Identification and characterization of a gene encoding drought-inducible protein localizing in the bundle sheath cell of sugarcane

Drought and salinity stresses induced physio-biochemical changes in sugarcane: an overview of tolerance mechanism and mitigating approaches

Sugarcane productivity is being hampered globally under changing environmental scenarios like drought and salinity. The highly complex nature of the plant responses against these stresses is determined by a variety of factors such as genotype, developmental phase of the plant, progression rate and stress, intensity, and duration. These factors influence plant responses and can determine whether mitigation approaches associated with acclimation are implemented. In this review, we attempt to summarize the effects of drought and salinity on sugarcane growth, specifically on the plant's responses at various levels, viz., physiological, biochemical, and metabolic responses, to these stresses. Furthermore, mitigation strategies for dealing with these stresses have been discussed. Despite sugarcane's complex genomes, conventional breeding approaches can be utilized in conjunction with molecular breeding and omics technologies to develop drought- and salinity-tolerant cultivars. The significant role of plant growth-promoting bacteria in sustaining sugarcane productivity under drought and salinity cannot be overlooked.

Modern Biotechnologies: Innovative and Sustainable Approaches for the Improvement of Sugarcane Tolerance to Environmental Stresses

Sugarcane (Saccharum spp.) is one of the most important industrial cash crops, contributing to the world sugar industry and biofuel production. It has been cultivated and improved from prehistoric times through natural selection and conventional breeding and, more recently, using the modern tools of genetic engineering and biotechnology. However, the heterogenicity, complex poly-aneuploid genome and susceptibility of sugarcane to different biotic and abiotic stresses represent impediments that require us to pay greater attention to the improvement of the sugarcane crop. Compared to traditional breeding, recent advances in breeding technologies (molecular marker-assisted breeding, sugarcane transformation, genome-editing and multiple omics technologies) can potentially improve sugarcane, especially against environmental stressors. This article will focus on efficient modern breeding technologies, which provide crucial clues for the engineering of sugarcane cultivars resistant to environmental stresses.

Comparative analysis of drought-responsive transcriptomes of sugarcane genotypes with differential tolerance to drought

Water stress causes considerable yield losses in sugarcane. To investigate differentially expressed genes under water stress, two sugarcane genotypes were subjected to three water-deficit levels (mild, moderate, and severe) and subsequent recovery and leaf transcriptome was generated using Illumina NextSeq sequencing. Among the differentially expressed genes, the tolerant genotype Co 06022 generated 2970 unigenes (p ≤ 0.05, functionally known, non-redundant DEGs) at 2-day stress, and there was a progressive decrease in the expressed genes as the stress period increased with 2109 unigenes at 6-day stress and 2307 unigenes at 10-day stress. There was considerable reduction at recovery with 1334 unigenes expressed at 10 days after recovery. However, in the susceptible genotype Co 8021, the number of unigenes expressed at 2 days was lower (2025) than the tolerant genotype and a further reduction was seen at 6-day stress (1552). During recovery, more differentially expressed genes were observed in the susceptible cultivar indicating that the cultivar has to activate more functions/processes to recover from the damage caused by stress. Comparison of DEGs between all stages of stress and recovery in both genotypes revealed that, the commonly up- and down-regulated genes across different stages were approximately double in the tolerant genotype. The most enriched gene ontology classes were heme binding, peroxidase activity and metal ion binding in the biological process and response to oxidative stress, hydrogen peroxide catabolic process and response to stress in the molecular function category. The cellular component was enriched with DEGs involved in extracellular region followed by integral component of membrane. The KEGG pathway analysis revealed important metabolic activities and functionally important genes involved in mitigating water-deficit stress in both the varieties. In addition, several unannotated genes in important pathways were detected and together may provide novel insights into water-deficit tolerance mechanisms in sugarcane. The reliability of the observed expression patterns was confirmed by qRT-PCR. The results of this study will help to identify useful genes for improving drought tolerance in sugarcane.

