Differential expression of poplar sucrose nonfermenting1-related protein kinase 2 genes in response to abiotic stress and abscisic acid

Heterogeneous Expression of Arabidopsis Subclass II of SNF1-Related Kinase 2 Improves Drought Tolerance via Stomatal Regulation in Poplar

Abscisic acid (ABA) is the most important phytohormone involved in the response to drought stress. Subclass II of SNF1-related kinase 2 (SnRK2) is an important signaling kinase related to ABA signal transduction. It regulates the phosphorylation of the target transcription factors controlling the transcription of a wide range of ABA-responsive genes in Arabidopsis thaliana. The transgenic poplars (Populus tremula × P. tremuloides, clone T89) ectopically overexpressing AtSnRK2.8, encoding a subclass II SnRK2 kinase of A. thaliana, have been engineered but almost no change in its transcriptome was observed. In this study, we evaluated osmotic stress tolerance and stomatal behavior of the transgenic poplars maintained in the netted greenhouse. The transgenic poplars, line S22, showed a significantly higher tolerance to 20% PEG treatment than non-transgenic controls. The stomatal conductance of the transgenic poplars tended to be lower than the non-transgenic control. Microscopic observations of leaf imprints revealed that the transgenic poplars had significantly higher stomatal closures under the stress treatment than the non-transgenic control. In addition, the stomatal index was lower in the transgenic poplars than in the non-transgenic controls regardless of the stress treatment. These results suggested that AtSnRK2.8 is involved in the regulation of stomatal behavior. Furthermore, the transgenic poplars overexpressing AtSnRK2.8 might have improved abiotic stress tolerance through this stomatal regulation.

Comprehensive identification and expression analyses of the SnRK gene family in Casuarina equisetifolia in response to salt stress

BACKGROUND

Sucrose nonfermenting-1 (SNF1)-related protein kinases (SnRKs) play crucial roles in plant signaling pathways and stress adaptive responses by activating protein phosphorylation pathways. However, there have been no comprehensive studies of the SnRK gene family in the widely planted salt-tolerant tree species Casuarina equisetifolia. Here, we comprehensively analyze this gene family in C. equisetifolia using genome-wide identification, characterization, and profiling of expression changes in response to salt stress.

RESULTS

A total of 26 CeqSnRK genes were identified, which were divided into three subfamilies (SnRK1, SnRK2, and SnRK3). The intron-exon structures and protein‑motif compositions were similar within each subgroup but differed among groups. Ka/Ks ratio analysis indicated that the CeqSnRK family has undergone purifying selection, and cis-regulatory element analysis suggested that these genes may be involved in plant development and responses to various environmental stresses. A heat map was generated using quantitative real‑time PCR (RT-qPCR) data from 26 CeqSnRK genes, suggesting that they were expressed in different tissues. We also examined the expression of all CeqSnRK genes under exposure to different salt concentrations using RT-qPCR, finding that most CeqSnRK genes were regulated by different salt treatments. Moreover, co-expression network analysis revealed synergistic effects among CeqSnRK genes.

CONCLUSIONS

Several CeqSnRK genes (CeqSnRK3.7, CeqSnRK3.16, CeqSnRK3.17) were up-regulated following salt treatment. Among them, CeqSnRK3.16 expression was significantly up-regulated under various salt treatments, identifying this as a candidate gene salt stress tolerance gene. In addition, CeqSnRK3.16 showed significant expression change correlations with multiple genes under salt stress, indicating that it might exhibit synergistic effects with other genes in response to salt stress. This comprehensive analysis will provide a theoretical reference for CeqSnRK gene functional verification and the role of these genes in salt tolerance.

Impact of abiotic stress on the regulation of cell wall biosynthesis in Populus trichocarpa

Growth of biomass for lignocellulosic biofuels and biomaterials may take place on land unsuitable for foods, meaning the biomass plants are exposed to increased abiotic stresses. Thus, the understanding how this affects biomass composition and quality is important for downstream bioprocessing. Here, we analyzed the effect of drought and salt stress on cell wall biosynthesis in young shoots and xylem tissues of Populus trichocarpa using transcriptomic and biochemical methods. Following exposure to abiotic stress, stem tissues reduced vessel sizes, and young shoots increased xylem formation. Compositional analyses revealed a reduction in the total amount of cell wall polysaccharides. In contrast, the total lignin amount was unchanged, while the ratio of S/G lignin was significantly decreased in young shoots. Consistent with these observations, transcriptome analyses show that the expression of a subset of cell wall-related genes is tightly regulated by drought and salt stresses. In particular, the expression of a part of genes encoding key enzymes for S-lignin biosynthesis, caffeic acid O-methyltransferase and ferulate 5-hydroxylase, was decreased, suggesting the lower S/G ratio could be partly attributed to the down-regulation of these genes. Together, our data identifies a transcriptional abiotic stress response strategy in poplar, which results in adaptive changes to the plant cell wall.

The Sucrose Non-Fermenting 1-Related Protein Kinase 2 (SnRK2) Genes Are Multifaceted Players in Plant Growth, Development and Response to Environmental Stimuli

Reversible protein phosphorylation orchestrated by protein kinases and phosphatases is a major regulatory event in plants and animals. The SnRK2 subfamily consists of plant-specific protein kinases in the Ser/Thr protein kinase superfamily. Early observations indicated that SnRK2s are mainly involved in response to abiotic stress. Recent evidence shows that SnRK2s are multifarious players in a variety of biological processes. Here, we summarize the considerable knowledge of SnRK2s, including evolution, classification, biological functions and regulatory mechanisms at the epigenetic, post-transcriptional and post-translation levels.

