Molecular and Physiological Mechanisms to Mitigate Abiotic Stress Conditions in Plants

Agriculture production faces many abiotic stresses, mainly drought, salinity, low and high temperature. These abiotic stresses inhibit plants' genetic potential, which is the cause of huge reduction in crop productivity, decrease potent yields for important crop plants by more than 50% and imbalance agriculture's sustainability. They lead to changes in the physio-morphological, molecular, and biochemical nature of the plants and change plants' regular metabolism, which makes them a leading cause of losses in crop productivity. These changes in plant systems also help to mitigate abiotic stress conditions. To initiate the signal during stress conditions, sensor molecules of the plant perceive the stress signal from the outside and commence a signaling cascade to send a message and stimulate nuclear transcription factors to provoke specific gene expression. To mitigate the abiotic stress, plants contain several methods of avoidance, adaption, and acclimation. In addition to these, to manage stress conditions, plants possess several tolerance mechanisms which involve ion transporters, osmoprotectants, proteins, and other factors associated with transcriptional control, and signaling cascades are stimulated to offset abiotic stress-associated biochemical and molecular changes. Plant growth and survival depends on the ability to respond to the stress stimulus, produce the signal, and start suitable biochemical and physiological changes. Various important factors, such as the biochemical, physiological, and molecular mechanisms of plants, including the use of microbiomes and nanotechnology to combat abiotic stresses, are highlighted in this article.

Over-Expression of a Melon Y3SK2-Type LEA Gene Confers Drought and Salt Tolerance in Transgenic Tobacco Plants

Climate change, with its attendant negative effects, is expected to hamper agricultural production in the coming years. To counteract these negative effects, breeding of environmentally resilient plants via conventional means and genetic engineering is necessary. Stress defense genes are valuable tools by which this can be achieved. Here we report the successful cloning and functional characterization of a melon Y3SK2-type dehydrin gene, designated as CmLEA-S. We generated CmLEA-S overexpressing transgenic tobacco lines and performed in vitro and in vivo drought and salt stress analyses. Seeds of transgenic tobacco plants grown on 10% polyethylene glycol (PEG) showed significantly higher germination rates relative to wild-type seeds. In the same way, transgenic seeds grown on 150 mM sodium chloride (NaCl) recorded significantly higher germination percentages compared with wild-type plants. The fresh weights and root lengths of young transgenic plants subjected to drought stress were significantly higher than that of wild-type plants. Similarly, the fresh weights and root lengths of transgenic seedlings subjected to salt stress treatments were also significantly higher than wild-type plants. Moreover, transgenic plants subjected to drought and salt stresses in vivo showed fewer signs of wilting and chlorosis, respectively. Biochemical assays revealed that transgenic plants accumulated more proline and less malondialdehyde (MDA) compared with wild-type plants under both drought and salt stress conditions. Finally, the enzymatic activities of ascorbate peroxidase (APX) and catalase (CAT) were enhanced in drought- and salt-stressed transgenic lines. These results suggest that the CmLEA-S gene could be used as a potential candidate gene for crop improvement.

Novel Maize NAC Transcriptional Repressor ZmNAC071 Confers Enhanced Sensitivity to ABA and Osmotic Stress by Downregulating Stress-Responsive Genes in Transgenic Arabidopsis

NAC TFs play crucial roles in response to abiotic stresses in plants. Here, ZmNAC071 was identified as a nuclear located transcriptional repressor. Overexpression of ZmNAC071 in Arabidopsis enhanced sensitivity of transgenic plants to ABA and osmotic stress. The expression levels of SODs, PODs, P5CSs, and AtMYB61 were inhibited by ZmNAC071, which results in reduced ROS scavenging and proline content, increased ROS level, and water loss. Besides, the expression levels of some ABA or abiotic stress-related genes, like ABIs, RD29A, DREBs, and LEAs were also significantly inhibited by ZmNAC071. Yeast one-hybrid assay demonstrated that ZmNAC071 specifically bound to the cis-acting elements containing CGT[G/A] core sequences in the promoter of stress-related genes, suggesting that ZmNAC071 may participate in the regulation of transcription of these genes through recognizing the core sequences CGT[G/A]. These results will facilitate further studies concerning the cis-elements and downstream genes targeted by ZmNAC071 in maize.

Arabidopsis thaliana Trihelix Transcription Factor AST1 Mediates Salt and Osmotic Stress Tolerance by Binding to a Novel AGAG-Box and Some GT Motifs

Trihelix transcription factors are characterized by containing a conserved trihelix (helix-loop-helix-loop-helix) domain that binds to GT elements required for light response, and they play roles in light stress and in abiotic stress responses. However, only a few of them have been functionally characterized. In the present study, we characterized the function of AST1 (Arabidopsis SIP1 clade Trihelix1) in response to salt and osmotic stress. AST1 shows transcriptional activation activity, and its expression is induced by osmotic and salt stress. A conserved sequence highly present in the promoters of genes regulated by AST1 was identified, which was bound by AST1, and termed the AGAG-box with the sequence [A/G][G/A][A/T]GAGAG. Additionally, AST1 also binds to some GT motifs including the sequence of GGTAATT, TACAGT, GGTAAAT and GGTAAA, but failed in binding to the sequence of GTTAC and GGTTAA. Chromatin immunoprecipitation combined with quantitative real-time reverse transcription-PCR analysis suggested that AST1 binds to the AGAG-box and/or some GT motifs to regulate the expression of stress tolerance genes, resulting in reduced reactive oxygen species, Na+ accumulation, stomatal apertures, lipid peroxidation, cell death and water loss rate, and increased proline content and reactive oxygen species scavenging capability. These physiological changes affected by AST1 finally improve salt and osmotic tolerance.