Overexpression of Sucrose Phosphate Synthase Enhanced Sucrose Content and Biomass Production in Transgenic Sugarcane

Sucrose phosphate synthase (SPS) is a key enzyme in sucrose synthesis, which controls sucrose content in plants. This study was designed to examine the efficacy of the overexpression of SoSPS1 gene on sucrose accumulation and carbon partitioning in transgenic sugarcane. The overexpression of SoSPS1 gene increased SPS activity and sucrose content in transgenic sugarcane leaves. More importantly, the overexpression enhanced soluble acid invertase (SAI) activity concomitant with the increase of glucose and fructose levels in the leaves, whereas sucrose synthase activity exhibited almost no change. In the stalk, a similar correlation was observed, but a higher correlation was noted between SPS activity and sugar content. These results suggest that SPS overexpression has both direct and indirect effects on sugar concentration and SAI activity in sugarcane. In addition, SPS overexpression resulted in a significant increase in plant height and stalk number in some transgenic lines compared to those in non-transgenic control. Taken together, these results strongly suggest that enhancing SPS activity is a useful strategy for improving sugarcane yield.

Sugarcane Omics: An Update on the Current Status of Research and Crop Improvement

Sugarcane is an important crop from Poaceae family, contributing about 80% of the total world's sucrose with an annual value of around US$150 billion. In addition, sugarcane is utilized as a raw material for the production of bioethanol, which is an alternate source of renewable energy. Moving towards sugarcane omics, a remarkable success has been achieved in gene transfer from a wide variety of plant and non-plant sources to sugarcane, with the accessibility of efficient transformation systems, selectable marker genes, and genetic engineering gears. Genetic engineering techniques make possible to clone and characterize useful genes and also to improve commercially important traits in elite sugarcane clones that subsequently lead to the development of an ideal cultivar. Sugarcane is a complex polyploidy crop, and hence no single technique has been found to be the best for the confirmation of polygenic and phenotypic characteristics. To better understand the application of basic omics in sugarcane regarding agronomic characters and industrial quality traits as well as responses to diverse biotic and abiotic stresses, it is important to explore the physiology, genome structure, functional integrity, and collinearity of sugarcane with other more or less similar crops/plants. Genetic improvements in this crop are hampered by its complex genome, low fertility ratio, longer production cycle, and susceptibility to several biotic and abiotic stresses. Biotechnology interventions are expected to pave the way for addressing these obstacles and improving sugarcane crop. Thus, this review article highlights up to date information with respect to how advanced data of omics (genomics, transcriptomic, proteomics and metabolomics) can be employed to improve sugarcane crops.

Sugarcane proteomics: An update on current status, challenges, and future prospects

Sugarcane is one of the most important commercial crops cultivated worldwide for the production of crystal sugar, ethanol, and other related by-products. Unlike other comparable monocots like sorghum, maize, and rice, sugarcane genome by virtue of its polyploidy nature remains yet to be fully deciphered. Proteomics-an established complementary tool to genomics is at its infancy in sugarcane as compared to the other monocots. However, with the surge in genomics research accomplished by next-generation sequencing platforms, sugarcane proteomics has gained momentum. This review summarizes the available literature from 1970 to 2014, which ensures a comprehensive coverage on sugarcane proteomics-a topic first of its kind to be reviewed. We herewith compiled substantial contributions in different areas of sugarcane proteomics, which include abiotic and biotic stresses, cell wall, organelle, and structural proteomics. The past decade has witnessed a paradigm shift in the pace with which sugarcane proteomics is progressing, as evident by the number of research publications. In addition to extensively reviewing the progress made thus far, we intend to highlight the scope in sugarcane proteomics, with an aspiration to instigate focused research on sugarcane to harness its full potential for the human welfare.

Label-Free Quantitative Proteomics of Embryogenic and Non-Embryogenic Callus during Sugarcane Somatic Embryogenesis

The development of somatic cells in to embryogenic cells occurs in several stages and ends in somatic embryo formation, though most of these biochemical and molecular changes have yet to be elucidated. Somatic embryogenesis coupled with genetic transformation could be a biotechnological tool to improve potential crop yields potential in sugarcane cultivars. The objective of this study was to observe somatic embryo development and to identify differentially expressed proteins in embryogenic (E) and non-embryogenic (NE) callus during maturation treatment. E and NE callus were cultured on maturation culture medium supplemented with different concentrations (0.0, 0.75, 1.5 and 2.0 g L(-1)) of activated charcoal (AC). Somatic embryo formation and differential protein expression were evaluated at days 0 and 21 using shotgun proteomic analyses. Treatment with 1.5 g L(-1) AC resulted in higher somatic embryo maturation rates (158 somatic embryos in 14 days) in E callus but has no effect in NE callus. A total of 752 co-expressed proteins were identified through the SUCEST (The Sugarcane EST Project), including many housekeeping proteins. E callus showed 65 exclusive proteins on day 0, including dehydrogenase, desiccation-related protein, callose synthase 1 and nitric oxide synthase. After 21 days on maturation treatment, 14 exclusive proteins were identified in E callus, including catalase and secreted protein. NE callus showed 23 exclusive proteins on day 0 and 10 exclusive proteins after 21 days on maturation treatment, including many proteins related to protein degradation. The induction of maturation leads to somatic embryo development, which likely depends on the expression of specific proteins throughout the process, as seen in E callus under maturation treatment. On the other hand, some exclusive proteins can also specifically prevent of somatic embryos development, as seen in the NE callus.

