GsSRK, a G-type lectin S-receptor-like serine/threonine protein kinase,is a positive regulator of plant tolerance to salt stress

Receptor-like protein kinases (RLKs) play vital roles in sensing outside signals, yet little is known about RLKs functions and roles in stress signal perception and transduction in plants, especially in wild soybean. Through the microarray analysis, GsSRK was identified as an alkaline (NaHCO₃)-responsive gene, and was subsequently isolated from Glycine soja by homologous cloning. GsSRK encodes a 93.22 kDa protein with a highly conserved serine/threonine protein kinase catalytic domain, a G-type lectin region, and an S-locus region. Real-time PCR results showed that the expression levels of GsSRK were largely induced by ABA, salt, and drought stresses. Over expression of GsSRK in Arabidopsis promoted seed germination, as well as primary root and rosette leaf growth during the early stages of salt stress. Compared to the wild type Arabidopsis, GsSRK overexpressors exhibited enhanced salt tolerance and higher yields under salt stress, with higher chlorophyll content, lower ion leakage, higher plant height, and more siliques at the adult developmental stage. Our studies suggest that GsSRK plays a crucial role in plant response to salt stress.     

Overproduction of the Membrane-bound Receptor-like Protein Kinase 1, RPK1, Enhances Abiotic Stress Tolerance in Arabidopsis

RPK1 (receptor-like protein kinase 1) localizes to the plasma membrane and functions as a regulator of abscisic acid (ABA) signaling in Arabidopsis. In our current study, we investigated the effect of RPK1 disruption and overproduction upon plant responses to drought stress. Transgenic Arabidopsis overexpressing the RPK1 protein showed increased ABA sensitivity in their root growth and stomatal closure and also displayed less transpirational water loss. In contrast, a mutant lacking RPK1 function, rpk1-1, was found to be resistant to ABA during these processes and showed increased water loss. RPK1 overproduction in these transgenic plants thus increased their tolerance to drought stress. We performed microarray analysis of RPK1 transgenic plants and observed enhanced expression of several stress-responsive genes, such as Cor15a, Cor15b, and rd29A, in addition to H₂O₂-responsive genes. Consistently, the expression levels of ABA/stress-responsive genes in rpk1-1 had decreased compared with wild type. The results suggest that the overproduction of RPK1 enhances both the ABA and drought stress signaling pathways. Furthermore, the leaves of the rpk1-1 plants exhibit higher sensitivity to oxidative stress upon ABA-pretreatment, whereas transgenic plants overproducing RPK1 manifest increased tolerance to this stress. Our current data suggest therefore that RPK1 overproduction controls reactive oxygen species homeostasis and enhances both water and oxidative stress tolerance in Arabidopsis.     

Overexpression of maize mitogen-activated protein kinase gene, ZmSIMK1 in Arabidopsis increases tolerance to salt stress

Mitogen-activated protein kinase (MAPK) cascades play a remarkably crucial role in plants. It has been studied intensively in model plants Arabidopsis, tobacco and rice. However, the function of MAPKs in maize (Zea mays L.) has not been well documented. ZmSIMK1 (Zea mays salt-induced mitogen-activated protein kinase 1) is a previously identified MAPK gene in maize. In this research, we charactered ZmSIMK1 and showed that ZmSIMK1 was involved in Arabidopsis salt stress. The genomic organization of ZmSIMK1 gene and its expression in maize have been analyzed. In order to investigate the function of ZmSIMK1, we generated transgenic Arabidopsis constitutively overexpressing ZmSIMK1. Ectopic expression of ZmSIMK1 in Arabidopsis resulted in increased resistance against salt stress. Importantly, ZmSIMK1-overexpressing Arabidopsis exhibited constitutive expression of stress-responsive marker genes, RD29A and P5CS1. Furthermore, RD29A and P5CS1 were upregulated under salt stress. These results suggest that ZmSIMK1 may play an important role in plant salt stress.     

