Plant dehydrins and stress tolerance: Versatile proteins for complex mechanisms

Dehydrins (DHNs), or group 2 LEA (Late Embryogenesis Abundant) proteins, play a fundamental role in plant response and adaptation to abiotic stresses. They accumulate typically in maturing seeds or are induced in vegetative tissues following salinity, dehydration, cold and freezing stress. The generally accepted classification of dehydrins is based on their structural features, such as the presence of conserved sequences, designated as Y, S and K segments. The K segment representing a highly conserved 15 amino acid motif forming amphiphilic a-helix is especially important since it has been found in all dehydrins. Since more than 20 y, they are thought to play an important protective role during cellular dehydration but their precise function remains unclear. This review outlines the current status of the progress made toward the structural, physico-chemical and functional characterization of plant dehydrins and how these features could be exploited in improving stress tolerance in plants.Key words: abiotic stress, dehydration stress, drought, cold acclimation, freezing tolerance, LEA proteins, dehydrins     

Expression of the barley dehydrin multigene family and the development of freezing tolerance

Dehydrins (DHNs; LEA D11) are one of the typical families of plant proteins that accumulate in response to dehydration, low temperature, osmotic stress or treatment with abscisic acid (ABA), or during seed maturation. We previously found that three genes encoding low-molecular-weight DHNs (Dhn1, Dhn2 and Dhn9) map within a 15-cM region of barley chromosome 5H that overlaps a QTL for winterhardiness, while other Dhn genes encoding low- and high-molecular-weight DHNs are located on chromosomes 3H, 4H and 6H. Here we examine the expression of specific Dhn genes under conditions associated with expression of the winterhardiness phenotype. Plants grown at 4 degrees C or in the field in Riverside, California developed similar, modest levels of freezing tolerance, coinciding with little low-MW Dhn gene activity. Dicktoo (the more tolerant cultivar) and Morex (the less tolerant) grown in Saskatoon, Canada expressed higher levels of expression of genes for low-MW DHNs than did the same cultivars in Riverside, with expression being higher in Dicktoo than Morex. Dehydration or freeze-thaw also evoked expression of genes for low MW DHNs, suggesting that the dehydration component of freeze-thaw in the field induces low expression of genes encoding low-MW DHNs. These observations are consistent with the hypothesis that the major chilling-induced DHNs help to prime plant cells for acclimation to more intense cold, which then involves adaptation to dehydration during freeze-thaw cycling. A role for chromosome 5H-encoded DHNs in acclimation to more intense cold seems possible, even though it is not the basis of the major heritable variation in winterhardiness within the Dicktoo x Morex population.     

Abiotic stress response in the moss <Emphasis Type="Italic">Physcomitrella patens</Emphasis>: evidence for an evolutionary alteration in signaling pathways in land plants

The mechanisms plants use to adapt to abiotic stress have been widely studied in a number of seed plants. Major research has been focused on the isolation of stress-responsive genes as a means to understand the molecular events underlying the adaptation process. To study stress-related gene regulation in the moss Physcomitrella patens we have isolated two cDNAs showing homology to highly conserved small hydrophobic proteins from different seed plants. The corresponding genes are up-regulated by dehydration, salt, sorbitol, cold and the hormone abscisic acid, indicating overlapping pathways are involved in the control of these genes. Based on the molecular characterization of the moss homologs we propose that signaling pathways in response to abiotic stress may have been altered during the evolution of land plants.Abbreviation ABA Abscisic acid - EST Expressed sequence tag     

CYP707A3, a major ABA 8'-hydroxylase involved in dehydration and rehydration response in Arabidopsis thaliana

Abscisic acid (ABA) catabolism is one of the determinants of endogenous ABA levels affecting numerous aspects of plant growth and abiotic stress responses. The major ABA catabolic pathway is triggered by ABA 8'-hydroxylation catalysed by the cytochrome P450 CYP707A family. Among four members of Arabidopsis CYP707As, the expression of CYP707A3 was most highly induced in response to both dehydration and subsequent rehydration. A T-DNA insertional cyp707a3-1 mutant contained higher ABA levels in turgid plants, which showed a reduced transpiration rate and hypersensitivity to exogenous ABA during early seedling growth. On dehydration, the cyp707a3-1 mutant accumulated a higher amount of stress-induced ABA than the wild type, an event that occurred relatively later and was coincident with slow drought induction of CYP707A3. The cyp707a3 mutant plants exhibited both exaggerated ABA-inducible gene expression and enhanced drought tolerance. Conversely, constitutive expression of CYP707A3 relieved growth retardation by ABA, increased transpiration, and a reduction of endogenous ABA in both turgid and dehydrated plants. Taken together, our results indicate that CYP707A3 plays an important role in determining threshold levels of ABA during dehydration and after rehydration.     