Tomato ABSCISIC ACID STRESS RIPENING (ASR) gene family revisited

Tomato ABSCISIC ACID RIPENING 1 (ASR1) was the first cloned plant ASR gene. ASR orthologs were then cloned from a large number of monocot, dicot and gymnosperm plants, where they are mostly involved in response to abiotic (drought and salinity) stress and fruit ripening. The tomato genome encodes five ASR genes: ASR1, 2, 3 and 5 encode low-molecular-weight proteins (ca. 110 amino acid residues each), whereas ASR4 encodes a 297-residue polypeptide. Information on the expression of the tomato ASR gene family is scarce. We used quantitative RT-PCR to assay the expression of this gene family in plant development and in response to salt and osmotic stresses. ASR1 and ASR4 were the main expressed genes in all tested organs and conditions, whereas ASR2 and ASR3/5 expression was two to three orders of magnitude lower (with the exception of cotyledons). ASR1 is expressed in all plant tissues tested whereas ASR4 expression is limited to photosynthetic organs and stamens. Essentially, ASR1 accounted for most of ASR gene expression in roots, stems and fruits at all developmental stages, whereas ASR4 was the major gene expressed in cotyledons and young and fully developed leaves. Both ASR1 and ASR4 were expressed in flower organs, with ASR1 expression dominating in stamens and pistils, ASR4 in sepals and petals. Steady-state levels of ASR1 and ASR4 were upregulated in plant vegetative organs following exposure to salt stress, osmotic stress or the plant abiotic stress hormone abscisic acid (ABA). Tomato plants overexpressing ASR1 displayed enhanced survival rates under conditions of water stress, whereas ASR1-antisense plants displayed marginal hypersensitivity to water withholding.

Subtractive hybridization-mediated analysis of genes and in silico prediction of associated microRNAs under waterlogged conditions in sugarcane (Saccharum spp.)

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Waterlogging adversely affects sugarcane productivity and quality.

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A subtractive cDNA library was prepared from sugarcane leaf tissue and sequenced to generate ESTs.

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EST sequences were used to identify transcripts induced by waterlogging.

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The sequenced clones were classified by predicted functions and stress-related genes formed the largest class.

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EST sequences were also used to identify putative novel microRNAs and their targets.

Sugarcane is an important tropical cash crop meeting 75% of world sugar demand and it is fast becoming an energy crop for the production of bio-fuel ethanol. A considerable area under sugarcane is prone to waterlogging which adversely affects both cane productivity and quality. In an effort to elucidate the genes underlying plant responses to waterlogging, a subtractive cDNA library was prepared from leaf tissue. cDNA clones were sequenced and annotated for their putative functions. Major groups of ESTs were related to stress (15%), catalytic activity (13%), cell growth (10%) and transport related proteins (6%). A few stress-related genes were identified, including senescence-associated protein, dehydration-responsive family protein, and heat shock cognate 70 kDa protein. A bioinformatics search was carried out to discover novel microRNAs (miRNAs) that can be regulated in sugarcane plants subjected to waterlogging stress. Taking advantage of the presence of miRNA precursors in the related sorghum genome, seven candidate mature miRNAs were identified in sugarcane. The application of subtraction technology allowed the identification of differentially expressed sequences and novel miRNAs in sugarcane under waterlogging stress. The comparative global transcript profiling in sugarcane plants undertaken in this study suggests that proteins associated with stress response, signal transduction, metabolic activity and ion transport play important role in conferring waterlogging tolerance in sugarcane.

