The expression of small heat shock proteins in seeds responds to discrete developmental signals and suggests a general protective role in desiccation tolerance

Regulatory network analysis reveals novel regulators of seed desiccation tolerance in Arabidopsis thaliana

Desiccation tolerance (DT) is a remarkable process that allows seeds in the dry state to remain viable for long periods of time that in some instances exceed 1,000 y. It has been postulated that seed DT evolved by rewiring the regulatory and signaling networks that controlled vegetative DT, which itself emerged as a crucial adaptive trait of early land plants. Understanding the networks that regulate seed desiccation tolerance in model plant systems would provide the tools to understand an evolutionary process that played a crucial role in the diversification of flowering plants. In this work, we used an integrated approach that included genomics, bioinformatics, metabolomics, and molecular genetics to identify and validate molecular networks that control the acquisition of DT in Arabidopsis seeds. Two DT-specific transcriptional subnetworks were identified related to storage of reserve compounds and cellular protection mechanisms that act downstream of the embryo development master regulators LEAFY COTYLEDON 1 and 2, FUSCA 3, and ABSCICIC ACID INSENSITIVE 3. Among the transcription factors identified as major nodes in the DT regulatory subnetworks, PLATZ1, PLATZ2, and AGL67 were confirmed by knockout mutants and overexpression in a desiccation-intolerant mutant background to play an important role in seed DT. Additionally, we found that constitutive expression of PLATZ1 in WT plants confers partial DT in vegetative tissues.

Rice PROTEIN l-ISOASPARTYL METHYLTRANSFERASE isoforms differentially accumulate during seed maturation to restrict deleterious isoAsp and reactive oxygen species accumulation and are implicated in seed vigor and longevity

PROTEIN l-ISOASPARTYL O-METHYLTRANSFERASE (PIMT) is a protein-repairing enzyme involved in seed vigor and longevity. However, the regulation of PIMT isoforms during seed development and the mechanism of PIMT-mediated improvement of seed vigor and longevity are largely unknown. In this study in rice (Oryza sativa), we demonstrate the dynamics and correlation of isoaspartyl (isoAsp)-repairing demands and PIMT activity, and their implications, during seed development, germination and aging, through biochemical, molecular and genetic studies. Molecular and biochemical analyses revealed that rice possesses various biochemically active and inactive PIMT isoforms. Transcript and western blot analyses clearly showed the seed development stage and tissue-specific accumulation of active isoforms. Immunolocalization studies revealed distinct isoform expression in embryo and aleurone layers. Further analyses of transgenic lines for each OsPIMT isoform revealed a clear role in the restriction of deleterious isoAsp and age-induced reactive oxygen species (ROS) accumulation to improve seed vigor and longevity. Collectively, our data suggest that a PIMT-mediated, protein repair mechanism is initiated during seed development in rice, with each isoform playing a distinct, yet coordinated, role. Our results also raise the intriguing possibility that PIMT repairs antioxidative enzymes and proteins which restrict ROS accumulation, lipid peroxidation, etc. in seed, particularly during aging, thus contributing to seed vigor and longevity.

HsfA2 Controls the Activity of Developmentally and Stress-Regulated Heat Stress Protection Mechanisms in Tomato Male Reproductive Tissues

Male reproductive tissues are more sensitive to heat stress (HS) compared to vegetative tissues, but the basis of this phenomenon is poorly understood. Heat stress transcription factors (Hsfs) regulate the transcriptional changes required for protection from HS In tomato (Solanum lycopersicum), HsfA2 acts as coactivator of HsfA1a and is one of the major Hsfs accumulating in response to elevated temperatures. The contribution of HsfA2 in heat stress response (HSR) and thermotolerance was investigated in different tissues of transgenic tomato plants with suppressed HsfA2 levels (A2AS). Global transcriptome analysis and immunodetection of two major Hsps in vegetative and reproductive tissues showed that HsfA2 regulates subsets of HS-induced genes in a tissue-specific manner. Accumulation of HsfA2 by a moderate HS treatment enhances the capacity of seedlings to cope with a subsequent severe HS, suggesting an important role for HsfA2 in regulating acquired thermotolerance. In pollen, HsfA2 is an important coactivator of HsfA1a during HSR HsfA2 suppression reduces the viability and germination rate of pollen that received the stress during the stages of meiosis and microspore formation but had no effect on more advanced stages. In general, pollen meiocytes and microspores are characterized by increased susceptibility to HS due to their lower capacity to induce a strong HSR This sensitivity is partially mitigated by the developmentally regulated expression of HsfA2 and several HS-responsive genes mediated by HsfA1a under nonstress conditions. Thereby, HsfA2 is an important factor for the priming process that sustains pollen thermotolerance during microsporogenesis.

Arabidopsis AtDjA3 Null Mutant Shows Increased Sensitivity to Abscisic Acid, Salt, and Osmotic Stress in Germination and Post-germination Stages

DnaJ proteins are essential co-chaperones involved in abiotic and biotic stress responses. Arabidopsis AtDjA3 gene encodes a molecular co-chaperone of 420 amino acids, which belongs to the J-protein family. In this study, we report the functional characterization of the AtDjA3 gene using the Arabidopsis knockout line designated j3 and the 35S::AtDjA3 overexpression lines. Loss of AtDjA3 function was associated with small seed production. In fact, j3 mutant seeds showed a reduction of 24% in seed weight compared to Col-0 seeds. Expression analysis showed that the AtDjA3 gene was modulated in response to NaCl, glucose, and abscisic acid (ABA). The j3 line had increased sensitivity to NaCl and glucose treatments in the germination and cotyledon development in comparison to parental Col-0. Furthermore, the j3 mutant line exhibited higher ABA sensitivity in comparison to parental Col-0 and 35S::AtDjA3 overexpression lines. In addition, we examined the expression of ABI3 gene, which is a central regulator in ABA signaling, in j3 mutant and 35S::AtDjA3 overexpression lines. Under 5 μM ABA treatment at 24 h, j3 mutant seedlings displayed higher ABI3 expression, whereas in 35S::AtDjA3 overexpression lines, ABI3 gene expression was repressed. Taken together, these results demonstrate that the AtDjA3 gene is involved in seed development and abiotic stress tolerance.

The Arabidopsis DELAY OF GERMINATION 1 gene affects ABSCISIC ACID INSENSITIVE 5 (ABI5) expression and genetically interacts with ABI3 during Arabidopsis seed development

The seed expressed gene DELAY OF GERMINATION (DOG) 1 is absolutely required for the induction of dormancy. Next to a non-dormant phenotype, the dog1-1 mutant is also characterized by a reduced seed longevity suggesting that DOG1 may affect additional seed processes as well. This aspect however, has been hardly studied and is poorly understood. To uncover additional roles of DOG1 in seeds we performed a detailed analysis of the dog1 mutant using both transcriptomics and metabolomics to investigate the molecular consequences of a dysfunctional DOG1 gene. Further, we used a genetic approach taking advantage of the weak aba insensitive (abi) 3-1 allele as a sensitized genetic background in a cross with dog1-1. DOG1 affects the expression of hundreds of genes including LATE EMBRYOGENESIS ABUNDANT and HEAT SHOCK PROTEIN genes which are affected by DOG1 partly via control of ABI5 expression. Furthermore, the content of a subset of primary metabolites, which normally accumulate during seed maturation, was found to be affected in the dog1-1 mutant. Surprisingly, the abi3-1 dog1-1 double mutant produced green seeds which are highly ABA insensitive, phenocopying severe abi3 mutants, indicating that dog1-1 acts as an enhancer of the weak abi3-1 allele and thus revealing a genetic interaction between both genes. Analysis of the dog1 and dog1 abi3 mutants revealed additional seed phenotypes and therefore we hypothesize that DOG1 function is not limited to dormancy but that it is required for multiple aspects of seed maturation, in part by interfering with ABA signalling components.