A novel Glycine soja cysteine proteinase inhibitor GsCPI14, interacting with the calcium/calmodulin-binding receptor-like kinase GsCBRLK,regulated plant tolerance to alkali stress

It has been well demonstrated that cystatins regulated plant stress tolerance through inhibiting the cysteine proteinase activity under environmental stress. However, there was limited information about the role of cystatins in plant alkali stress response, especially in wild soybean. Here, in this study, we focused on the biological characterization of a novel Glycine soja cystatin protein GsCPI14, which interacted with the calcium/calmodulin-binding receptor-like kinase GsCBRLK and positively regulated plant alkali stress tolerance. The protein–protein interaction between GsCBRLK and GsCPI14 was confirmed by using split-ubiquitin based membrane yeast two-hybrid analysis and bimolecular fluorescence complementation assay. Expression of GsCPI14 was greatly induced by salt, ABA and alkali stress in G. soja, and GsCBRLK overexpression (OX) in Glycine max promoted the stress induction of GmCPI14 expression under stress conditions. Furthermore, we found that GsCPI14-eGFP fusion protein localized in the entire Arabidopsis protoplast and onion epidermal cell, and GsCPI14 showed ubiquitous expression in different tissues of G. soja. In addition, we gave evidence that the GST-GsCPI14 fusion protein inhibited the proteolytic activity of papain in vitro. At last, we demonstrated that OX of GsCPI14 in Arabidopsis promoted the seed germination under alkali stress, as evidenced by higher germination rates. GsCPI14 transgenic Arabidopsis seedlings also displayed better growth performance and physiological index under alkali stress. Taken together, results presented in this study demonstrated that the G. soja cysteine proteinase inhibitor GsCPI14 interacted with the calcium/calmodulin-binding receptor-like kinase GsCBRLK and regulated plant tolerance to alkali stress.     

Effect of plant growth promoting <Emphasis Type="Italic">Pseudomonas</Emphasis> spp. on compatible solutes,antioxidant status and plant growth of maize under drought stress

Drought is one of the major abiotic stresses affecting yield of dryland crops. Rhizobacterial populations of stressed soils are adapted and tolerant to stress and can be screened for isolation of efficient stress adaptive/tolerant, plant growth promoting rhizobacterial (PGPR) strains that can be used as inoculants for crops grown in stressed ecosystems. The effect of inoculation of five drought tolerant plant growth promoting Pseudomonas spp. strains namely P. entomophila strain BV-P13, P. stutzeri strain GRFHAP-P14, P. putida strain GAP-P45, P. syringae strain GRFHYTP52, and P. monteilli strain WAPP53 on growth, osmoregulation and antioxidant status of maize seedlings under drought stress conditions was investigated. Drought stress induced by withholding irrigation had drastic effects on growth of maize seedlings. However seed bacterization of maize with Pseudomonas spp. strains improved plant biomass, relative water content, leaf water potential, root adhering soil/root tissue ratio, aggregate stability and mean weight diameter and decreased leaf water loss. The inoculated plants showed higher levels of proline, sugars, free amino acids under drought stress. However protein and starch content was reduced under drought stress conditions. Inoculation decreased electrolyte leakage compared to uninoculated seedlings under drought stress. As compared to uninoculated seedlings, inoculated seedlings showed significantly lower activities of antioxidant enzymes, ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX) under drought stress, indicating that inoculated seedlings felt less stress as compared to uninoculated seedlings. The strain GAP-P45 was found to be the best in terms of influencing growth and biochemical and physiological status of the seedlings under drought stress. The study reports the potential of rhizobacteria in alleviating drought stress effects in maize.     

