Arabidopsis mutants deregulated in RCI2A expression reveal new signaling pathways in abiotic stress responses

To uncover new pathways involved in low-temperature signal transduction, we screened for mutants altered in cold-induced expression of RCI2A, an Arabidopsis gene that is not a member of the CBF/DREB1 regulon and is induced not only by low temperature but also by abscisic acid (ABA), dehydration (DH) and NaCl. This was accomplished by generating a line of Arabidopsis carrying a transgene consisting of the RCI2A promoter fused to the firefly luciferase coding sequence. A number of mutants showing low or high RCI2A expression in response to low temperature were identified. These mutants also displayed deregulated RCI2A expression in response to ABA, DH or NaCl. Interestingly, however, they were not altered in stress-induced expression of RD29A, a CBF/DREB1-target gene, suggesting that the mutations affect signaling intermediates of CBF/DREB1-independent regulatory pathways. Several mutants showed alterations in their tolerance to freezing, DH or salt stress, as well as in their ABA sensitivity, which indicates that the signaling intermediates defined by the corresponding mutations play an important role in Arabidopsis tolerance to abiotic stresses. Based on the mutants identified, we discuss the involvement of CBF/DREB1-independent pathways in modulating stress signaling.     

Glycine-rich RNA-binding protein 7 affects abiotic stress responses by regulating stomata opening and closing in Arabidopsis thaliana

Despite the fact that glycine-rich RNA-binding proteins (GRPs) have been implicated in the responses of plants to environmental stresses, their physiological functions and mechanisms of action in stress responses remain largely unknown. Here, we assessed the functional roles of GRP7, one of the eight GRP family members in Arabidopsis thaliana , on seed germination, seedling growth, and stress tolerance under high salinity, drought, or cold stress conditions. The transgenic Arabidopsis plants overexpressing GRP7 under the control of the cauliflower mosaic virus 35S promoter displayed retarded germination and poorer seedling growth compared with the wild-type plants and T-DNA insertional mutant lines under high salinity or dehydration stress conditions. By contrast, GRP7 overexpression conferred freezing tolerance in Arabidopsis plants. GRP7 is expressed abundantly in the guard cells, and has been shown to influence the opening and closing of the stomata, in accordance with the prevailing stress conditions. GRP7 is localized to both the nucleus and the cytoplasm, and is involved in the export of mRNAs from the nucleus to the cytoplasm under cold stress conditions. Collectively, these results provide compelling evidence that GRP7 affects the growth and stress tolerance of Arabidopsis plants under high salt and dehydration stress conditions, and also confers freezing tolerance, particularly via the regulation of stomatal opening and closing in the guard cells.     

Functions and interaction of plant lipid signalling under abiotic stresses

Lipids are the primary form of energy storage and a major component of plasma membranes, which form the interface between the cell and the extracellular environment. Several lipids — including phosphoinositide, phosphatidic acid, sphingolipids, lysophospholipids, oxylipins, and free fatty acids — also serve as substrates for the generation of signalling molecules. Abiotic stresses, such as drought and temperature stress, are known to affect plant growth. In addition, abiotic stresses can activate certain lipid-dependent signalling pathways that control the expression of stress-responsive genes and contribute to plant stress adaptation. Many studies have focused either on the enzymatic production and metabolism of lipids, or on the mechanisms of abiotic stress response. However, there is little information regarding the roles of plant lipids in plant responses to abiotic stress. In this review, we describe the metabolism of plant lipids and discuss their involvement in plant responses to abiotic stress. As such, this review provides crucial background for further research on the interactions between plant lipids and abiotic stress.     

Comprehensive Expression Analysis of Rice Armadillo Gene Family During Abiotic Stress and Development

Genes in the Armadillo (ARM)-repeat superfamily encode proteins with a range of developmental and physiological processes in unicellular and multicellular eukaryotes. These 42 amino acid, long tandem repeat-containing proteins have been abundantly recognized in many plant species. Previous studies have confirmed that Armadillo proteins constitute a multigene family in Arabidopsis. In this study, we performed a computational analysis in the rice genome (Oryza sativa L. subsp. japonica), and identified 158 genes of Armadillo superfamily. Phylogenetic study classified them into several arbitrary groups based on a varying number of non-conserved ARM repeats and accessory domain(s) associated with them. An in-depth analysis of gene expression through microarray and Q-PCR revealed a number of ARM proteins expressing differentially in abiotic stresses and developmental conditions, suggesting a potential roles of this superfamily in development and stress signalling. Comparative phylogenetic analysis between Arabidopsis and rice Armadillo genes revealed a high degree of evolutionary conservation between the orthologues in two plant species. The non-synonymous and synonymous substitutions per site ratios (Ka/Ks) of duplicated gene pairs indicate a purifying selection. This genome-wide identification and expression analysis provides a basis for further functional analysis of Armadillo genes under abiotic stress and reproductive developmental condition in the plant lineage.     

