Purification and in vitro chaperone activity of a class I small heat-shock protein abundant in recalcitrant chestnut seeds

Amino acid composition and nutritional value evaluation of Chinese chestnut (Castanea mollissima Blume) and its protein subunit

The amino acid composition, nutritional value and proteins subunit of chestnuts (Castanea mollissima Blume) from three regions of China (Henan, Hunan, and Guangdong) were investigated. Experimental results showed that the albumin fraction dominated the chestnut protein composition, but globulin and prolamin were nondetectable. All the Chinese chestnut proteins had a nutritionally balanced amino acid composition. Their amino acid score (AAS) could reach the FAO/WHO (2013) requirement for essential amino acids for older children, adolescents and adults. Leucine was the first limiting amino acid for all the Chinese chestnut protein isolates and digestible indispensable amino acid scores (DIAAS) were more than 90 (Leu). The sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) spectra showed that different Chinese chestnut protein isolates had almost the same band components, which were mainly comprised of seven polypeptide segments with the molecular weights of (91–93) kDa, (70–72) kDa, (53–55) kDa, 37 kDa, (27–33) kDa, 20 kDa and (5–15) kDa. The amino acid compositions and the abundance of low molecular weight proteins indicated that Chinese chestnut could be utilized as a good source of plant protein for human nutrition.

Chinese chestnut (DIAAS > 90%) with low molecular weight subunits is a good source of plant protein for human nutrition.

The CsHSP17.2 molecular chaperone is essential for thermotolerance in Camellia sinensis

Small heat shock proteins (sHSPs) play important roles in responses to heat stress. However, the functions of sHSPs in tea plants (Camellia sinensis) remain uncharacterized. A novel sHSP gene, designated CsHSP17.2, was isolated from tea plants. Subcellular localization analyses indicated that the CsHSP17.2 protein was present in the cytosol and the nucleus. CsHSP17.2 expression was significantly up-regulated by heat stress but was unaffected by low temperature. The CsHSP17.2 transcript levels increased following salt and polyethylene glycol 6000 treatments but decreased in the presence of abscisic acid. The molecular chaperone activity of CsHSP17.2 was demonstrated in vitro. Transgenic Escherichia coli and Pichia pastoris expressing CsHSP17.2 exhibited enhanced thermotolerance. The transgenic Arabidopsis thaliana exhibited higher maximum photochemical efficiencies, greater soluble protein proline contents, higher germination rates and higher hypocotyl elongation length than the wild-type controls. The expression levels of several HS-responsive genes increased in transgenic A. thaliana plants. Additionally, the CsHSP17.2 promoter is highly responsive to high-temperature stress in A. thaliana. Our results suggest that CsHSP17.2 may act as a molecular chaperone to mediate heat tolerance by maintaining maximum photochemical efficiency and protein synthesis, enhancing the scavenging of reactive oxygen species and inducing the expression of HS-responsive genes.

Physiological aspects of seed recalcitrance: a case study on the tree Aesculus hippocastanum

Recalcitrant seeds are typical of some tropical and subtropical trees. Their post-shedding life activity proceeds in humid air and wet litter. They are desiccation sensitive and, for this reason, have a short life span and need some special procedures for cryopreservation. This review is devoted to the post-shedding life strategy of recalcitrant seeds, which includes the maintenance of high hydration status, metabolic readiness and ability to rapidly germinate before desiccation-induced damage exerts a lethal effect. The main physiological aspects of recalcitrant seeds are considered starting from mature seeds, followed during dormancy if occurs and resulting in germination. The collected data embrace the metabolic processes in embryonic axes and whole seeds. The up-to-date results are integrated covering the main metabolic processes, namely water status and transport, protein and carbohydrate metabolism, antioxidant defense, axis-cotyledon relations, hormonal control and germination. Among the representatives of various taxa, the seeds of which exhibit recalcitrance, attention was given to horse chestnut seeds as one of most studied recalcitrants.

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.

Plantation forestry under global warming: hybrid poplars with improved thermotolerance provide new insights on the in vivo function of small heat shock protein chaperones