Transcriptomic Profiling Reveals Metabolic and Regulatory Pathways in the Desiccation Tolerance of Mungbean (Vigna radiata [L.] R. Wilczek)

Mungbean (Vigna radiate L. Wilczek) is an important legume crop for its valuable nutritional and health benefits. Desiccation tolerance (DT) is a capacity of seeds to survive and maintain physiological activities during storage and under stress conditions. Many studies of DT have been reported in other legume crop, such as soybean and Medicago truncatula with little studies in the mungbean. In this study, the transcript profiles of mungbean seeds under different imbibition times were investigated for DT using RNA-sequencing (RNA-seq). A total of 3210 differentially expressed genes (DEGs) were found at the key period of DT (3-18 h of imbibition). Gene ontology (GO) and KEGG analysis showed that the terms of "response to stimulus," "transcription regulator," "methylation," and "starch and sucrose metabolism" were enriched for DT. Clustering analysis also showed that many transcription factors (MYB, AP2, and NAC), HSPs, embryogenesis abundant (LEA) proteins, and genes encoding methyltransferase and histone were differentially expressed. Nine of these DEGs were further validated by quantitative RT-PCR (qRT-PCR). Our study extends our knowledge of mungbean transcriptomes and further provides insight into the molecular mechanism of DT as well as new strategies for developing drought-tolerant crops.

Surviving metabolic arrest: photosynthesis during desiccation and rehydration in resurrection plants

Photosynthesis is the key process that is affected by dehydration in plants. Desiccation-tolerant resurrection plants can survive conditions of very low relative water content. During desiccation, photosynthesis is not operational, but is recovered within a short period after rehydration. While homoiochlorophyllous resurrection plants retain their photosynthetic apparatus during desiccation, poikilochlorophyllous resurrection species dismantle chloroplasts and degrade chlorophyll but resynthesize them again during rehydration. Dismantling the chloroplasts avoids the photooxidative stress in poikilochlorophyllous resurrection plants, whereas it is minimized in homoiochlorophyllous plants through the synthesis of antioxidant enzymes and protective proteins or metabolites. Although the cellular protection mechanisms in both of these species vary, these mechanisms protect cells from desiccation-induced damage and restore photosynthesis upon rehydration. Several of the proteins synthesized during dehydration are localized in chloroplasts and are believed to play major roles in the protection of photosynthetic structures and in recovery in resurrection species. This review focuses on the strategies of resurrection plants in terms of how they protect their photosynthetic apparatus from oxidative stress during desiccation without membrane damage and with full recovery during rehydration. We review the role of the dehydration-induced protection mechanisms in chloroplasts and how photosynthesis is restored during rehydration.

Arabidopsis seed-specific vacuolar aquaporins are involved in maintaining seed longevity under the control of ABSCISIC ACID INSENSITIVE 3

The tonoplast intrinsic proteins TIP3;1 and TIP3;2 are specifically expressed during seed maturation and localized to the seed protein storage vacuole membrane. However, the function and physiological roles of TIP3s are still largely unknown. The seed performance of TIP3 knockdown mutants was analysed using the controlled deterioration test. The tip3;1/tip3;2 double mutant was affected in seed longevity and accumulated high levels of hydrogen peroxide compared with the wild type, suggesting that TIP3s function in seed longevity. The transcription factor ABSCISIC ACID INSENSITIVE 3 (ABI3) is known to be involved in seed desiccation tolerance and seed longevity. TIP3 transcript and protein levels were significantly reduced in abi3-6 mutant seeds. TIP3;1 and TIP3;2 promoters could be activated by ABI3 in the presence of abscisic acid (ABA) in Arabidopsis protoplasts. TIP3 proteins were detected in the protoplasts transiently expressing ABI3 and in ABI3-overexpressing seedlings when treated with ABA. Furthermore, ABI3 directly binds to the RY motif of the TIP3 promoters. Therefore, seed-specific TIP3s may help maintain seed longevity under the expressional control of ABI3 during seed maturation and are members of the ABI3-mediated seed longevity pathway together with small heat shock proteins and late embryo abundant proteins.

Hsp transcript induction is correlated with physiological changes under drought stress in Indian mustard

Brassica juncea is an important oilseed crop and drought stress is major abiotic stress that limits its growth and productivity. RH0116 (drought tolerant) and RH8812 (drought sensitive) genotypes were undertaken to study some of the physiological parameters and hsp gene expression related to stress tolerance under drought stress conditions. Differential response in terms of seed germination, electrolyte leakage, RWC, osmotic potential was observed in the selected genotypes. In vitro seed germination studies using PEG stress treatments indicated reduced seed germination with increasing levels of stress treatment. Electrolyte leakage increased, whereas, relative water content and osmotic potential decreased in stressed seedlings. Expression of hsp gene was found to be upregulated during drought stress as the transcripts were present only in the stressed plants and disappeared upon rehydration. The drought tolerant variety showed higher transcript accumulation as compared to the sensitive variety. The study showed that drought induced changes in gene expression in two contrasting genotypes were consistent with the physiological response.

Variability within a pea core collection of LEAM and HSP22, two mitochondrial seed proteins involved in stress tolerance

LEAM, a late embryogenesis abundant protein, and HSP22, a small heat shock protein, were shown to accumulate in the mitochondria during pea (Pisum sativum L.) seed development, where they are expected to contribute to desiccation tolerance. Here, their expression was examined in seeds of 89 pea genotypes by Western blot analysis. All genotypes expressed LEAM and HSP22 in similar amounts. In contrast with HSP22, LEAM displayed different isoforms according to apparent molecular mass. Each of the 89 genotypes harboured a single LEAM isoform. Genomic and RT-PCR analysis revealed four LEAM genes differing by a small variable indel in the coding region. These variations were consistent with the apparent molecular mass of each isoform. Indels, which occurred in repeated domains, did not alter the main properties of LEAM. Structural modelling indicated that the class A α-helix structure, which allows interactions with the mitochondrial inner membrane in the dry state, was preserved in all isoforms, suggesting functionality is maintained. The overall results point out the essential character of LEAM and HSP22 in pea seeds. LEAM variability is discussed in terms of pea breeding history as well as LEA gene evolution mechanisms.

Molecular Cloning and Differential Expression of Cytosolic Class I Small Hsp Gene Family in Pennisetum glaucum (L.)

Small heat shock protein (Hsp) family genes have been reported in several plant species that function as molecular chaperones to protect proteins from being denatured in extreme conditions. As a first step towards the isolation and characterization of genes that contribute to combating abiotic stresses particularly heat stress, construction and screening of the subtracted complementary DNA (cDNA) library is reported here. In this study, a subtractive heat stress cDNA library was constructed that was used to isolate members of small Hsps (sHsps) using PgsHsp17.9A gene as a probe. As a result, a total of 150 cDNA clones were isolated from the subtracted cDNA library screening, leading to 121 high-quality expressed sequence tags (ESTs), with an average size of 450 bp, comprising of 15 contigs, and majority of these isolated sHsp genes belong to cytosolic class I (CI) family. In silico sequence analysis of CI-sHsp family genes revealed that the length of sHsp proteins varied from 151 to 159 amino acids and showed large variation in isoelectric point value (5.03 to 10.05) and a narrow range of molecular weight (16.09 to 17.94 kDa). The real-time PCR results demonstrated that CI-sHsp genes are differentially expressed in Pennisetum leaves under different abiotic stress conditions particularly at high temperature. The results presented in this study provide basic information on PgCI-sHsp family genes and form the foundation for future functional studies of these genes.