New insights into aluminum tolerance in rice: the ASR5 protein binds the STAR1 promoter and other aluminum-responsive genes

Aluminum (Al) toxicity in plants is one of the primary constraints in crop production. Al³⁺, the most toxic form of Al, is released into soil under acidic conditions and causes extensive damage to plants, especially in the roots. In rice, Al tolerance requires the ASR5 gene, but the molecular function of ASR5 has remained unknown. Here, we perform genome-wide analyses to identify ASR5-dependent Al-responsive genes in rice. Based on ASR5_RNAi silencing in plants, a global transcriptome analysis identified a total of 961 genes that were responsive to Al treatment in wild-type rice roots. Of these genes, 909 did not respond to Al in the ASR5_RNAi plants, indicating a central role for ASR5 in Al-responsive gene expression. Under normal conditions, without Al treatment, the ASR5_RNAi plants expressed 1.756 genes differentially compared to the wild-type plants, and 446 of these genes responded to Al treatment in the wild-type plants. Chromatin immunoprecipitation followed by deep sequencing identified 104 putative target genes that were directly regulated by ASR5 binding to their promoters, including the STAR1 gene, which encodes an ABC transporter required for Al tolerance. Motif analysis of the binding peak sequences revealed the binding motif for ASR5, which was confirmed via in vitro DNA-binding assays using the STAR1 promoter. These results demonstrate that ASR5 acts as a key transcription factor that is essential for Al-responsive gene expression and Al tolerance in rice.

Involvement of ASR genes in aluminium tolerance mechanisms in rice

Among cereal crops, rice is considered the most tolerant to aluminium (Al). However, variability among rice genotypes leads to remarkable differences in the degree of Al tolerance for distinct cultivars. A number of studies have demonstrated that rice plants achieve Al tolerance through an unknown mechanism that is independent of root tip Al exclusion. We have analysed expression changes of the rice ASR gene family as a function of Al treatment. The gene ASR5 was differentially regulated in the Al-tolerant rice ssp. Japonica cv. Nipponbare. However, ASR5 expression did not respond to Al exposure in Indica cv. Taim rice roots, which are highly Al sensitive. Transgenic plants carrying RNAi constructs that targeted the ASR genes were obtained, and increased Al susceptibility was observed in T1 plants. Embryogenic calli of transgenic rice carrying an ASR5-green fluorescent protein fusion revealed that ASR5 was localized in both the nucleus and cytoplasm. Using a proteomic approach to compare non-transformed and ASR-RNAi plants, a total of 41 proteins with contrasting expression patterns were identified. We suggest that the ASR5 protein acts as a transcription factor to regulate the expression of different genes that collectively protect rice cells from Al-induced stress responses.

Dissection of the transcriptional regulation of grape ASR and response to glucose and abscisic acid

Despite the fact that the precise physiological function of ASRs [abscisic acid (ABA), stress, ripening] remains unknown, they have been suggested to play a dual role in the plant response to environmental cues, as highly hydrophilic proteins for direct protection, as well as transcription factors involved in the regulation of gene expression. To investigate further the biological positioning of grape ASR in the hormonal and metabolic signal network, three promoters corresponding to its cDNA were isolated and submited to a detailed in silico and functional analysis. The results obtained provided evidence for the allelic polymorphism of the grape ASR gene, the organ-preferential expression conferred on the GUS reporter gene, and the specific phloem tissue localization revealed by in situ hybridization. The study of glucose and ABA signalling in its transcriptional control, by transfection of grape protoplasts using the dual luciferase system, revealed the complexity of ASR gene expression regulation. A model was proposed allowing a discussion of the place of ASR in the fine tuning of hormonal and metabolic signalling involved in the integration of environmental cues by the plant organism.

The ZmASR1 protein influences branched-chain amino acid biosynthesis and maintains kernel yield in maize under water-limited conditions