CBL1, a calcium sensor that differentially regulates salt,drought, and cold responses in Arabidopsis

Although calcium is a critical component in the signal transduction pathways that lead to stress gene expression in higher plants, little is known about the molecular mechanism underlying calcium function. It is believed that cellular calcium changes are perceived by sensor molecules, including calcium binding proteins. The calcineurin B-like (CBL) protein family represents a unique group of calcium sensors in plants. A member of the family, CBL1, is highly inducible by multiple stress signals, implicating CBL1 in stress response pathways. When the CBL1 protein level was increased in transgenic Arabidopsis plants, it altered the stress response pathways in these plants. Although drought-induced gene expression was enhanced, gene induction by cold was inhibited. In addition, CBL1-overexpressing plants showed enhanced tolerance to salt and drought but reduced tolerance to freezing. By contrast, cbl1 null mutant plants showed enhanced cold induction and reduced drought induction of stress genes. The mutant plants displayed less tolerance to salt and drought but enhanced tolerance to freezing. These studies suggest that CBL1 functions as a positive regulator of salt and drought responses and a negative regulator of cold response in plants.     

Constitutive expression of CaSRP1, a hot pepper small rubber particle protein homolog, resulted in fast growth and improved drought tolerance in transgenic Arabidopsis plants

Transient and long-term shortages of fresh water are major adverse environmental factors that cause dramatic reductions in crop production and distribution globally. In this study, we isolated a full-length CaSRP1 (Capsicum annuum stress-related protein 1) cDNA, which was rapidly induced by dehydration in hot pepper plants. The predicted CaSRP1 protein sequence exhibited significant amino acid identity to putative stress-related proteins and the small rubber particle protein (SRPP) found in rubber trees (Hevea brasiliensis). To study the cellular functions of CaSRP1, transgenic Arabidopsis plants (35S:CaSRP1) that constitutively expressed the CaSRP1 gene were constructed. Overexpression of CaSRP1 resulted in enhanced root and shoot growth and earlier bolting in the transgenic plants relative to wild-type plants. In addition, 35S:CaSRP1 overexpressors exhibited enhanced tolerance to drought stress as compared to the control plants. These results suggest that CaSRP1 plays dual functions as a positive factor for tissue growth and development and for drought-defensive responses. A possible cellular function of SRPP homologs in non-rubber-producing plants in relation to drought stress tolerance is discussed.     

低温诱导蛋白及其与植物的耐寒性研究进展

低温诱导蛋白是植物在温度逆境条件下诱导产生的一系列蛋白,以抗冻蛋白、脱水蛋白、热激蛋白和热稳定蛋白较多,而且低温诱导蛋白质一旦在体内形成,植物体就会尽快地适应外界环境,表现出较强的抗逆性.本文对几种主要的低温诱导蛋白——抗冻蛋白、脱水蛋白、热激蛋白和热稳定蛋白的特性及其与植物耐寒性的关系研究进行综述,以期为进一步阐明植物耐寒的分子机制以及提高植物耐寒力研究提供新的思路.     

A poplar DRE-binding protein gene, PeDREB2L, is involved in regulation of defense response against abiotic stress

Dehydration responsive element (DRE)-binding proteins comprise a family of proteins that have important roles in abiotic stress processes. The present study aimed at elucidating the molecular basis of the stress tolerance mechanism in poplar. We isolated a gene, PeDREB2L, from the desert tree Populus euphratica Oliva. PeDREB2L contains a conserved AP2/ERF domain, a nuclear localization signal at the N-terminus and a serine-rich region at the C-terminus. PeDREB2L binds specifically to DRE elements and is expressed ubiquitously in all tissues, including roots, stems, and leaves. Expression of PeDREB2L in leaves increased under dehydration, salt, and abscisic acid stress treatments. Furthermore, PeDREB2L-GFP ectopically expressed in Arabidopsis gave strong green fluorescent protein signals in the nucleus of transgenic Arabidopsis under unstressed conditions and resulted in an improved tolerance against drought and freezing under stress conditions, while the Arabidopsis protein gene DREB2A-GFP used as a control only gave a weak fluorescent. We expected that the PeDREB2L might be useful in improving abiotic stress tolerance in transgenic plants.     