Proteomic analysis reveals differential accumulation of small heat shock proteins and late embryogenesis abundant proteins between ABA-deficient mutant vp5 seeds and wild-type Vp5 seeds in maize

ABA is a major plant hormone that plays important roles during many phases of plant life cycle, including seed development, maturity and dormancy, and especially the acquisition of desiccation tolerance. Understanding of the molecular basis of ABA-mediated plant response to stress is of interest not only in basic research on plant adaptation but also in applied research on plant productivity. Maize mutant viviparous-5 (vp5), deficient in ABA biosynthesis in seeds, is a useful material for studying ABA-mediated response in maize. Due to carotenoid deficiency, vp5 endosperm is white, compared to yellow Vp5 endosperm. However, the background difference at proteome level between vp5 and Vp5 seeds is unclear. This study aimed to characterize proteome alterations of maize vp5 seeds and to identify ABA-dependent proteins during seed maturation. We compared the embryo and endosperm proteomes of vp5 and Vp5 seeds by gel-based proteomics. Up to 46 protein spots, most in embryos, were found to be differentially accumulated between vp5 and Vp5. The identified proteins included small heat shock proteins (sHSPs), late embryogenesis abundant (LEA) proteins, stress proteins, storage proteins and enzymes among others. However, EMB564, the most abundant LEA protein in maize embryo, accumulated in comparable levels between vp5 and Vp5 embryos, which contrasted to previously characterized, greatly lowered expression of emb564 mRNA in vp5 embryos. Moreover, LEA proteins and sHSPs displayed differential accumulations in vp5 embryos: six out of eight identified LEA proteins decreased while nine sHSPs increased in abundance. Finally, we discussed the possible causes of global proteome alterations, especially the observed differential accumulation of identified LEA proteins and sHSPs in vp5 embryos. The data derived from this study provides new insight into ABA-dependent proteins and ABA-mediated response during maize seed maturation.

Differentially expressed seed aging responsive heat shock protein OsHSP18.2 implicates in seed vigor, longevity and improves germination and seedling establishment under abiotic stress

Small heat shock proteins (sHSPs) are a diverse group of proteins and are highly abundant in plant species. Although majority of these sHSPs were shown to express specifically in seed, their potential function in seed physiology remains to be fully explored. Our proteomic analysis revealed that OsHSP18.2, a class II cytosolic HSP is an aging responsive protein as its abundance significantly increased after artificial aging in rice seeds. OsHSP18.2 transcript was found to markedly increase at the late maturation stage being highly abundant in dry seeds and sharply decreased after germination. Our biochemical study clearly demonstrated that OsHSP18.2 forms homooligomeric complex and is dodecameric in nature and functions as a molecular chaperone. OsHSP18.2 displayed chaperone activity as it was effective in preventing thermal inactivation of Citrate Synthase. Further, to analyze the function of this protein in seed physiology, seed specific Arabidopsis overexpression lines for OsHSP18.2 were generated. Our subsequent functional analysis clearly demonstrated that OsHSP18.2 has ability to improve seed vigor and longevity by reducing deleterious ROS accumulation in seeds. In addition, transformed Arabidopsis seeds also displayed better performance in germination and cotyledon emergence under adverse conditions. Collectively, our work demonstrates that OsHSP18.2 is an aging responsive protein which functions as a molecular chaperone and possibly protect and stabilize the cellular proteins from irreversible damage particularly during maturation drying, desiccation and aging in seeds by restricting ROS accumulation and thereby improves seed vigor, longevity and seedling establishment.

Environmentally induced transgenerational changes in seed longevity: maternal and genetic influence

BACKGROUND AND AIMS

Seed longevity, a fundamental plant trait for ex situ conservation and persistence in the soil of many species, varies across populations and generations that experience different climates. This study investigates the extent to which differences in seed longevity are due to genetic differences and/or modified by adaptive responses to environmental changes.

METHODS

Seeds of two wild populations of Silene vulgaris from alpine (wA) and lowland (wL) locations and seeds originating from their cultivation in a lowland common garden for two generations (cA1, cL1, cA2 and cL2) were exposed to controlled ageing at 45 °C, 60 % relative humidity and regularly sampled for germination and relative mRNA quantification (SvHSP17.4 and SvNRPD12).

KEY RESULTS

The parental plant growth environment affected the longevity of seeds with high plasticity. Seeds of wL were significantly longer lived than those of wA. However, when alpine plants were grown in the common garden, longevity doubled for the first generation of seeds produced (cA1). Conversely, longevity was similar in all lowland seed lots and did not increase in the second generation of seeds produced from alpine plants grown in the common garden (cA2). Analysis of parental effects on mRNA seed provisioning indicated that the accumulation of gene transcripts involved in tolerance to heat stress was highest in wL, cL1 and cL2, followed by cA1, cA2 and wA.

CONCLUSIONS

Seed longevity has a genetic basis, but may show strong adaptive responses, which are associated with differential accumulation of mRNA via parental effects. Adaptive adjustments of seed longevity due to transgenerational plasticity may play a fundamental role in the survival and persistence of the species in the face of future environmental challenges. The results suggest that regeneration location may have important implications for the conservation of alpine plants held in seed banks.

Proteomic identification of differentially expressed proteins during the acquisition of somatic embryogenesis in oil palm (Elaeis guineensis Jacq.)

In the present study we have identified and characterized the proteins expressed during different developmental stages of Elaeis guineensis calli obtained from zygotic embryos. We were interested in the possible proteomic changes that would occur during the acquisition of somatic embryogenesis and therefore samples were collected from zygotic embryos (E1), swollen explants 14days (E2) in induction medium, primary callus (E3), and pro-embryogenic callus (E4). The samples were grinded in liquid nitrogen, followed by total protein extraction using phenol and extraction buffer. Proteins were analyzed by two-dimensional electrophoresis (2-DE) and the differentially expressed protein spots were analyzed by MALDI-TOF mass spectrometry (MS and MS/MS). Interestingly, we have identified proteins, which can be used as potential candidates for future studies aiming at the development of biomarkers for embryogenesis acquisition and for the different stages leading to pro-embryogenic callus formation such as type IIIa membrane protein cp-wap13, fructokinase and PR proteins. The results obtained shed some light on the biochemical events involved in the process of somatic embryogenesis of E. guineensis obtained from zygotic embryos. The use of stage-specific protein markers can help monitor cell differentiation and contribute to improve the protocols for successfully cloning the species.

BIOLOGICAL SIGNIFICANCE

Understanding the fate and dynamics of cells and tissues during callus formation is essential to understand totipotency and the mechanisms involved during acquisition of somatic embryogenesis (SE). In this study we have investigated the early stages of somatic embryogenesis induction in oil palm and have identified potential markers as well as proteins potentially involved in embryogenic competence acquisition. The use of these proteins can help improve tissue culture protocols in order to increase regeneration rates. This article is part of a Special Issue entitled: Environmental and structural proteomics.

Unraveling regulation of the small heat shock proteins by the heat shock factor HvHsfB2c in barley: its implications in drought stress response and seed development

The rapid increase in heat shock proteins upon exposure to damaging stresses and during plant development related to desiccation events reveal their dual importance in plant development and stress tolerance. Genome-wide sequence survey identified 20 non-redundant small heat shock proteins (sHsp) and 22 heat shock factor (Hsf) genes in barley. While all three major classes (A, B, C) of Hsfs are localized in nucleus, the 20 sHsp gene family members are localized in different cell organelles like cytoplasm, mitochondria, plastid and peroxisomes. Hsf and sHsp members are differentially regulated during drought and at different seed developmental stages suggesting the importance of chaperone role under drought as well as seed development. In silico cis-regulatory motif analysis of Hsf promoters showed an enrichment with abscisic acid responsive cis-elements (ABRE), implying regulatory role of ABA in mediating transcriptional response of HvsHsf genes. Gene regulatory network analysis identified HvHsfB2c as potential central regulator of the seed-specific expression of several HvsHsps including 17.5CI sHsp. These results indicate that HvHsfB2c is co-expressed in the central hub of small Hsps and therefore it may be regulating the expression of several HvsHsp subclasses HvHsp16.88-CI, HvHsp17.5-CI and HvHsp17.7-CI. The in vivo relevance of binding specificity of HvHsfB2C transcription factor to HSE-element present in the promoter of HvSHP17.5-CI under heat stress exposure is confirmed by gel shift and LUC-reporter assays. Further, we isolated 477 bp cDNA from barley encoding a 17.5 sHsp polypeptide, which was predominantly upregulated under drought stress treatments and also preferentially expressed in developing seeds. Recombinant HvsHsp17.5-CI protein was expressed in E. coli and purified to homogeneity, which displayed in vitro chaperone activity. The predicted structural model of HvsHsp-17.5-CI protein suggests that the α-crystallin domain is evolutionarily highly conserved.

