dose-dependent manner in the roots and hypocotyls of Arabidopsis plants. AtEXPA5 is also downregulated in plants that overexpress AtRALF1, and it is upregulated in plants in which the AtRALF1 gene is partially silenced. The AtRALF1 peptide is also able to repress AtEXPA5 induction following a pre-treatment with BL. A schematic diagram showing the gene regulatory network connecting the recently reported genes with the regulation of cell expansion by AtEXPA5 is presented.     

Involvement of nitric oxide and auxin in signal transduction of copper-induced morphological responses in Arabidopsis seedlings

Background and Aims

Plants are able to adapt to the environment dynamically through regulation of their growth and development. Excess copper (Cu²⁺), a toxic heavy metal, induces morphological alterations in plant organs; however, the underlying mechanisms are still unclear. With this in mind, the multiple signalling functions of nitric oxide (NO) in plant cells and its possible regulatory role and relationship with auxin were examined during Cu²⁺-induced morphological responses.

Methods

Endogenous auxin distribution was determined by microscopic observation of X-Gluc-stained DR5::GUS arabidopsis, and the levels of NO, superoxide and peroxynitrite were detected by fluorescence microscopy. As well as wild-type, NO-overproducer (nox1) and -deficient (nia1nia2 and nia1nia2noa1-2) arabidopsis plants were used.

Key Results

Cu²⁺ at a concentration of 50 µm resulted in a large reduction in cotyledon area and hypocotyl and primary root lengths, accompanied by an increase in auxin levels. In cotyledons, a low Cu²⁺ concentration promoted NO accumulation, which was arrested by nitric oxide synthase or nitrate reductase inhibitors. The 5-μm Cu²⁺-induced NO synthesis was not detectable in nia1nia2 or nia1nia2noa1-2 plants. In roots, Cu²⁺ caused a decrease of the NO level which was not associated with superoxide and peroxynitrite formation. Inhibition of auxin transport resulted in an increase in NO levels, while exogenous application of an NO donor reduced DR5::GUS expression. The elongation processes of nox1 were not sensitive to Cu²⁺, but NO-deficient plants showed diverse growth responses.

Conclusions

In plant organs, Cu²⁺ excess results in severe morphological responses during which the endogenous hormonal balance and signal transduction are affected. Auxin and NO negatively regulate each other''s level and NO intensifies the metal-induced cotyledon expansion, but mitigates elongation processes under Cu²⁺ exposure.     

Portrait of the expansin superfamily in Physcomitrella patens: comparisons with angiosperm expansins

BACKGROUND AND AIMS: Expansins are plant cell wall loosening proteins important in a variety of physiological processes. They comprise a large superfamily of genes consisting of four families (EXPA, EXPB, EXLA and EXLB) whose evolutionary relationships have been well characterized in angiosperms, but not in basal land plants. This work attempts to connect the expansin superfamily in bryophytes with the evolutionary history of this superfamily in angiosperms. METHODS: The expansin superfamily in Physcomitrella patens has been assembled from the Physcomitrella sequencing project data generated by the Joint Genome Institute and compared with angiosperm expansin superfamilies. Phylogenetic, motif, intron and distance analyses have been used for this purpose. KEY RESULTS: A gene superfamily is revealed that contains similar numbers of genes as found in arabidopsis, but lacking EXLA or EXLB genes. This similarity in gene numbers exists even though expansin evolution in Physcomitrella diverged from the angiosperm line approx. 400 million years ago. Phylogenetic analyses suggest that there were a minimum of two EXPA genes and one EXPB gene in the last common ancestor of angiosperms and Physcomitrella. Motif analysis seems to suggest that EXPA protein function is similar in bryophytes and angiosperms, but that EXPB function may be altered. CONCLUSIONS: The EXPA genes of Physcomitrella are likely to have maintained the same biochemical function as angiosperm expansins despite their independent evolutionary history. Changes seen at normally conserved residues in the Physcomitrella EXPB family suggest a possible change in function as one mode of evolution in this family.     

The cell wall pectic polymer rhamnogalacturonan-II is required for proper pollen tube elongation: implications of a putative sialyltransferase-like protein

Background and Aims

Rhamnogalacturonan-II (RG-II) is one of the pectin motifs found in the cell wall of all land plants. It contains sugars such as 2-keto-3-deoxy-d-lyxo-heptulosaric acid (Dha) and 2-keto-3-deoxy-d-manno-octulosonic acid (Kdo), and within the wall RG-II is mostly found as a dimer via a borate diester cross-link. To date, little is known regarding the biosynthesis of this motif. Here, after a brief review of our current knowledge on RG-II structure, biosynthesis and function in plants, this study explores the implications of the presence of a Golgi-localized sialyltransferase-like 2 (SIA2) protein that is possibly involved in the transfer of Dha or Kdo in the RG-II of Arabidopsis thaliana pollen tubes, a fast-growing cell type used as a model for the study of cell elongation.

Methods

Two heterozygous mutant lines of arabidopsis (sia2-1+/– and qrt1 × sia2-2+/–) were investigated. sia2-2+/– was in a quartet1 background and the inserted T-DNA contained the reporter gene β-glucuronidase (GUS) under the pollen-specific promoter LAT52. Pollen germination and pollen tube phenotype and growth were analysed both in vitro and in vivo by microscopy.