Abscisic acid-, stress-, and ripening-induced (ASR) proteins were first described about 15 years ago as accumulating to high levels during plant developmental processes and in response to diverse stresses. Currently, the effects of ASRs on water deficit tolerance and the ways in which their physiological and biochemical functions lead to this stress tolerance remain poorly understood. Here, we characterized the ASR gene family from maize (Zea mays), which contains nine paralogous genes, and showed that maize ASR1 (ZmASR1) was encoded by one of the most highly expressed paralogs. Ectopic expression of ZmASR1 had a large overall impact on maize yield that was maintained under water-limited stress conditions in the field. Comparative transcriptomic and proteomic analyses of wild-type and ZmASR1-overexpressing leaves led to the identification of three transcripts and 16 proteins up- or down-regulated by ZmASR1. The majority of them were involved in primary and/or cellular metabolic processes, including branched-chain amino acid (BCAA) biosynthesis. Metabolomic and transcript analyses further indicated that ZmASR1-overexpressing plants showed a decrease in BCAA compounds and changes in BCAA-related gene expression in comparison with wild-type plants. Interestingly, within-group correlation matrix analysis revealed a close link between 13 decreased metabolites in ZmASR1-overexpressing leaves, including two BCAAs. Among these 13 metabolites, six were previously shown to be negatively correlated to biomass, suggesting that ZmASR1-dependent regulation of these 13 metabolites might contribute to regulate leaf growth, resulting in improvement in kernel yield.

Embryogenic callus proliferation and regeneration conditions for genetic transformation of diverse sugarcane cultivars

Amenability to tissue culture stages required for gene transfer, selection and plant regeneration are the main determinants of genetic transformation efficiency via particle bombardment into sugarcane. The technique is moving from the experimental phase, where it is sufficient to work in a few amenable genotypes, to practical application in a diverse and changing set of elite cultivars. Therefore, we investigated the response to callus initiation, proliferation, regeneration and selection steps required for microprojectile-mediated transformation, in a diverse set of Australian sugarcane cultivars. 12 of 16 tested cultivars were sufficiently amenable to existing routine tissue-culture conditions for practical genetic transformation. Three cultivars required adjustments to 2,4-D levels during callus proliferation, geneticin concentration during selection, and/or light intensity during regeneration. One cultivar gave an extreme necrotic response in leaf spindle explants and produced no callus tissue under the tested culture conditions. It was helpful to obtain spindle explants for tissue culture from plants with good water supply for growth, especially for genotypes that were harder to culture. It was generally possible to obtain several independent transgenic plants per bombardment, with time in callus culture limited to 11-15 weeks. A caution with this efficient transformation system is that separate shoots arose from different primary transformed cells in more than half of tested calli after selection for geneticin resistance. The results across this diverse cultivar set are likely to be a useful guide to key variables for rapid optimisation of tissue culture conditions for efficient genetic transformation of other sugarcane cultivars.

Sugarcane proteomics: establishment of a protein extraction method for 2-DE in stalk tissues and initiation of sugarcane proteome reference map

Sugarcane is an important commercial crop cultivated for its stalks and sugar is a prized commodity essential in human nutrition. Proteomics of sugarcane is in its infancy, especially when dealing with the stalk tissues, where there is no study to date. A systematic proteome analysis of stalk tissue yet remains to be investigated in sugarcane, wherein the stalk tissue is well known for its rigidity, fibrous nature, and the presence of oxidative enzymes, phenolic compounds and extreme levels of carbohydrates, thus making the protein extraction complicated. Here, we evaluated five different protein extraction methods in sugarcane stalk tissues. These methods are as follows: direct extraction using lysis buffer (LB), TCA/acetone precipitation followed by solubilization in LB, LB containing thiourea (LBT), and LBT containing tris, and phenol extraction. Both quantitative and qualitative protein analyses were performed for each method. 2-DE analysis of extracted total proteins revealed distinct differences in protein patterns among the methods, which might be due to their physicochemical limitations. Based on the 2-D gel protein profiles, TCA/acetone precipitation-LBT and phenol extraction methods showed good results. The phenol method showed a shift in pI values of proteins on 2-D gel, which was mostly overcome by the use of 2-D cleanup kit after protein extraction. Among all the methods tested, 2-D cleanup-phenol method was found to be the most suitable for producing high number of good-quality spots and reproducibility. In total, 30 and 12 protein spots commonly present in LB, LBT and phenol methods, and LBT method were selected and subjected to eLD-IT-TOF-MS/MS and nESI-LC-MS/MS analyses, respectively, and a reference map has been established for sugarcane stalk tissue proteome. A total of 36 nonredundant proteins were identified. This is a very first basic study on sugarcane stalk proteome analysis and will promote the unexplored areas of sugarcane proteome research.