RNA-Seq Analysis Reveals MAPKKK Family Members Related to Drought Tolerance in Maize

The mitogen-activated protein kinase (MAPK) cascade is an evolutionarily conserved signal transduction pathway that is involved in plant development and stress responses. As the first component of this phosphorelay cascade, mitogen-activated protein kinase kinase kinases (MAPKKKs) act as adaptors linking upstream signaling steps to the core MAPK cascade to promote the appropriate cellular responses; however, the functions of MAPKKKs in maize are unclear. Here, we identified 71 MAPKKK genes, of which 14 were novel, based on a computational analysis of the maize (Zea mays L.) genome. Using an RNA-seq analysis in the leaf, stem and root of maize under well-watered and drought-stress conditions, we identified 5,866 differentially expressed genes (DEGs), including 8 MAPKKK genes responsive to drought stress. Many of the DEGs were enriched in processes such as drought stress, abiotic stimulus, oxidation-reduction, and metabolic processes. The other way round, DEGs involved in processes such as oxidation, photosynthesis, and starch, proline, ethylene, and salicylic acid metabolism were clearly co-expressed with the MAPKKK genes. Furthermore, a quantitative real-time PCR (qRT-PCR) analysis was performed to assess the relative expression levels of MAPKKKs. Correlation analysis revealed that there was a significant correlation between expression levels of two MAPKKKs and relative biomass responsive to drought in 8 inbred lines. Our results indicate that MAPKKKs may have important regulatory functions in drought tolerance in maize.     

AtCPK6, a functionally redundant and positive regulator involved in salt/drought stress tolerance in Arabidopsis

The calcium-dependent protein kinase (CDPK) family is needed in plant signaling during various physiological pathways. The Arabidopsis AtCPK6 gene belongs to the subclass of stress-inducible CDPKs, which is stimulated by salt and osmotic stress. To elucidate the physiological function of AtCPK6, transgenic Arabidopsis plants under the control of double CaMV 35S promoter were obtained. AtCPK6 over-expressing plants showed enhanced tolerance to salt/drought stresses. The elevated tolerance of the AtCPK6 over-expressing plants was confirmed by the change of proline and malondialdehyde (MDA). Real-time PCR analyses revealed that the expression levels of several stress-regulated genes were altered in AtCPK6 over-expressing plants. However, cpk6 mutant displayed no obvious difference with control. These results are likely to indicate that AtCPK6 is functionally redundant and a positive regulator involved in the tolerance to salt/drought stress in Arabidopsis.     

Potential role of D-<Emphasis Type="Italic">myo</Emphasis>-inositol-3-phosphate synthase and 14-3-3 genes in the crosstalk between <Emphasis Type="Italic">Zea mays</Emphasis> and <Emphasis Type="Italic">Rhizophagus intraradices</Emphasis> under drought stress

Arbuscular mycorrhizal (AM) symbiosis is known to stimulate plant drought tolerance. However, the mechanisms underlying the synergistic responses of the symbiotic partners to drought stress are largely unknown. A split-root experiment was designed to investigate the molecular interactions between a host plant and an AM fungus (AMF) under drought stress. In the two-compartment cultivation system, an entire or only a half root system of a maize plant was inoculated with an AMF, Rhizophagus intraradices, in the presence of localized or systemic drought treatment. Plant physiological parameters including growth, water status, and phosphorus concentration, and the expression of drought tolerance-related genes in both roots and R. intraradices were recorded. Although mycorrhizal inoculation in either one or both compartments systemically decreased abscisic acid (ABA) content in the whole root system subjected to systemic or local drought stress, we observed local and/or systemic AM effects on root physiological traits and the expression of functional genes in both roots and R. intraradices. Interestingly, the simultaneous increase in the expression of plant genes encoding D-myo-inositol-3-phosphate synthase (IPS) and 14-3-3-like protein GF14 (14-3GF), which were responsible for ABA signal transduction, was found to be involved in the activation of 14-3-3 protein and aquaporins (GintAQPF1 and GintAQPF2) in R. intraradices. These findings suggest that coexpression of IPS and 14-3GF is responsible for the crosstalk between maize and R. intraradices under drought stress, and potentially induces the synergistic actions of the symbiotic partners in enhancing plant drought tolerance.