Key Results

Self-pollination of heterozygous lines produced no homozygous plants in the progeny, which may suggest that the mutation could be lethal. Heterozygous mutants displayed a much lower germination rate overall and exhibited a substantial delay in germination (20 h of delay to reach 30 % of pollen grain germination compared with the wild type). In both lines, mutant pollen grains that were able to produce a tube had tubes that were either bursting, abnormal (swollen or dichotomous branching tip) or much shorter compared with wild-type pollen tubes. In vivo, mutant pollen tubes were restricted to the style, whereas the wild-type pollen tubes were detected at the base of the ovary.

Conclusions

This study highlights that the mutation in arabidopsis SIA2 encoding a sialyltransferase-like protein that may transfer Dha or Kdo on the RG-II motif has a dramatic effect on the stability of the pollen tube cell wall.     

ZEITLUPE positively regulates hypocotyl elongation at warm temperature under light in Arabidopsis thaliana

Hypocotyl cell elongation has been studied as a model to understand how cellular expansion contributes to plant organ growth. Hypocotyl elongation is affected by multiple environmental factors, including light quantity and light quality. Red light inhibits hypocotyl growth via the phytochrome signaling pathways. Proteins of the FLAVIN-BINDING KELCH REPEAT F-BOX 1 / LOV KELCH PROTEIN 2 / ZEITLUPE family are positive regulators of hypocotyl elongation under red light in Arabidopsis. These proteins were suggested to reduce phytochrome-mediated inhibition of hypocotyl elongation. Here, we show that ZEITLUPE also functions as a positive regulator in warmth-induced hypocotyl elongation under light in Arabidopsis.     

The pectic disaccharides lepidimoic acid and β-d-xylopyranosyl-(1→3)-d-galacturonic acid occur in cress-seed exudate but lack allelochemical activity

Background and aims Cress-seed (Lepidium sativum) exudate exerts an allelochemical effect, promoting excessive hypocotyl elongation and inhibiting root growth in neighbouring Amaranthus caudatus seedlings. We investigated acidic disaccharides present in cress-seed exudate, testing the proposal that the allelochemical is an oligosaccharin—lepidimoic acid (LMA; 4-deoxy-β-l-threo-hex-4-enopyranuronosyl-(1→2)-l-rhamnose).Methods Cress-seed exudate was variously treated [heating, ethanolic precipitation, solvent partitioning, high-voltage paper electrophoresis and gel-permeation chromatography (GPC)], and the products were bioassayed for effects on dark-grown Amaranthus seedlings. Two acidic disaccharides, including LMA, were isolated and characterized by electrophoresis, thin-layer chromatography (TLC) and nuclear magnetic resonance (NMR) spectroscopy, and then bioassayed.Key Results Cress-seed exudate contained low-M_r, hydrophilic, heat-stable material that strongly promoted Amaranthus hypocotyl elongation and inhibited root growth, but that separated from LMA on electrophoresis and GPC. Cress-seed exudate contained ∼250 µm LMA, whose TLC and electrophoretic mobilities, susceptibility to mild acid hydrolysis and NMR spectra are reported. A second acidic disaccharide, present at ∼120 µm, was similarly characterized, and shown to be β-d-xylopyranosyl-(1→3)-d-galacturonic acid (Xyl→GalA), a repeat unit of xylogalacturonan. Purified LMA and Xyl→GalA when applied at 360 and 740 µm, respectively, only slightly promoted Amaranthus hypocotyl growth, but equally promoted root growth and thus had no effect on the hypocotyl:root ratio, unlike total cress-seed exudate.Conclusions LMA is present in cress seeds, probably formed by rhamnogalacturonan lyase action on rhamnogalacturonan-I during seed development. Our results contradict the hypothesis that LMA is a cress allelochemical that appreciably perturbs the growth of potentially competing seedlings. Since LMA and Xyl→GalA slightly promoted both hypocotyl and root elongation, their effect could be nutritional. We conclude that rhamnogalacturonan-I and xylogalacturonan (pectin domains) are not sources of oligosaccharins with allelochemical activity, and the biological roles (if any) of the disaccharides derived from them are unknown. The main allelochemical principle in cress-seed exudate remains to be identified.     

Expansins: roles in plant growth and potential applications in crop improvement

Key message

Results from various expansin related studies have demonstrated that expansins present an opportunity to improve various crops in many different aspects ranging from yield and fruit ripening to improved stress tolerance.

Abstract

The recent advances in expansin studies were reviewed. Besides producing the strength that is needed by the plants, cell walls define cell shape, cell size and cell function. Expansins are cell wall proteins which consist of four sub families; α-expansin, β-expansin, expansin-like A and expansin-like B. These proteins mediate cell wall loosening and they are present in all plants and in some microbial organisms and other organisms like snails. Decades after their initial discovery in cucumber, it is now clear that these small proteins have diverse biological roles in plants. Through their ability to enable the local sliding of wall polymers by reducing adhesion between adjacent wall polysaccharides and the part they play in cell wall remodeling after cytokinesis, it is now clear that expansins are required in almost all plant physiological development aspects from germination to fruiting. This is shown by the various reports from different studies using various molecular biology approaches such as gene achieve these many roles through their non-enzymatic wall loosening ability. This paper reviews and summarizes some of the reported functions of expansins and outlines the potential uses of expansins in crop improvement programs.