A seed preferential heat shock transcription factor from wheat provides abiotic stress tolerance and yield enhancement in transgenic Arabidopsis under heat stress environment

Reduction in crop yield and quality due to various abiotic stresses is a worldwide phenomenon. In the present investigation, a heat shock factor (HSF) gene expressing preferentially in developing seed tissues of wheat grown under high temperatures was cloned. This newly identified heat shock factor possesses the characteristic domains of class A type plant HSFs and shows high similarity to rice OsHsfA2d, hence named as TaHsfA2d. The transcription factor activity of TaHsfA2d was confirmed through transactivation assay in yeast. Transgenic Arabidopsis plants overexpressing TaHsfA2d not only possess higher tolerance towards high temperature but also showed considerable tolerance to salinity and drought stresses, they also showed higher yield and biomass accumulation under constant heat stress conditions. Analysis of putative target genes of AtHSFA2 through quantitative RT-PCR showed higher and constitutive expression of several abiotic stress responsive genes in transgenic Arabidopsis plants over-expressing TaHsfA2d. Under stress conditions, TaHsfA2d can also functionally complement the T-DNA insertion mutants of AtHsfA2, although partially. These observations suggest that TaHsfA2d may be useful in molecular breeding of crop plants, especially wheat, to improve yield under abiotic stress conditions.

Recovery from heat, salt and osmotic stress in Physcomitrella patens requires a functional small heat shock protein PpHsp16.4

BACKGROUND

Plant small heat shock proteins (sHsps) accumulate in response to various environmental stresses, including heat, drought, salt and oxidative stress. Numerous studies suggest a role for these proteins in stress tolerance by preventing stress-induced protein aggregation as well as by facilitating protein refolding by other chaperones. However, in vivo evidence for the involvement of sHsps in tolerance to different stress factors is still missing, mainly due to the lack of appropriate mutants in specific sHsp genes.

RESULTS

In this study we characterized the function of a sHsp in abiotic stress tolerance in the moss Physcomitrella patens, a model for primitive land plants. Using suppression subtractive hybridization, we isolated an abscisic acid-upregulated gene from P. patens encoding a 16.4 kDa cytosolic class II sHsp. PpHsp16.4 was also induced by salicylic acid, dithiothreitol (DTT) and by exposure to various stimuli, including osmotic and salt stress, but not by oxidative stress-inducing compounds. Expression of the gene was maintained upon stress relief, suggesting a role for this protein in the recovery stage. PpHsp16.4 is encoded by two identical genes arranged in tandem in the genome. Targeted disruption of both genes resulted in the inability of plants to recover from heat, salt and osmotic stress. In vivo localization studies revealed that PpHsp16.4 localized in cytosolic granules in the vicinity of chloroplasts under non stress conditions, suggesting possible distinct roles for this protein under stress and optimal growth.

CONCLUSIONS

We identified a member of the class II sHsp family that showed hormonal and abiotic stress gene regulation. Induction of the gene by DTT treatment suggests that damaged proteins may act as signals for the stress-induction of PpHsp16.4. The product of this gene was shown to localize in cytosolic granules near the chloroplasts, suggesting a role for the protein in association with these organelles. Our study provides the first direct genetic evidence for a role of a sHsp in osmotic and salt stress tolerance, and supports a function for this protein particularly during the stress recovery stage of P. patens.

Ensuring Reproduction at High Temperatures: The Heat Stress Response during Anther and Pollen Development

Sexual reproduction in flowering plants is very sensitive to environmental stresses, particularly to thermal insults which frequently occur when plants grow in field conditions in the warm season. Although abnormalities in both male and female reproductive organs due to high temperatures have been described in several crops, the failure to set fruits has mainly been attributed to the high sensitivity of developing anthers and pollen grains, particularly at certain developmental stages. A global view of the molecular mechanisms involved in the response to high temperatures in the male reproductive organs will be presented in this review. In addition, transcriptome and proteomic data, currently available, will be discussed in the light of physiological and metabolic changes occurring during anther and pollen development. A deep understanding of the molecular mechanisms involved in the stress response to high temperatures in flowers and, particularly, in the male reproductive organs will be a major step towards development of effective breeding strategies for high and stable production in crop plants.

Proteomic perspective of Quercus suber somatic embryogenesis

Quercus suber L. is a forest tree with remarkable ecological, social and economic value in the southern Europe ecosystems. To circumvent the difficulties of breeding such long-lived species like Q. suber in a conventional fashion, clonal propagation of Q. suber elite trees can be carried out, although this process is sometimes unsuccessful. To help decipher the complex program underlying the development of Q. suber somatic embryos from the first early stage until maturity, a proteomic approach based on DIGE and MALDI-MS has been envisaged. Results highlighted several key processes involved in the three developmental stages (proliferative, cotyledonary and mature) of Q. suber somatic embryogenesis studied. Results show that the proliferation stage is characterized by fermentation as an alternative energy source at the first steps of somatic embryo development, as well as by up-regulation of proteins involved in cell division. In this stage reactive oxygen species play a role in proliferation, while other proteins like CAD and PR5 seem to be implied in embryonic competence. In the transition to the cotyledonary stage diverse ROS detoxification enzymes are activated and reserve products (mainly carbohydrates and proteins) are accumulated, whereas energy production is increased probably to participate in the synthesis of primary metabolites such as amino acids and fatty acids. Finally, in the mature stage ethylene accumulation regulates embryo development.

BIOLOGICAL SIGNIFICANCE

Quercus suber L. is a forest tree with remarkable ecological, social and economic value in the southern Europe ecosystems. To circumvent the difficulties of breeding such long-lived species like Q. suber in a conventional fashion, clonal propagation of Q. suber elite trees can be carried out, although this process is sometimes unsuccessful. To help decipher the complex program underlying the development of Q. suber somatic embryos from the first early stage until maturity, in deep studies become necessary. This article is part of a Special Issue entitled: Translational Plant Proteomics.

The evolution of desiccation tolerance in angiosperm plants: a rare yet common phenomenon

In a minute proportion of angiosperm species, rehydrating foliage can revive from airdryness or even from equilibration with air of ~0% RH. Such desiccation tolerance is known from vegetative cells of some species of algae and of major groups close to the evolutionary path of the angiosperms. It is also found in the reproductive structures of some algae, moss spores and probably the aerial spores of other terrestrial cryptogamic taxa. The occurrence of desiccation tolerance in the seed plants is overwhelmingly in the aerial reproductive structures; the pollen and seed embryos. Spatially and temporally, pollen and embryos are close ontogenetic derivatives of the angiosperm microspores and megaspores respectively. This suggests that the desiccation tolerance of pollen and embryos derives from the desiccation tolerance of the spores of antecedent taxa and that the basic pollen/embryo mechanism of desiccation tolerance has eventually become expressed also in the vegetative tissue of certain angiosperm species whose drought avoidance is inadequate in micro-habitats that suffer extremely xeric episodes. The protective compounds and processes that contribute to desiccation tolerance in angiosperms are found in the modern groups related to the evolutionary path leading to the angiosperms and are also present in the algae and in the cyanobacteria. The mechanism of desiccation tolerance in the angiosperms thus appears to have its origins in algal ancestors and possibly in the endosymbiotic cyanobacteria-related progenitor of chloroplasts and the bacteria-related progenitor of mitochondria. The mechanism may involve the regulation and timing of the accumulation of protective compounds and of other contributing substances and processes.