Drought and heat stress-related proteins: an update about their functional relevance in imparting stress tolerance in agricultural crops

We describe here the recent developments about the involvement of diverse stress-related proteins in sensing, signaling, and defending the cells in plants in response to drought or/and heat stress. In the current era of global climate drift, plant growth and productivity are often limited by various environmental stresses, especially drought and heat. Adaptation to abiotic stress is a multigenic process involving maintenance of homeostasis for proper survival under adverse environment. It has been widely observed that a series of proteins respond to heat and drought conditions at both transcriptional and translational levels. The proteins are involved in various signaling events, act as key transcriptional activators and saviors of plants under extreme environments. A detailed insight about the functional aspects of diverse stress-responsive proteins may assist in unraveling various stress resilience mechanisms in plants. Furthermore, by identifying the metabolic proteins associated with drought and heat tolerance, tolerant varieties can be produced through transgenic/recombinant technologies. A large number of regulatory and functional stress-associated proteins are reported to participate in response to heat and drought stresses, such as protein kinases, phosphatases, transcription factors, and late embryogenesis abundant proteins, dehydrins, osmotins, and heat shock proteins, which may be similar or unique to stress treatments. Few studies have revealed that cellular response to combined drought and heat stresses is distinctive, compared to their individual treatments. In this review, we would mainly focus on the new developments about various stress sensors and receptors, transcription factors, chaperones, and stress-associated proteins involved in drought or/and heat stresses, and their possible role in augmenting stress tolerance in crops.

Genome-wide identification and characterization of the CsSnRK2 family in Camellia sinensis

The sucrose nonfermenting 1 (SNF1)-related protein kinase 2 (SnRK2) genes play central roles in plant stress signal transduction. In this study, 8 SnRK2 genes were identified from the tea plant genome database and named CsSnRK2.1-8. Phylogenetic analysis showed that the CsSnRK2 genes were classifiable into three groups, similar to those of Arabidopsis thaliana, Oryza sativa and maize. The coding sequences (CDSs) of all CsSnRK2s were separated by eight introns, and their exon-intron organizations exhibited high similarity to those of other plants. The fluorescence of GFP fused with CsSnRK2.3 was detected in only the cytoplasm, while the rest of the proteins showed GFP signal in both the nucleus and the cytoplasm. The results of the expression patterns of the CsSnRK2 genes showed that CsSnRK2s were differentially induced by salt, polyethylene glycol (PEG) and abscisic acid (ABA) stress. Interestingly, The expression of CsSnRK2.3 was inhibited by ABA, suggesting the complicated roles of CsSnRK2s in the ABA signal transduction pathway. Some CsSnRK2 gene pairs showed significant expression change correlations under stresses, indicating that CsSnRK2s might exhibit synergistic effects of signal regulation in response to various stresses. In summary, this comprehensive analysis will facilitate further studies of the SnRK2 family of Camellia sinensis and provide useful information for the functional validation of CsSnRK2s.

Expression profiling of genes encoding ABA route components in response to dehydration or various light conditions in poplar buds and leaves

In this report, the members of PP2C, SnRK2a and Rboh oxidase families from Arabidopsis and poplar were studied in silico, and the expression profiles of the some of them were specified in Populus tremula buds and adult leaves. In poplars, the counterparts of ABI1- and ABI2-like protein phosphatases are lacking, but poplar genomes encode three HAB-like proteins denoted in this work as HAB1, HAB3a and HAB3b, and the counterparts of the two latter ones are absent in Arabidopsis. Nonetheless, they may be present in other species. In poplars, SnRK2 subclass III includes two SnRK2.6-like protein kinases denoted by us as SnRK2.6a and SnRK2.6b, and only one SnRK2.2 corresponding to SnRK2.2 and SnRK2.3 ones from Arabidopsis. In contrast to Arabidopsis, the poplar Rboh family involves two RbohD- and RbohF-like proteins denoted here as RbohD1 and RbohD2, and RbohF1 and RbohF2, respectively. The expressions of genes encoding the above components of the ABA route were studied in Populus tremula dehydrated buds and adult leaves not subjected to stress but exposed to natural daylight or to darkness, and to inhibition of ethylene biosynthesis or signaling route by cobalt or silver ions, respectively. In leaves, the light conditions seemed to be the most pronounced factor, from among the studied stimuli, controlling the expression Ptre-HAB3a, Ptre-HAB1, Ptre-SnRK2.6a and Ptre-RbohF2 genes, their expression was upregulated in darkness. This observation implies that these genes may be important for dark-induced stomatal closure regulation. Ethylene negatively affected the expression of three studied Rboh genes and Ptre-HAB1one but only at daylight, whereas its positive effect on the of Ptre-HAB3a was shown in the dark exposed leaves. In buds, three studied Rboh genes took part in the early response to dehydration, however their participation involved the visibly highest level of the Ptre-RbohD1 transcripts, followed by Ptre-RbohF2 and the lowest one of Ptre-RbohF1. Nonetheless, the further stress-induced superoxide anion generation seemed to depend on the enhanced expression of the Ptre-RbohD1 and Ptre-RbohF2 genes only, still with a significantly higher level of the Ptre-RbohD1 one. Ptre-RbohD2 transcripts were found neither in leaves nor in buds. The expression of the other genes discussed in the present work was either slightly upregulated at moderate stress or did not significantly change in response to dehydration. The protein kinase activity of overexpressed Ptre-SnRK2.6a and Ptre-SnRK2.6b was confirmed in in vitro protein kinase assay and compared to that of SnRK2.6/OST1 one from Arabidopsis.