Glutathione peroxidase genes in Arabidopsis are ubiquitous and regulated by abiotic stresses through diverse signaling pathways

Glutathione peroxidases (GPXs) are a group of enzymes that protect cells against oxidative damage generated by reactive oxygen species (ROS). The presence of GPXs in plants has been reported by several groups, but the roles of individual members of this family in a single plant species have not been studied. A family of seven related proteins named AtGPX1- AtGPX7 in Arabidopsis was identified, and the genomic organization of this family was reported. The putative subcellular localizations of the encoded proteins are the cytosol, chloroplast, mitochondria, and endoplasmic reticulum. Expressed sequence tags (ESTs) for all the genes except AtGPX7 were identified. Expression analysis of AtGPX genes in Arabidopsis tissues was performed, and different patterns were detected. Interestingly, several genes were up-regulated coordinately in response to abiotic stresses. AtGPX6, like human phospholipid hydroperoxide GPX (PHGPX), possibly encodes mitochondrial and cytosolic isoforms by alternative initiation. In addition, this gene showed the strongest responses under most abiotic stresses tested. AtGPX6::GUS analysis in transgenic Arabidopsis showed that AtGPX6 is highly expressed throughout development in most tissues, thus supporting an important role for this gene in protection against oxidative damage. The different effects of salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and auxin on the expression of the genes indicate that the AtGPX family is regulated by multiple signaling pathways. Analysis of the upstream region of the AtGPX genes revealed the presence of multiple conserved motifs, and some of them resembled antioxidant-responsive elements found in plant and human promoters. The potential regulatory role of specific sequences is discussed.     

Strigolactones in plant adaptation to abiotic stresses: An emerging avenue of plant research

Phytohormones play central roles in boosting plant tolerance to environmental stresses, which negatively affect plant productivity and threaten future food security. Strigolactones (SLs), a class of carotenoid‐derived phytohormones, were initially discovered as an “ecological signal” for parasitic seed germination and establishment of symbiotic relationship between plants and beneficial microbes. Subsequent characterizations have described their functional roles in various developmental processes, including root development, shoot branching, reproductive development, and leaf senescence. SLs have recently drawn much attention due to their essential roles in the regulation of various physiological and molecular processes during the adaptation of plants to abiotic stresses. Reports suggest that the production of SLs in plants is strictly regulated and dependent on the type of stresses that plants confront at various stages of development. Recently, evidence for crosstalk between SLs and other phytohormones, such as abscisic acid, in responses to abiotic stresses suggests that SLs actively participate within regulatory networks of plant stress adaptation that are governed by phytohormones. Moreover, the prospective roles of SLs in the management of plant growth and development under adverse environmental conditions have been suggested. In this review, we provide a comprehensive discussion pertaining to SL‐mediated plant responses and adaptation to abiotic stresses.     

植物蛋白激酶介导的非生物胁迫和激素信号转导途径的研究进展

干旱、盐渍、低温和高温等非生物胁迫严重影响植物的生长发育和作物的产量。在长期的进化过程中,植物逐渐形成了对外部刺激快速感知和主动适应的能力,其中植物体内逆境信号的传递在植物快速感知外部刺激和主动适应非生物胁迫过程中起着非常重要的作用。蛋白激酶和蛋白磷酸酶催化的蛋白质磷酸化和去磷酸化是植物体内存在的最普遍且最重要的信号转导调节方式。其中,蛋白激酶的主要作用是将ATP或GTP上的γ磷酸基团转移到特定的底物蛋白上,使蛋白磷酸化,被磷酸化的蛋白发挥相应的生理功能。近年来,利用生物技术和基因工程等手段从细胞、分子水平上研究有关蛋白激酶的抗逆机理,通过基因沉默、基因过表达等策略提高植物的抗逆性成为国内外抗逆分子生物学与分子育种学研究的热点。本文主要对植物蛋白激酶在介导非生物胁迫和激素信号通路中的作用进行综述,为进一步研究植物蛋白激酶功能提供有价值的信息。     

A novel high-throughput genetic screen for stress-responsive mutants of Arabidopsis thaliana reveals new loci involving stress responses