Gasotransmitters are emerging as new guard cell signaling molecules and regulators of leaf gas exchange

Specialized guard cells modulate plant gas exchange through the regulation of stomatal aperture. The size of the stomatal pore is a direct function of the volume of the guard cells. The transport of solutes across channels in plasma membrane is a crucial process in the maintenance of guard cell water status. The fine tuned regulation of that transport requires an integrated convergence of multiple endogenous and exogenous signals perceived at both the cellular and the whole plant level. Gasotransmitters are novel signaling molecules with key functions in guard cell physiology. Three gasotransmitters, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H(2)S) are involved in guard cell regulatory processes. These molecules are endogenously produced by plant cells and are part of the guard cells responses to drought stress conditions through ABA-dependent pathways. In this review, we summarize the current knowledge of gasotransmitters as versatile molecules interacting with different components of guard cell signaling network and propose them as players in new paradigms to study ABA-independent guard cell responses to water deficit.

PROTEIN L-ISOASPARTYL METHYLTRANSFERASE2 is differentially expressed in chickpea and enhances seed vigor and longevity by reducing abnormal isoaspartyl accumulation predominantly in seed nuclear proteins

PROTEIN l-ISOASPARTYL METHYLTRANSFERASE (PIMT) is a widely distributed protein-repairing enzyme that catalyzes the conversion of abnormal l-isoaspartyl residues in spontaneously damaged proteins to normal aspartyl residues. This enzyme is encoded by two divergent genes (PIMT1 and PIMT2) in plants, unlike many other organisms. While the biological role of PIMT1 has been elucidated, the role and significance of the PIMT2 gene in plants is not well defined. Here, we isolated the PIMT2 gene (CaPIMT2) from chickpea (Cicer arietinum), which exhibits a significant increase in isoaspartyl residues in seed proteins coupled with reduced germination vigor under artificial aging conditions. The CaPIMT2 gene is found to be highly divergent and encodes two possible isoforms (CaPIMT2 and CaPIMT2') differing by two amino acids in the region I catalytic domain through alternative splicing. Unlike CaPIMT1, both isoforms possess a unique 56-amino acid amino terminus and exhibit similar yet distinct enzymatic properties. Expression analysis revealed that CaPIMT2 is differentially regulated by stresses and abscisic acid. Confocal visualization of stably expressed green fluorescent protein-fused PIMT proteins and cell fractionation-immunoblot analysis revealed that apart from the plasma membrane, both CaPIMT2 isoforms localize predominantly in the nucleus, while CaPIMT1 localizes in the cytosol. Remarkably, CaPIMT2 enhances seed vigor and longevity by repairing abnormal isoaspartyl residues predominantly in nuclear proteins upon seed-specific expression in Arabidopsis (Arabidopsis thaliana), while CaPIMT1 enhances seed vigor and longevity by repairing such abnormal proteins mainly in the cytosolic fraction. Together, our data suggest that CaPIMT2 has most likely evolved through gene duplication, followed by subfunctionalization to specialize in repairing the nuclear proteome.

The evolution, function, structure, and expression of the plant sHSPs

Small heat shock proteins are a diverse, ancient, and important family of proteins. All organisms possess small heat shock proteins (sHSPs), indicating that these proteins evolved very early in the history of life prior to the divergence of the three domains of life (Archaea, Bacteria, and Eukarya). Comparing the structures of sHSPs from diverse organisms across these three domains reveals that despite considerable amino acid divergence, many structural features are conserved. Comparisons of the sHSPs from diverse organisms reveal conserved structural features including an oligomeric form with a β-sandwich that forms a hollow ball. This conservation occurs despite significant divergence in primary sequences. It is well established that sHSPs are molecular chaperones that prevent misfolding and irreversible aggregation of their client proteins. Most notably, the sHSPs are extremely diverse and variable in plants. Some plants have >30 individual sHSPs. Land plants, unlike other groups, possess distinct sHSP subfamilies. Most are highly up-regulated in response to heat and other stressors. Others are selectively expressed in seeds and pollen, and a few are constitutively expressed. As a family, sHSPs have a clear role in thermotolerance, but attributing specific effects to individual proteins has proved challenging. Considerable progress has been made during the last 15 years in understanding the sHSPs. However, answers to many important questions remain elusive, suggesting that the next 15 years will be at least equally rewarding.

The wheat chloroplastic small heat shock protein (sHSP26) is involved in seed maturation and germination and imparts tolerance to heat stress

The nuclear-encoded chloroplast small heat shock proteins (sHSPs) are present in all plant species from algae to angiosperms. Expression analysis shows that the wheat chloroplastic sHSP (HSP26) is highly inducible by heat stress in almost all the vegetative and generative tissues and is also expressed constitutively in certain developmental growth stages. We characterize wheat chloroplastic sHSP 26 through transgenic approach using Arabidopsis and report cloning of the promoter and its characterization. Transgenic Arabidopsis plants were substantially tolerant under continuous high temperature regimen than wild-type plants, as measured by photosystem II (PSII) activity, accumulation of more photosynthetic pigments, higher biomass and seed yield. Transgenic plants produced bold seeds under high temperature, having higher germination potential than the wild-type plants. Further, antisense Arabidopsis plants showed negligible tolerance even for non-lethal heat shock, impaired in basal thermo-tolerance, and accumulated less biomass and seed yield under normal growth conditions. Promoter analysis revealed the presence of several heat and other abiotic stress responsive cis-acting elements along with developmental stage and tissue-specific elements. Analysis of promoter through GUS reporter system in both transgenic rice and Arabidopsis further confirms the role of chloroplastic sHsp26 in heat and other abiotic stresses as well as during seed maturation and germination. Genome-wide expression analysis of overexpression Arabidopsis plants revealed that the transcriptome remained unchanged in the transgenic plants and the tolerance was due to the overexpression of chloroplastic heat shock protein (HSP) only.

The Opuntia streptacantha OpsHSP18 gene confers salt and osmotic stress tolerance in Arabidopsis thaliana

Abiotic stress limits seed germination, plant growth, flowering and fruit quality, causing economic decrease. Small Heat Shock Proteins (sHSPs) are chaperons with roles in stress tolerance. Herein, we report the functional characterization of a cytosolic class CI sHSP (OpsHSP18) from Opuntia streptacantha during seed germination in Arabidopsis thaliana transgenic lines subjected to different stress and hormone treatments. The over-expression of the OpsHSP18 gene in A. thaliana increased the seed germination rate under salt (NaCl) and osmotic (glucose and mannitol) stress, and in ABA treatments, compared with WT. On the other hand, the over-expression of the OpsHSP18 gene enhanced tolerance to salt (150 mM NaCl) and osmotic (274 mM mannitol) stress in Arabidopsis seedlings treated during 14 and 21 days, respectively. These plants showed increased survival rates (52.00 and 73.33%, respectively) with respect to the WT (18.75 and 53.75%, respectively). Thus, our results show that OpsHSP18 gene might have an important role in abiotic stress tolerance, in particular in seed germination and survival rate of Arabidopsis plants under unfavorable conditions.