Identification and characterization of a gibberellin-regulated protein, which is ASR5, in the basal region of rice leaf sheaths

Gibberellins (GAs) regulate growth and development in higher plants. To identify GA-regulated proteins during rice leaf sheath elongation, a proteomic approach was used. Proteins from the basal region of leaf sheath in rice seedling treated with GA(3) were analyzed by fluorescence two-dimensional difference gel electrophoresis. The levels of abscisic acid-stress-ripening-inducible 5 protein (ASR5), elongation factor-1 beta, translationally controlled tumor protein, fructose-bisphosphate aldolase and a novel protein increased; whereas the level of RuBisCO subunit binding-protein decreased by GA(3) treatment. ASR5 out of these six proteins was significantly regulated by GA(3) at the protein level but not at the mRNA level in the basal region of leaf sheaths. Since this protein is regulated not only by abscisic acid but also by GA(3), these results indicate that ASR5 might be involved in plant growth in addition to stress in the basal regions of leaf sheaths.

Tomato leaf spatial expression of stress-induced Asr genes

Asr1 and Asr2 are water stress-inducible genes belonging to the Asr gene family, which transcriptionally regulate a sugar transporter gene, at least in grape. Using an in situ RNA hybridization methodology, we determined that, in basal conditions, expression of Asr2 in tomato leaves is detected in the phloem tissue, particularly in companion phloem cells. When plants are exposed to water stress, Asr2 expression is contained in companion cells but expands occasionally to mesophyll cells. In contrast, Asr1 transcript localization seems to be sparse in leaf vascular tissue under both non-stress and stress conditions. The occurrence of Asr transcripts precisely in companion cells is in accordance with the cell type specificity reported for hexose-transporter protein molecules in grape encoded by the only Asr-target gene known to date. The results are discussed in light of the reported scarcity of plasmodesmata between companion cells and the rest of leaf tissue in the family Solanaceae.

ci21A/Asr1 expression influences glucose accumulation in potato tubers

Asr genes are exclusively found in the genomes of higher plants. In many species, this gene family is expressed under abiotic stress conditions and during fruit ripening. The encoded proteins have nuclear localisation and consequently a transcription factor function has been suggested. Interestingly, yeast-one-hybrid experiments revealed that a grape ASR binds to the promoter of a hexose transporter gene (VvHT1). However, the role of these proteins in planta is still elusive. By using a reverse genetics approach in potato we found that modification of Asr1 expression has no incidence on the aerial phenotype of the plant but exerts a dramatic effect in tuber. Asr1 antisense potatoes displayed decreased tuber fresh weight whereas Asr1 overexpressors had a diminished number of tubers. Moreover, overexpression lines showed lower transcript levels of a plasma membrane hexose transporter and a concomitant decrease in glucose content in parenchyma cells of potato tubers. On the same hand glucose uptake rate was also reduced in one of the overexpressing lines. It thus seems likely that Asr1 is involved in the control of hexose uptake in heterotrophic organs. In addition, the transgenic plants were characterized by several other changes in steady state metabolite levels. Results presented here support a role for ci21A/Asr1 in glucose metabolism of potato tuber.

Desiccation and zinc binding induce transition of tomato abscisic acid stress ripening 1, a water stress- and salt stress-regulated plant-specific protein, from unfolded to folded state

Abscisic acid stress ripening 1 (ASR1) is a low molecular weight plant-specific protein encoded by an abiotic stress-regulated gene. Overexpression of ASR1 in transgenic plants increases their salt tolerance. The ASR1 protein possesses a zinc-dependent DNA-binding activity. The DNA-binding site was mapped to the central part of the polypeptide using truncated forms of the protein. Two additional zinc-binding sites were shown to be localized at the amino terminus of the polypeptide. ASR1 protein is presumed to be an intrinsically unstructured protein using a number of prediction algorithms. The degree of order of ASR1 was determined experimentally using nontagged recombinant protein expressed in Escherichia coli and purified to homogeneity. Purified ASR1 was shown to be unfolded using dynamic light scattering, gel filtration, microcalorimetry, circular dichroism, and Fourier transform infrared spectrometry. The protein was shown to be monomeric by analytical ultracentrifugation. Addition of zinc ions resulted in a global change in ASR1 structure from monomer to homodimer. Upon binding of zinc ions, the protein becomes ordered as shown by Fourier transform infrared spectrometry and microcalorimetry, concomitant with dimerization. Tomato (Solanum lycopersicum) leaf soluble ASR1 is unstructured in the absence of added zinc and gains structure upon binding of the metal ion. The effect of zinc binding on ASR1 folding and dimerization is discussed.