Climate-driven heat stress is a key factor affecting forest plantation yields. While its effects are expected to worsen during this century, breeding more tolerant genotypes has proven elusive. We report here a substantial and durable increase in the thermotolerance of hybrid poplar (Populus tremula×Populus alba) through overexpression of a major small heat shock protein (sHSP) with convenient features. Experimental evidence was obtained linking protective effects in the transgenic events with the unique chaperone activity of sHSPs. In addition, significant positive correlations were observed between phenotype strength and heterologous sHSP accumulation. The remarkable baseline levels of transgene product (up to 1.8% of total leaf protein) have not been reported in analogous studies with herbaceous species. As judged by protein analyses, such an accumulation is not matched either by endogenous sHSPs in both heat-stressed poplar plants and field-grown adult trees. Quantitative real time-polymerase chain reaction analyses supported these observations and allowed us to identify the poplar members most responsive to heat stress. Interestingly, sHSP overaccumulation was not associated with pleiotropic effects that might decrease yields. The poplar lines developed here also outperformed controls under in vitro and ex vitro culture conditions (callus biomass, shoot production, and ex vitro survival), even in the absence of thermal stress. These results reinforce the feasibility of improving valuable genotypes for plantation forestry, a field where in vitro recalcitrance, long breeding cycles, and other practical factors constrain conventional genetic approaches. They also provide new insights into the biological functions of the least understood family of heat shock protein chaperones.

Proteomic analysis of date palm (Phoenix dactylifera L.) responses to endophytic colonization by entomopathogenic fungi

The entomopathogenic fungi Beauveria bassiana, Lecanicillium dimorphum and L. cf. psalliotae can survive and colonize living palm tissue as endophytes. The molecular interaction between these biocontrol agent fungi and the date palm Phoenix dactylifera L. was investigated using proteomic techniques. Field date palms inoculated with these fungi were analyzed 15 and 30 days after inoculation in two independent bioassays. In vitro date palms were also inoculated with B. bassiana or L. cf. psalliotae. Qualitative and quantitative differences in protein accumulation between controls (not inoculated) and inoculated palms were found using 2-DE analysis, and some of these responsive proteins could be identified using MALDI/TOF-TOF. Proteins involved in plant defence or stress response were induced in P. dactylifera leaves as a response to endophytic colonization by entomopathogenic fungi in field date palms. Proteins related with photosynthesis and energy metabolism were also affected by entomopathogenic fungi colonization. A myosin heavy chain-like protein was accumulated in in vitro palms inoculated with these fungi. This suggests that endophytic colonization by these entomopathogenic fungi modulates plant defence responses and energy metabolism in field date palms and possibly modulates the expression of cell division-related proteins in in vitro palms at proteomic level.

The molecular basis for stress-induced acquisition of somatic embryogenesis

Somatic embryogenesis (SE) has been studied as a model system for understanding of molecular events in the physiology, biochemistry, and biology areas occurring during plant embryo development. Stresses are also the factors that have been increasingly recognized as having important role in the induction of SE. Plant growth regulators such as 2,4-dichlorophenoxyacetic acid (2,4-D), ABA, ethylene, and high concentrations of 2,4-D are known as stress-related substances for acquisition of embryogenic competence by plant cells. Gene expression analysis in both the proteome and transcriptome levels have led to the identification and characterization of some stress-related genes and proteins associated with SE. This review focuses on the molecular basis for stress-induced acquisition of SE.

Non-redundant functions of sHSP-CIs in acquired thermotolerance and their role in early seed development in Arabidopsis

Heat-shock proteins (HSPs) are a group of evolutionarily conserved polypeptides whose expression is induced in all organisms in response to environmental stresses and during various developmental processes. In this work, we show that the rose (Rosa hybrida) cytoplasmic 17.5-kDa Class I small HSP (sHSP17.5-CI, accession number: BQ103946) increases dramatically during flower development, and accumulates in closed bud petals and leaves only in response to heat stress. mRNA for a putative ortholog of this protein is also found in petals, but not leaves, of Arabidopsis (Arabidopsis thaliana) plants grown under optimal conditions, and it accumulates in leaves in response to heat stress. Analysis of Arabidopsis T-DNA insertion lines affected at three homologous genes revealed that their acquired thermotolerance, as measured by hypocotyl-elongation assay, is impaired. The correlation between sHSP-CI accumulation and expansion of rose petal cells, impairment of acquired thermotolerance, and defects in early embryogenesis of the double mutants (hsp17.4/hsp17.6A), all suggest that sHSP-CI proteins play a role in protecting cell proteins at various developmental stages, whereas in hypocotyl elongation they have a non-redundant function in acquired thermotolerance but have a redundant function in early embryogenesis.