Temporal profiling of the heat-stable proteome during late maturation of Medicago truncatula seeds identifies a restricted subset of late embryogenesis abundant proteins associated with longevity

Developing seeds accumulate late embryogenesis abundant (LEA) proteins, a family of intrinsically disordered and hydrophilic proteins that confer cellular protection upon stress. Many different LEA proteins exist in seeds, but their relative contribution to seed desiccation tolerance or longevity (duration of survival) is not yet investigated. To address this, a reference map of LEA proteins was established by proteomics on a hydrophilic protein fraction from mature Medicago truncatula seeds and identified 35 polypeptides encoded by 16 LEA genes. Spatial and temporal expression profiles of the LEA polypeptides were obtained during the long maturation phase during which desiccation tolerance and longevity are sequentially acquired until pod abscission and final maturation drying occurs. Five LEA polypeptides, representing 6% of the total LEA intensity, accumulated upon acquisition of desiccation tolerance. The gradual 30-fold increase in longevity correlated with the accumulation of four LEA polypeptides, representing 35% of LEA in mature seeds, and with two chaperone-related polypeptides. The majority of LEA polypeptides increased around pod abscission during final maturation drying. The differential accumulation profiles of the LEA polypeptides suggest different roles in seed physiology, with a small subset of LEA and other proteins with chaperone-like functions correlating with desiccation tolerance and longevity.

Water status and associated processes mark critical stages in pollen development and functioning

BACKGROUND

The male gametophyte developmental programme can be divided into five phases which differ in relation to the environment and pollen hydration state: (1) pollen develops inside the anther immersed in locular fluid, which conveys substances from the mother plant--the microsporogenesis phase; (2) locular fluid disappears by reabsorption and/or evaporation before the anther opens and the maturing pollen grains undergo dehydration--the dehydration phase; (3) the anther opens and pollen may be dispersed immediately, or be held by, for example, pollenkitt (as occurs in almost all entomophilous species) for later dispersion--the presentation phase; (4) pollen is dispersed by different agents, remaining exposed to the environment for different periods--the dispersal phase; and (5) pollen lands on a stigma and, in the case of a compatible stigma and suitable conditions, undergoes rehydration and starts germination--the pollen-stigma interaction phase.

SCOPE

This review highlights the issue of pollen water status and indicates the various mechanisms used by pollen grains during their five developmental phases to adjust to changes in water content and maintain internal stability.

CONCLUSIONS

Pollen water status is co-ordinated through structural, physiological and molecular mechanisms. The structural components participating in regulation of the pollen water level, during both dehydration and rehydration, include the exine (the outer wall of the pollen grain) and the vacuole. Recent data suggest the involvement of water channels in pollen water transport and the existence of several molecular mechanisms for pollen osmoregulation and to protect cellular components (proteins and membranes) under water stress. It is suggested that pollen grains will use these mechanisms, which have a developmental role, to cope with environmental stress conditions.

The absence of heat shock protein HSP101 affects the proteome of mature and germinating maize embryos

Maize heat shock protein HSP101 accumulates during embryo maturation and desiccation and persists at high levels during the first 24 h following kernel imbibition in the absence of heat stress. This protein has a known function in disaggregation of high molecular weight complexes and has been proposed to be a translational regulator of specific mRNAs. Here, a global proteomic approach was used to identify changes in the maize proteome due to the absence of HSP101 in embryos from mature-dry or 24 h-imbibed kernels. A total of 26 protein spots from the mature dry embryo exhibited statistically significant expression changes in the L10 inbred hsp101 mutant (hsp101-m5::Mu1/hsp101-m5::Mu1) line as compared to the corresponding wild type (Hsp101/Hsp101). Additional six spots reproducibly showed qualitative changes between the mutant and wild-type mature and germinating embryos. Several chaperones, translation-related proteins, actin, and enzymes participating in cytokinin metabolism were identified in these spots by tandem mass-spectrometry (MS). The proteomic changes partially explain the altered root growth and architecture observed in young hsp101 mutant seedlings. In addition, specific protein de novo synthesis was altered in the 24 h-imbibed mutant embryos indicating that maize HSP101 functions as both chaperone and translational regulator during germination. Supporting this, HSP101 was found as part of Cap-binding and translation initiation complexes during early kernel imbibition. Overall, these findings expose the relevance of maize HSP101 for protein synthesis and balance mechanisms during germination.

Small heat shock protein expression and functions during development

The expression of small heat shock proteins is tightly regulated during development in multiple organisms. As housekeeping proteins, small heat shock proteins help protect cells from apoptosis, stabilize the cytoskeleton and contribute to proteostasis. Consistently, depletion of one small heat shock protein is usually not detrimental due to a certain level of redundancy between the functions of each small heat shock protein. However, while their stress-induced expression is regulated by heat shock factors, their constitutive expression is under the control of other specific transcription factors, suggesting the existence of very specialized functions. This review focuses on the expression patterns and functions of small heat shock proteins in various organisms during development. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Genome-wide transcriptomic analysis of cotton under drought stress reveal significant down-regulation of genes and pathways involved in fibre elongation and up-regulation of defense responsive genes

Cotton is an important source of natural fibre used in the textile industry and the productivity of the crop is adversely affected by drought stress. High throughput transcriptomic analyses were used to identify genes involved in fibre development. However, not much information is available on cotton genome response in developing fibres under drought stress. In the present study a genome wide transcriptome analysis was carried out to identify differentially expressed genes at various stages of fibre growth under drought stress. Our study identified a number of genes differentially expressed during fibre elongation as compared to other stages. High level up-regulation of genes encoding for enzymes involved in pectin modification and cytoskeleton proteins was observed at fibre initiation stage. While a large number of genes encoding transcription factors (AP2-EREBP, WRKY, NAC and C2H2), osmoprotectants, ion transporters and heat shock proteins and pathways involved in hormone (ABA, ethylene and JA) biosynthesis and signal transduction were up-regulated and genes involved in phenylpropanoid and flavonoid biosynthesis, pentose and glucuronate interconversions and starch and sucrose metabolism pathways were down-regulated during fibre elongation. This study showed that drought has relatively less impact on fibre initiation but has profound effect on fibre elongation by down-regulating important genes involved in cell wall loosening and expansion process. The comprehensive transcriptome analysis under drought stress has provided valuable information on differentially expressed genes and pathways during fibre development that will be useful in developing drought tolerant cotton cultivars without compromising fibre quality.

Identification and expression analysis of two small heat shock protein cDNAs from developing seeds of biodiesel feedstock plant Jatropha curcas

Plant small heat shock proteins (sHSPs) are known to be important for environmental stress tolerance and involved in various developmental processes. In this study, two full-length cDNAs encoding sHSPs, designated JcHSP-1 and JcHSP-2, were identified and characterized from developing seeds of a promising biodiesel feedstock plant Jatropha curcas by expressed sequence tag (EST) sequencing of embryo cDNA libraries and rapid amplification of cDNA ends (RACE). JcHSP-1 and JcHSP-2 contained open-reading frames encoding sHSPs of 219 and 157 amino acids, with predicted molecular weights of 24.42kDa and 18.02kDa, respectively. Sequence alignment indicated that both JcHSP-1 and JcHSP-2 shared high similarity with other plant sHSPs. Real-time quantitative RT-PCR analysis showed that the transcriptional level of both JcHSP-1 and JcHSP-2 increased along with natural dehydration process during seed development. A sharp increase of JcHSP-2 transcripts occurred in response to water content dropping from 42% in mature seeds to 12% in dry seeds. Western blot analysis revealed that the accumulation profile of two cross-reacting proteins, whose molecular weight corresponding to the calculated size of JcHSP-1 and JcHSP-2, respectively, was well consistent with the mRNA expression pattern of JcHSP-1 and JcHSP-2 in jatropha seeds during maturation and natural dehydration. These results indicated that both JcHSPs might play an important role in cell protection and seed development during maturation of J. curcas seeds.

Transcriptome analysis of chestnut (Castanea sativa) tree buds suggests a putative role for epigenetic control of bud dormancy

BACKGROUND AND AIMS

Recent papers indicated that epigenetic control is involved in transitions in bud dormancy, purportedly controlling gene expression. The present study aimed to identify genes that are differentially expressed in dormant and non-dormant Castanea sativa buds.