Activation sequence-1 (as-1) cognate promoter elements are widespread in the promoters of plant defense-related genes as well as in plant pathogen promoters, and may play important roles in the activation of defense-related genes. The as-1-type elements are highly responsive to multiple stress stimuli such as jasmonic acid (JA), salicylic acid (SA), H(2)O(2), xenobiotics and heavy metals, and therefore provide a unique opportunity for identifying additional signaling components and cross-talk points in the various signaling networks. A single as-1-type cis-element-driven GUS reporter Arabidopsis line responsive to JA, SA, H(2)O(2), xenobiotics and heavy metals was constructed for mutagenesis. A large-scale T-DNA mutagenesis has been conducted in the reporter background, and an efficient high-throughput mutant screen was established for isolating mutants with altered responses to the stress chemicals. A number of mutants with altered stress responses were obtained, some of which appear to identify new components in the as-1-based signal transduction pathways. We characterized a mutant (Delta8L4) with a T-DNA insertion in the coding sequence of the gene At4g24275. The as-1-regulated gene expression and GUS reporter gene expression were altered in the Delta8L4 mutant, but there was no change in the expression of genes lacking as-1 elements in their promoters. The phenotype observed with the Delta8L4 mutant was further verified using RNAi plants for At4g24275 (8L4-RNAi), suggesting the feasibility of use of this high-throughput mutant screening in isolating stress-signaling mutants.     

Germination variation in Arabidopsis thaliana accessions under moderate osmotic and salt stresses

Background

Water and salt stresses are two important environmental factors that limit the germination of seeds in most ecological environments. Most studies conducted so far to address the genetic basis of the above phenomenon have used stress conditions that are much more extreme than those found in natural environments. Furthermore, although an excess of ions and water restrictions have similar osmotic effects on germination, the common and divergent signalling components mediating the effects of both factors remain unknown.

Methods

The germination of seeds was compared under solutions of NaCl (50 mm) and polyethylene glycol (PEG, −0·6 MPa), that establish mild stress conditions, in 28 Arabidopsis thaliana accessions. Because Bayreuth (Bay) and Shadara (Sha) accessions showed contrasting sensitivity responses to both stresses, a quantitative trait locus (QTL) analysis was carried out using Bay × Sha recombinant inbred lines (RILs) to identify loci involved in the control of germination under mild salt and osmotic stresses.

Key Results

Two loci associated with the salt sensitivity response, named SSR1 and SSR2 QTLs, and four loci for the osmotic sensitivity response, named OSR1–OSR4 QTLs, were mapped. The effects of the SSR1 QTL on toxic salt sensitivity, and the osmotic contribution of OSR1, were confirmed by heterogeneous inbred families (HIFs). Whilst the SSR1 QTL had a significant effect under a wide range of NaCl concentrations, the OSR1 QTL was confirmed only under moderate drought stress. Interestingly the OSR1 QTL also showed pleiotropic effects on biomass accumulation in response to water deficit.

Conclusions

The regulation of germination under moderate salt and osmotic stresses involves the action of independent major loci, revealing the existence of loci specifically associated with the toxic component of salt and not just its osmotic effect. Furthermore, this work demonstrates that novel loci control germination under osmotic stress conditions simulating more realistic ecological environments as found by populations of seeds in nature.     

Arabidopsis metacaspase 2d is a positive mediator of cell death induced during biotic and abiotic stresses

Cysteine proteases such as caspases play important roles in programmed cell death (PCD) of metazoans. Plant metacaspases (MCPs), a family of cysteine proteases structurally related to caspases, have been hypothesized to be ancestors of metazoan caspases, despite their different substrate specificity. Arabidopsis thaliana contains six type II MCP genes (AtMCP2a-f). Whether and how these individual members are involved in controlling PCD in plants remains largely unknown. Here we investigated the function and regulation of AtMCP2d, the predominant and constitutively expressed member of type II MCPs, in stress-inducible PCD. Two AtMCP2d mutants (mcp2d-1 and mcp2d-3) exhibited reduced sensitivity to PCD-inducing mycotoxin fumonisin B1 as well as oxidative stress inducers, whereas AtMCP2d over-expressors were more sensitive to these agents, and exhibited accelerated cell-death progression. We found that AtMCP2d exclusively localizes to the cytosol, and its accumulation and self-processing patterns were age-dependent in leaves. Importantly, active proteolytic processing of AtMCP2d proteins dependent on its catalytic activity was observed in mature leaves during mycotoxin-induced cell death. We also found that mcp2d-1 leaves exhibited reduced cell death in response to Pseudomonas syringae carrying avirulent gene avrRpt2, and that self-processing of AtMCP2d was also detected in wild-type leaves in response to this pathogen. Furthermore, increases in processed AtMCP2d proteins were found to correlate with conditional cell-death induction in two lesion-mimic mutants (cpr22 and ssi4) that exhibit spontaneous cell-death phenotypes. Taken together, our data strongly suggest that AtMCP2d plays a positive regulatory role in biotic and abiotic stress-induced PCD.