An Ethylene-responsive Factor BpERF11 Negatively Modulates Salt and Osmotic Tolerance in Betula platyphylla

Ethylene responsive factors (ERFs) play important roles in the abiotic stress; however, only a few ERF genes from woody plants have been functionally characterized. In the present study, an ERF gene from Betula platyphylla (birch), BpERF11, was functionally characterized in response to abiotic stress. BpERF11 is a nuclear protein, which could specifically bind to GCC boxes and DRE motifs. BpERF11-overexpressing and BpERF11 RNA interference (RNAi) knockdown plants were generated for gain- and loss-of-function analysis. BpERF11 negatively regulates resistance to salt and severe osmotic stress, and the transgenic birch plants overexpressing BpERF11 shows increased electrolyte leakage and malondialdehyde (MDA) contents. BpERF11 inhibits the expression of an AtMYB61 homologous gene, resulting in increased stomatal aperture, which elevated the transpiration rate. Furthermore, BpERF11 downregulates the expression of P5CS, SOD and POD genes, but upregulates the expression of PRODH and P5CDH, which results in reduced proline levels and increased reactive oxygen species (ROS) accumulation. BpERF11 also significantly inhibits the expression of LEA and dehydrin genes that involve in abiotic stress tolerance. Therefore, BpERF11 serves as a transcription factor that negatively regulates salt and severe osmotic tolerance by modulating various physiological processes.

A bHLH gene from Tamarix hispida improves abiotic stress tolerance by enhancing osmotic potential and decreasing reactive oxygen species accumulation

Basic helix-loop-helix (bHLH) leucine-zipper transcription factors play important roles in abiotic stress responses. However, their specific roles in abiotic stress tolerance are not fully known. Here, we functionally characterized a bHLH gene, ThbHLH1, from Tamarix hispida in abiotic stress tolerance. ThbHLH1 specifically binds to G-box motif with the sequence of 'CACGTG'. Transiently transfected T. hispida plantlets with transiently overexpressed ThbHLH1 and RNAi-silenced ThbHLH1 were generated for gain- and loss-of-function analysis. Transgenic Arabidopsis thaliana lines overexpressing ThbHLH1 were generated to confirm the gain- and loss-of-function analysis. Overexpression of ThbHLH1 significantly elevates glycine betaine and proline levels, increases Ca(2+) concentration and enhances peroxidase (POD) and superoxide dismutase (SOD) activities to decrease reactive oxygen species (ROS) accumulation. Additionally, ThbHLH1 regulates the expression of the genes including P5CS, BADH, CaM, POD and SOD, to activate the above physiological changes, and also induces the expression of stress tolerance-related genes LEAs and HSPs. These data suggest that ThbHLH1 induces the expression of stress tolerance-related genes to improve abiotic stress tolerance by increasing osmotic potential, improving ROS scavenging capability and enhancing second messenger in stress signaling cascades.

Functional characterization of the late embryogenesis abundant (LEA) protein gene family from Pinus tabuliformis (Pinaceae) in Escherichia coli

Late embryogenesis abundant (LEA) proteins are a large and highly diverse gene family present in a wide range of plant species. LEAs are proposed to play a role in various stress tolerance responses. Our study represents the first-ever survey of LEA proteins and their encoding genes in a widely distributed pine (Pinus tabuliformis) in China. Twenty-three LEA genes were identified from the P. tabuliformis belonging to seven groups. Proteins with repeated motifs are an important feature specific to LEA groups. Ten of 23 pine LEA genes were selectively expressed in specific tissues, and showed expression divergence within each group. In addition, we selected 13 genes representing each group and introduced theses genes into Escherichia coli to assess the protective function of PtaLEA under heat and salt stresses. Compared with control cells, the E. coli cells expressing PtaLEA fusion protein exhibited enhanced salt and heat resistance and viability, indicating the protein may play a protective role in cells under stress conditions. Furthermore, among these enhanced tolerance genes, a certain extent of function divergence appeared within a gene group as well as between gene groups, suggesting potential functional diversity of this gene family in conifers.

A LEA gene regulates Cadmium tolerance by mediating physiological responses

In this study, the function of a LEA gene (TaLEA1) from Tamrix androssowii in response to heavy metal stress was characterized. Time-course expression analyses showed that NaCl, ZnCl₂, CuSO₄, and CdCl₂ considerably increased the expression levels of the TaLEA1 gene, thereby suggesting that this gene plays a role in the responses to these test stressors. To analyze the heavy metal stress-tolerance mechanism regulated by TaLEA1, TaLEA1-overexpressing transgenic poplar plants (Populus davidiana Dode × P. bollena Lauche) were generated. Significant differences were not observed between the proline content of the transgenic and wild-type (WT) plants before and after CdCl₂ stress. However, in comparison with the WT plants, the TaLEA1-transformed poplar plants had significantly higher superoxide dismutase (SOD) and peroxidase (POD) activities, and lower malondialdehyde (MDA) levels under CdCl₂ stress. Further, the transgenic plants showed better growth than the WT plants did, indicating that TaLEA1 provides tolerance to cadmium stress. These results suggest that TaLEA1 confers tolerance to cadmium stress by enhancing reactive oxygen species (ROS)-scavenging ability and decreasing lipid peroxidation. Subcellular-localization analysis showed that the TaLEA1 protein was distributed in the cytoplasm and nucleus.