METHODS

Two suppression subtractive hybridization cDNA libraries were constructed to characterize the transcriptomes of dormant apical buds of C. sativa, and buds in which dormancy was released.

KEY RESULTS

A total of 512 expressed sequence tags (ESTs) were generated in a forward and reverse subtractive hybridization experiment. Classification of these ESTs into functional groups demonstrated that dormant buds were predominantly characterized by genes associated with stress response, while non-dormant buds were characterized by genes associated with energy, protein synthesis and cellular components for development and growth. ESTs for a few genes involved in different forms of epigenetic modification were found in both libraries, suggesting a role for epigenetic control in bud dormancy different from that in growth. Genes encoding histone mono-ubiquitinase HUB2 and histone acetyltransferase GCN5L were associated with dormancy, while a gene encoding histone H3 kinase AUR3 was associated with growth. Real-time RT-PCR with a selection of genes involved in epigenetic modification and stress tolerance confirmed the expression of the majority of investigated genes in various stages of bud development, revealing a cyclical expression pattern concurring with the growth seasons for most genes. However, senescing leaves also showed an increased expression of several of the genes associated with dormancy, implying pleiotropy. Furthermore, a comparison between these subtraction cDNA libraries and the poplar bud dormancy transcriptome and arabidopsis transcriptomes for seed dormancy and non-dormancy indicated a common basis for dormancy in all three systems.

CONCLUSIONS

Bud dormancy and non-dormancy in C. sativa were characterized by distinct sets of genes and are likely to be under different epigenetic control.

Transcriptome analysis of chestnut (Castanea sativa) tree buds suggests a putative role for epigenetic control of bud dormancy

BACKGROUND AND AIMS

Recent papers indicated that epigenetic control is involved in transitions in bud dormancy, purportedly controlling gene expression. The present study aimed to identify genes that are differentially expressed in dormant and non-dormant Castanea sativa buds.

METHODS

Two suppression subtractive hybridization cDNA libraries were constructed to characterize the transcriptomes of dormant apical buds of C. sativa, and buds in which dormancy was released.

KEY RESULTS

A total of 512 expressed sequence tags (ESTs) were generated in a forward and reverse subtractive hybridization experiment. Classification of these ESTs into functional groups demonstrated that dormant buds were predominantly characterized by genes associated with stress response, while non-dormant buds were characterized by genes associated with energy, protein synthesis and cellular components for development and growth. ESTs for a few genes involved in different forms of epigenetic modification were found in both libraries, suggesting a role for epigenetic control in bud dormancy different from that in growth. Genes encoding histone mono-ubiquitinase HUB2 and histone acetyltransferase GCN5L were associated with dormancy, while a gene encoding histone H3 kinase AUR3 was associated with growth. Real-time RT-PCR with a selection of genes involved in epigenetic modification and stress tolerance confirmed the expression of the majority of investigated genes in various stages of bud development, revealing a cyclical expression pattern concurring with the growth seasons for most genes. However, senescing leaves also showed an increased expression of several of the genes associated with dormancy, implying pleiotropy. Furthermore, a comparison between these subtraction cDNA libraries and the poplar bud dormancy transcriptome and arabidopsis transcriptomes for seed dormancy and non-dormancy indicated a common basis for dormancy in all three systems.

CONCLUSIONS

Bud dormancy and non-dormancy in C. sativa were characterized by distinct sets of genes and are likely to be under different epigenetic control.

Proteome analysis of leaves of the desiccation-tolerant grass, Sporobolus stapfianus, in response to dehydration

Drought and its affects on agricultural production is a serious issue facing global efforts to increase food supplies and ensure food security for the growing world population. Understanding how plants respond to dehydration is an important prerequisite for developing strategies for crop improvement in drought tolerance. This has proved to be a difficult task as all of the current research plant models do not tolerate cellular dehydration well and, like all crops, they succumb to the effects of a relatively small water deficit of -4MPa or less. For these reasons many researchers have started to investigate the usefulness of resurrection plants, plants that can survive extremes of dehydration to the point of desiccation, to provide answers as to how plants tolerate water loss. We have chosen to investigate the leaf proteome response of the desiccation-tolerant grass Sporobolus stapfianus Gandoger to dehydration to a water content that encompasses the initiation of the cellular protection response evident in these plants. We used a combination of two-dimensional Difference Gel Electrophoresis (2D-DIGE) and liquid chromatography-tandem-mass spectrometry to compare the proteomes of young leaves from hydrated plants to those dehydrated to approximately 30% relative water content. High-resolution 2D-DIGE revealed 96 significantly different proteins and 82 of these spots yielded high-quality protein assignments by tandem-mass spectrometry. Inferences from the bioinformatic annotations of these proteins revealed the possible involvement of protein kinase-based signaling cascades and brassinosteroid involvement in the regulation of the cellular protection response. Enzymes of glycolysis, both cytoplasmic and plastidic, as well as five enzymes of the Calvin cycle increased in abundance. However, the RuBisCO large subunit and associated proteins were reduced, indicating a loss of carbon fixation but a continued need to supply the necessary carbon skeletons for the constituents involved in cell protection. Changes in abundance of several proteins that appear to have a function in chromatin structure and function indicate that these structures undergo significant changes as a result of dehydration. These observations give a unique "snap-shot" of the proteome of S. stapfianus at a critical point in the passage towards desiccation.

The induction of anhydrobiosis in the sleeping chironomid: current status of our knowledge

An African chironomid, Polypedilum vanderplanki, is the only insect known to be capable of extreme desiccation tolerance, or anhydrobiosis. In the 1950s and 1960s, Hinton strenuously studied anhydrobiosis in this insect from a physiological standpoint; however, nobody has afterward investigated the phenomenon. In 2000, research on mechanisms underlying anhydrobiosis was resumed due to successful establishment of a rearing system for P. vanderplanki. This review is focused on the latest findings on the physiological and molecular mechanisms underlying the induction of anhydrobiosis in P. vanderplanki. Early experiments demonstrated that the induction of anhydrobiosis was possible in isolated tissues and independent from the control of central nervous system. However, to achieve successful anhydrobiosis, larvae need a slow regime of desiccation, allowing them to synthesize molecules, which will protect cells and tissues against the deleterious effects of dehydration. Trehalose, a nonreducing disaccharide, which accumulates in P. vanderplanki larvae up to 20% of the dry body mass, is thought to replace the water in its tissues. Similarly, highly hydrophilic proteins called the late embryogenesis abundant (LEA) proteins are expressed in huge quantities and act as a molecular shield to protect biological molecules against aggregation and denaturation. This function is shared by heat shock proteins, which are also upregulated during the desiccation process. At the same time, desiccating larvae express various antioxidant molecules and enzymes, to cope with the massive oxidative stress, which is responsible for general damage to membranes, proteins, and DNA in dehydrating cells. Finally, specific water channels, called aquaporins, accelerate dehydration, and trehalose together with LEA proteins forms a glassy matrix, which protects the biological molecules and the structural integrity of larvae in the anhydrobiotic state.