Molecular chaperone activity of tomato (Lycopersicon esculentum) endoplasmic reticulum-located small heat shock protein

The gene encoding the small heat shock protein (sHSP), LeHSP21.5, has been previously cloned from tomato (GenBank accession no. AB026983). The deduced amino acid sequence of this tomato sHSP was most similar to that of other endoplasmic reticulum (ER)-localized sHSPs (ER-sHSP) and can be predicted to target the ER. We examined whether the gene product of LeHSP21.5 (probable ER-sHSP) can act as molecular chaperone. For functional analysis, LeHSP21.5 protein was expressed in Escherichia coli as His(6)-tagged protein in the C-terminal and purified. We confirmed that ER-sHSP could provide thermal protection of soluble proteins in vitro. We compared the thermal stability of E. coli strain BL21 (DE3) transformed with pET-ER-sHSP with the control E. coli strain BL21(DE3) transformed with only the pET vector under heat shock and IPTG-induced conditions. Most of the protein extracts from E. coli cells expressing ER-sHSP were protected from heat-induced denaturation, whereas extracts from cells not expressing ER-sHSP were very heat-sensitive under these conditions. A similar protective effect was observed when purified ER-sHSP was added to an E. coli cell extract. ER-sHSP prevented the thermal aggregation and inactivation of citrate synthase. These collective findings indicate that ER-sHSP can function as a molecular chaperone in vitro.

From Avicennia to Zizania: seed recalcitrance in perspective

BACKGROUND

Considered only in terms of tolerance of, or sensitivity to, desiccation (which is an oversimplification), orthodox seeds are those which tolerate dehydration and are storable in this condition, while highly recalcitrant seeds are damaged by loss of only a small proportion of water and are unstorable for practical purposes. Between these extremes, however, there may be a gradation of the responses to dehydration--and also to other factors--suggesting perhaps that seed behaviour might be best considered as constituting a continuum subtended by extreme orthodoxy and the highest degree of recalcitrance. As the characteristics of seeds of an increasing number of species are elucidated, non-orthodox seed behaviour is emerging as considerably more commonplace--and its basis far more complex--than previously suspected.

SCOPE

Whatever the post-harvest responses of seeds of individual species may be, they are the outcome of the properties of pre-shedding development, and a full understanding of the subtleties of various degrees of non-orthodox behaviour must await the identification of, and interaction among, all the factors conferring extreme orthodoxy. Appreciation of the phenomenon of recalcitrance is confounded by intra- and interseasonal variability across species, as well as within individual species. However, recent evidence suggests that provenance is a pivotal factor in determining the degree of recalcitrant behaviour exhibited by seeds of individual species. Non-orthodox--and, in particular, recalcitrant--seed behaviour is not merely a matter of desiccation sensitivity: the primary basis is that the seeds are actively metabolic when they are shed, in contrast to orthodox types which are quiescent. This affects all aspects of the handling and storage of recalcitrant seeds. In the short to medium term, recalcitrant seeds should be stored in as hydrated a condition as when they are shed, and at the lowest temperature not diminishing vigour or viability. Such hydrated storage has attendant problems of fungal proliferation which, unless minimized, will inevitably and significantly affect seed quality. The life span of seeds in hydrated storage even under the best conditions is variable among species, but is curtailed (days to months), and various approaches attempting to extend non-orthodox seed longevity are discussed. Conservation of the genetic resources by means other than seed storage is then briefly considered, with detail on the potential for, and difficulties with, cryostorage highlighted.

CONCLUSIONS

There appears to be little taxonomic relationship among species exhibiting the phenomenon of seed recalcitrance, suggesting that it is a derived trait, with tolerance having been lost a number of times. Although recalcitrant seededness is best represented in the mesic tropics, particularly among rainforest climax species, it does occur in cooler, drier and markedly seasonal habitats. The selective advantages of the trait are considered.

Protein cryoprotective activity of a cytosolic small heat shock protein that accumulates constitutively in chestnut stems and is up-regulated by low and high temperatures

Heat shock, and other stresses that cause protein misfolding and aggregation, trigger the accumulation of heat shock proteins (HSPs) in virtually all organisms. Among the HSPs of higher plants, those belonging to the small HSP (sHSP) family remain the least characterized in functional terms. We analyzed the occurrence of sHSPs in vegetative organs of Castanea sativa (sweet chestnut), a temperate woody species that exhibits remarkable freezing tolerance. A constitutive sHSP subject to seasonal periodic changes of abundance was immunodetected in stems. This protein was identified by matrix-assisted laser-desorption ionization time of flight mass spectrometry and internal peptide sequencing as CsHSP17.5, a cytosolic class I sHSP previously described in cotyledons. Expression of the corresponding gene in stems was confirmed through cDNA cloning and reverse transcription-PCR. Stem protein and mRNA profiles indicated that CsHSP17.5 is significantly up-regulated in spring and fall, reaching maximal levels in late summer and, especially, in winter. In addition, cold exposure was found to quickly activate shsp gene expression in both stems and roots of chestnut seedlings kept in growth chambers. Our main finding is that purified CsHSP17.5 is very effective in protecting the cold-labile enzyme lactate dehydrogenase from freeze-induced inactivation (on a molar basis, CsHSP17.5 is about 400 times more effective as cryoprotectant than hen egg-white lysozyme). Consistent with these observations, repeated freezing/thawing did not affect appreciably the chaperone activity of diluted CsHSP17.5 nor its ability to form dodecameric complexes in vitro. Taken together, these results substantiate the hypothesis that sHSPs can play relevant roles in the acquisition of freezing tolerance.