Transgenic banana plants overexpressing a native plasma membrane aquaporin MusaPIP1;2 display high tolerance levels to different abiotic stresses

Water transport across cellular membranes is regulated by a family of water channel proteins known as aquaporins (AQPs). As most abiotic stresses like suboptimal temperatures, drought or salinity result in cellular dehydration, it is imperative to study the cause–effect relationship between AQPs and the cellular consequences of abiotic stress stimuli. Although plant cells have a high isoform diversity of AQPs, the individual and integrated roles of individual AQPs in optimal and suboptimal physiological conditions remain unclear. Herein, we have identified a plasma membrane intrinsic protein gene (MusaPIP1;2) from banana and characterized it by overexpression in transgenic banana plants. Cellular localization assay performed using MusaPIP1;2::GFP fusion protein indicated that MusaPIP1;2 translocated to plasma membrane in transformed banana cells. Transgenic banana plants overexpressing MusaPIP1;2 constitutively displayed better abiotic stress survival characteristics. The transgenic lines had lower malondialdehyde levels, elevated proline and relative water content and higher photosynthetic efficiency as compared to equivalent controls under different abiotic stress conditions. Greenhouse‐maintained hardened transgenic plants showed faster recovery towards normal growth and development after cessation of abiotic stress stimuli, thereby underlining the importance of these plants in actual environmental conditions wherein the stress stimuli is often transient but severe. Further, transgenic plants where the overexpression of MusaPIP1;2 was made conditional by tagging it with a stress‐inducible native dehydrin promoter also showed similar stress tolerance characteristics in in vitro and in vivo assays. Plants developed in this study could potentially enable banana cultivation in areas where adverse environmental conditions hitherto preclude commercial banana cultivation.     

A cotton group C MAP kinase gene, GhMPK2, positively regulates salt and drought tolerance in tobacco

Mitogen-activated protein kinase (MAPK) cascades play important roles in mediating biotic and abiotic stress responses. In plants, MAPKs are classified into four major groups (A-D) according to their sequence homology and conserved phosphorylation motifs. Compared with well-studied MAPKs in groups A and B, little is known about group C. In this study, we functionally characterised a stress-responsive group C MAPK gene (GhMPK2) from cotton (Gossypium hirsutum). Northern blot analysis indicated that GhMPK2 was induced by abscisic acid (ABA) and abiotic stresses, such as NaCl, PEG, and dehydration. Subcellular localization analysis suggested that GhMPK2 may activate its specific targets in the nucleus. Constitutive overexpression of GhMPK2 in tobacco (Nicotiana tabacum) conferred reduced sensitivity to ABA during both seed germination and vegetative growth. Interestingly, transgenic plants had a decreased rate of water loss and exhibited enhanced drought and salt tolerance. Additionally, transgenic plants showed improved osmotic adjustment capacity, elevated proline accumulation and up-regulated expression of several stress-related genes, including DIN1, Osmotin and NtLEA5. β-glucuronidase (GUS) expression driven by the GhMPK2 promoter was clearly enhanced by treatment with NaCl, PEG, and ABA. These results strongly suggest that GhMPK2 positively regulates salt and drought tolerance in transgenic plants.     

XVSAP1 from Xerophyta viscosa improves osmotic-, salinity- and high-temperature-stress tolerance in Arabidopsis

XVSAP1, a gene isolated from a dehydrated Xerophyta viscosa cDNA library, was transformed into Arabidopsis thaliana by Ti plasmid-mediated transformation under the control of a cauliflower mosaic virus 35S promoter, a nos terminator and bar gene selection. Expression of XVSAP1 in Arabidopsis led to constitutive accumulation of the corresponding protein in the leaves. Transgenic Arabidopsis grown in tissue culture maintained higher growth rates during osmotic, high-salinity and high temperature stress, respectively. Non-transgenic plants had shorter roots, leaf expansion was inhibited and leaves were more chlorotic than those of the transgenic plants. This study demonstrates that XVSAP1 has a significant impact on dehydration, salinity and high-temperature stress tolerance in Arabidopsis.