Expression of heat shock protein-coding genes associated with anhydrobiosis in an African chironomid Polypedilum vanderplanki

In order to survive in extreme environments, organisms need to develop special adaptations both on physiological and molecular levels. The sleeping chironomid Polypedilum vanderplanki, inhabiting temporary water pools in semi-arid regions of Africa, is the only insect to have evolutionarily acquired the ability to withstand prolonged complete desiccation at larval stage, entering a state called anhydrobiosis. Even after years in a dry state, larvae are able to revive within a short period of time, completely restoring metabolism. Because of the possible involvement of stress proteins in the preservation of biomolecules during the anhydrobiosis of the sleeping chironomid, we have analyzed the expression of genes encoding six heat shock proteins (Pv-hsp90, Pv-hsp70, Pv-hsc70, Pv-hsp60, Pv-hsp20, and Pv-p23) and one heat shock factor (Pv-hsf1) in dehydrating, rehydrating, and heat-shocked larvae. All examined genes were significantly up-regulated in the larvae upon dehydration and several patterns of expression were detected. Gene transcript of Pv-hsf1 was up-regulated within 8 h of desiccation, followed by large shock proteins expression reaching peak at 24-48 h of desiccation. Heat-shock-responsive Pv-hsp70 and Pv-hsp60 showed a two-peak expression: in dehydrating and rehydrating larvae. Both small alpha-crystallin heat shock proteins (sHSP) transcripts were accumulated in the desiccated larvae, but showed different expression profiles. Both sHSP-coding genes were found to be heat-inducible, and Pv-hsp20 was up-regulated in the larvae at the early stage of desiccation. In contrast, expression of the second transcript, corresponding to Pv-p23, was limited to the late stages of desiccation, suggesting possible involvement of this protein in the glass-state formation in anhydrobiotic larvae. We discuss possible roles of proteins encoded by these stress genes during the different stages of anhydrobiosis in P. vanderplanki.

Transcriptome analysis of high-temperature stress in developing barley caryopses: early stress responses and effects on storage compound biosynthesis

High-temperature stress, like any abiotic stress, impairs the physiology and development of plants, including the stages of seed setting and ripening. We used the Affymetrix 22K Barley1 GeneChip microarray to investigate the response of developing barley (Hordeum vulgare) seeds, termed caryopses, after 0.5, 3, and 6 h of heat stress exposure; 958 induced and 1122 repressed genes exhibited spatial and temporal expression patterns that provide a detailed insight into the caryopses' early heat stress responses. Down-regulation of genes related to storage compound biosynthesis and cell growth provides evidence for a rapid impairment of the caryopsis' development. Increased levels of sugars and amino acids were indicative for both production of compatible solutes and feedback-induced accumulation of substrates for storage compound biosynthesis. Metadata analysis identified embryo and endosperm as primary locations of heat stress responses, indicating a strong impact of short-term heat stress on central developmental functions of the caryopsis. A comparison with heat stress responses in Arabidopsis shoots and drought stress responses in barley caryopses identified both conserved and presumably heat- and caryopsis-specific stress-responsive genes. Summarized, our data provide an important basis for further investigation of gene functions in order to aid an improved heat tolerance and reduced losses of yield in barley as a model for cereal crops.

Diverse roles and mechanisms of gene regulation by the Arabidopsis seed maturation master regulator FUS3 revealed by microarray analysis

The FUSCA3 (FUS3) transcription factor is considered a master regulator of seed maturation because a wide range of seed maturation events are impaired in its defective mutant. To identify comprehensively genes under the control of FUS3, two types of microarray experiments were performed. First, transgenic plants in which FUS3 expression could be induced by the application of estrogen (ESTR) were used to identify any genes up-regulated in young seedlings of Arabidopsis in response to the ectopic expression of FUS3. Secondly, the transcriptomes of the fus3 mutant and wild-type developing seeds were compared. The combined results of these experiments identified genes under the relatively immediate and robust control of FUS3 during seed development. The analysis has extended the range of identified gene types under the control of FUS3. The genes positively controlled by FUS3 are not confined to previously known seed maturation-related genes and include those involved in the production of secondary metabolites, such as glucosinolates, phenylpropanoids and flavonoids, and those involved in primary metabolism, such as photosynthesis and fatty acid biosynthesis. Furthermore, several different patterns were identified in the manner of ectopic activation by FUS3 with respect to the induction kinetics and ABA requirement of downstream gene induction depending on the nature of developmental regulation, suggesting mechanistic diversity of gene regulation by FUS3.

Quantitative trait loci associated with longevity of lettuce seeds under conventional and controlled deterioration storage conditions

Lettuce (Lactuca sativa L.) seeds have poor shelf life and exhibit thermoinhibition (fail to germinate) above ∼25°C. Seed priming (controlled hydration followed by drying) alleviates thermoinhibition by increasing the maximum germination temperature, but reduces lettuce seed longevity. Controlled deterioration (CD) or accelerated ageing storage conditions (i.e. elevated temperature and relative humidity) are used to study seed longevity and to predict potential seed lifetimes under conventional storage conditions. Seeds produced in 2002 and 2006 of a recombinant inbred line (RIL) population derived from a cross between L. sativa cv. Salinas×L. serriola accession UC96US23 were utilized to identify quantitative trait loci (QTLs) associated with seed longevity under CD and conventional storage conditions. Multiple longevity-associated QTLs were identified under both conventional and CD storage conditions for control (non-primed) and primed seeds. However, seed longevity was poorly correlated between the two storage conditions, suggesting that deterioration processes under CD conditions are not predictive of ageing in conventional storage conditions. Additionally, the same QTLs were not identified when RIL populations were grown in different years, indicating that lettuce seed longevity is strongly affected by production environment. Nonetheless, a major QTL on chromosome 4 [Seed longevity 4.1 (Slg4.1)] was responsible for almost 23% of the phenotypic variation in viability of the conventionally stored control seeds of the 2006 RIL population, with improved longevity conferred by the Salinas allele. QTL analyses may enable identification of mechanisms responsible for the sensitivity of primed seeds to CD conditions and breeding for improved seed longevity.

Substrates of the Arabidopsis thaliana protein isoaspartyl methyltransferase 1 identified using phage display and biopanning

The role of protein isoaspartyl methyltransferase (PIMT) in repairing a wide assortment of damaged proteins in a host of organisms has been inferred from the affinity of the enzyme for isoaspartyl residues in a plethora of amino acid contexts. The identification of PIMT target proteins in plant seeds, where the enzyme is highly active and proteome long-lived, has been hindered by large amounts of isoaspartate-containing storage proteins. Mature seed phage display libraries circumvented this problem. Inclusion of the PIMT co-substrate, S-adenosylmethionine (AdoMet), during panning permitted PIMT to retain aged phage in greater numbers than controls lacking co-substrate or when PIMT protein binding was poisoned with S-adenosyl homocysteine. After four rounds, phage titer plateaued in AdoMet-containing pans, whereas titer declined in both controls. This strategy identified 17 in-frame PIMT target proteins, including a cupin-family protein similar to those identified previously using on-blot methylation. All recovered phage had at least one susceptible Asp or Asn residue. Five targets were recovered independently. Two in-frame targets were produced in Escherichia coli as recombinant proteins and shown by on-blot methylation to acquire isoAsp, becoming a PIMT target. Both gained isoAsp rapidly in solution upon thermal insult. Mutant analysis of plants deficient in any of three in-frame PIMT targets resulted in demonstrable phenotypes. An over-representation of clones encoding proteins involved in protein production suggests that the translational apparatus comprises a subgroup for which PIMT-mediated repair is vital for orthodox seed longevity. Impaired PIMT activity would hinder protein function in these targets, possibly resulting in poor seed performance.

Gossypium arboreum GHSP26 enhances drought tolerance in Gossypium hirsutum

Heat-shock proteins (HSP) are molecular chaperones for protein molecules. These proteins play an important role in protein-protein interactions such as, folding and assisting in the establishment of proper protein conformation and prevention of unwanted protein aggregation. A small HSP gene GHSP26 present in Gossypium arboreum responds to dehydration. In the present study, an attempt was made to overcome the problem of drought stress in cotton. A cDNA of GHSP26 was isolated from G. arboreum, cloned in plant expression vector, pCAMBIA-1301 driven by the cauliflower mosaic virus 35S promoter and introduced into Gossypium hirsutum. The integration and expression studies of putative transgenic plants were performed through GUS assay; PCR from genomic DNA, and quantitative real-time PCR analysis. Transgenic cotton plants showed an enhanced drought tolerance, suggesting that GHSP26 may play a role in plant responsiveness to drought.