Mitochondrial small heat-shock protein enhances thermotolerance in tobacco plants

To clarify the role of mitochondrial small heat-shock protein (MT-sHSP) in the heat-shock response, we introduced the tomato (Lycopersicon esculentum) MT-sHSP gene under the control of the 35S promoter into tobacco (Nicotiana tabacum), and examined the thermotolerance of the transformed plants. Irrespective of the orientation, sense or antisense, of the gene, the transgenic plants exhibited a normal morphology and growth rate in the vegetative growth stage. When 4-week-old seedlings were exposed to sudden heat stress, the sense plants which overexpress the MT-sHSP gene exhibited thermotolerance, whereas the antisense plants in which the expression of the gene is suppressed exhibited susceptibility.

Plant responses to drought, salinity and extreme temperatures: towards genetic engineering for stress tolerance

Abiotic stresses, such as drought, salinity, extreme temperatures, chemical toxicity and oxidative stress are serious threats to agriculture and the natural status of the environment. Increased salinization of arable land is expected to have devastating global effects, resulting in 30% land loss within the next 25 years, and up to 50% by the year 2050. Therefore, breeding for drought and salinity stress tolerance in crop plants (for food supply) and in forest trees (a central component of the global ecosystem) should be given high research priority in plant biotechnology programs. Molecular control mechanisms for abiotic stress tolerance are based on the activation and regulation of specific stress-related genes. These genes are involved in the whole sequence of stress responses, such as signaling, transcriptional control, protection of membranes and proteins, and free-radical and toxic-compound scavenging. Recently, research into the molecular mechanisms of stress responses has started to bear fruit and, in parallel, genetic modification of stress tolerance has also shown promising results that may ultimately apply to agriculturally and ecologically important plants. The present review summarizes the recent advances in elucidating stress-response mechanisms and their biotechnological applications. Emphasis is placed on transgenic plants that have been engineered based on different stress-response mechanisms. The review examines the following aspects: regulatory controls, metabolite engineering, ion transport, antioxidants and detoxification, late embryogenesis abundant (LEA) and heat-shock proteins.

A 10-kDa class-CI sHsp protects E. coli from oxidative and high-temperature stress

We report on a new cDNA clone (Qshsp10.4-CI) of a Quercus suber L. class-CI small heat-shock protein (sHsp) obtained from cork (phellem), a highly oxidatively stressed plant tissue. The deduced gene product lacks the C-terminal extension and the consensus I region of the alpha-crystallin domain, being the most C-terminally truncated sHsp reported to date. In an attempt to prove that a protective function is possible for such a truncated sHsp, we overexpressed in Escherichia coli three recombinant sHsp-CIs, one (rQsHsp10.4-CI) showing the same truncation as Qshsp10.4-CI, a second (rN49) lacking the whole alpha-crystallin domain, and a third (rN153) consisting of a full-length sHsp-CI. The overexpression of rN153 and, remarkably, rQsHsp10.4-CI but not rN49 enhanced cell viability under high temperature and, interestingly, under oxidative stress. These results show that the C-terminal extension and the consensus I region of the alpha-crystallin domain are dispensable, but amino acids 1-41 of the alpha-crystallin domain (including the consensus II region) are essential for the protective activity of sHsp-CIs. On the other hand, two-dimensional immunodetection patterns showed accumulation of ca. 10-kDa sHsp-CI immunorelated polypeptides in cork and other oxidatively stressed tissues but not in control and heat-stressed tissues. We discuss the possible role of highly truncated sHsps in relation to oxidative stress.

Cryptobiosis--a peculiar state of biological organization

David Keilin (Proc. Roy. Soc. Lond. B, 150, 1959, 149-191) coined the term 'cryptobiosis' (hidden life) and defined it as 'the state of an organism when it shows no visible signs of life and when its metabolic activity becomes hardly measurable, or comes reversibly to a standstill.' I consider selected aspects of the 300 year history of research on this unusual state of biological organization. Cryptobiosis is peculiar in the sense that organisms capable of achieving it exhibit characteristics that differ dramatically from those of living ones, yet they are not dead either, so one may propose that cryptobiosis is a unique state of biological organization. I focus chiefly on animal anhydrobiosis, achieved by the reversible loss of almost all the organism's water. The adaptive biochemical and biophysical mechanisms allowing this to take place involve the participation of large concentrations of polyhydroxy compounds, chiefly the disaccharides trehalose or sucrose. Stress (heat shock) proteins might also be involved, although the details are poorly understood and seem to be organism-specific. Whether the removal of molecular oxygen (anoxybiosis) results in the reversible cessation of metabolism in adapted organisms is considered, with the result being 'yes and no', depending on how one defines metabolism. Basic research on cryptobiosis has resulted in unpredicted applications that are of substantial benefit to the human condition and a few of these are described briefly.

Developmental regulation of a gene coding for a low-molecular-weight heat shock protein during haustorium formation in the seedlings of a holoparasitic plant, Cuscuta japonica

Dodder (Cuscuta japonica), a holoparasitic angiosperm, develops haustoria that are essential for parasitism. We have previously demonstrated that in Cuscuta seedlings, haustorial formation could be induced efficiently by cooperative effects of far-red light and tactile stimuli in the absence of any host plant [Tada et al. (1996) Plant Cell Physiol. 37: 1049]. In this study, we performed differential display and isolated several cDNAs that were expressed differentially during haustorium development in the seedlings. Sequence similarities identified one of them as a gene encoding a 17-kDa low-molecular-weight heat shock protein (CJHSP17). Northern blot analysis revealed that CJHSP17 mRNAs constitutively accumulated in the seedlings in the absence of environmental stress, and that the transcripts dramatically decreased to undetectable levels prior to emergence of haustoria upon irradiation with far-red light in the presence of tactile stimuli. When treated with either of the two stimuli, the CJHSP17 transcript levels did not decrease and there was no differentiation of haustoria. Moreover, irradiation of red light immediately after far-red light completely repressed both the decrease of mRNAs and the subsequent formation of haustoria. These observations suggest the involvement of down-regulation of CJHSP17 in haustorium development in Cuscuta seedlings.

ATP and the core "alpha-Crystallin" domain of the small heat-shock protein alphaB-crystallin

Electrospray ionization mass spectrometry (ESI-LC/MS) of tryptic digests of human alphaB-crystallin in the presence and absence of ATP identified four residues located within the core "alpha-crystallin" domain, Lys(82), Lys(103), Arg(116), and Arg(123), that were shielded from the action of trypsin in the presence of ATP. In control experiments, chymotrypsin was used in place of trypsin. The chymotryptic fragments of human alphaB-crystallin produced in the presence and absence of ATP were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Seven chymotryptic cleavage sites, Trp(60), Phe(61), Phe(75), Phe(84), Phe(113), Phe(118), and Tyr(122), located near or within the core alpha-crystallin domain, were shielded from the action of chymotrypsin in the presence of ATP. Chemically similar analogs of ATP were less protective than ATP against proteolysis by trypsin or chymotrypsin. ATP had no effect on the enzymatic activity of trypsin and the K(m) for trypsin was 0.031 mM in the presence of ATP and 0.029 mM in the absence of ATP. The results demonstrated an ATP-dependent structural modification in the core alpha-crystallin domain conserved in nearly all identified small heat-shock proteins that act as molecular chaperones.

Heterologous expression of a plant small heat-shock protein enhances Escherichia coli viability under heat and cold stress

A small heat-shock protein (sHSP) that shows molecular chaperone activity in vitro was recently purified from mature chestnut (Castanea sativa) cotyledons. This protein, renamed here as CsHSP17. 5, belongs to cytosolic class I, as revealed by cDNA sequencing and immunoelectron microscopy. Recombinant CsHSP17.5 was overexpressed in Escherichia coli to study its possible function under stress conditions. Upon transfer from 37 degrees C to 50 degrees C, a temperature known to cause cell autolysis, those cells that accumulated CsHSP17.5 showed improved viability compared with control cultures. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of cell lysates suggested that such a protective effect in vivo is due to the ability of recombinant sHSP to maintain soluble cytosolic proteins in their native conformation, with little substrate specificity. To test the recent hypothesis that sHSPs may be involved in protection against cold stress, we also studied the viability of recombinant cells at 4 degrees C. Unlike the major heat-induced chaperone, GroEL/ES, the chestnut sHSP significantly enhanced cell survivability at this temperature. CsHSP17.5 thus represents an example of a HSP capable of protecting cells against both thermal extremes. Consistent with these findings, high-level induction of homologous transcripts was observed in vegetative tissues of chestnut plantlets exposed to either type of thermal stress but not salt stress.