Expression and localization of calcium-dependent protein kinase isoforms in chickpea

Calcium-dependent protein kinases in cotton: insights into early plant responses to salt stress

BACKGROUND

Soil salinization is one of the major environmental constraints to plant growth and agricultural production worldwide. Signaling components involving calcium (Ca²⁺) and the downstream calcium-dependent protein kinases (CPKs) play key roles in the perception and transduction of stress signals. However, the study of CPKs in cotton and their functions in response to salt stress remain unexplored.

RESULTS

A total of 98 predicted CPKs were identified from upland cotton (Gossypium hirsutum L. 'TM-1'), and phylogenetic analyses classified them into four groups. Gene family distribution studies have revealed the substantial impacts of the genome duplication events to the total number of GhCPKs. Transcriptome analyses showed a wide distribution of CPKs' expression among different organs. A total of 19 CPKs were selected for their rapid responses to salt stress at the transcriptional level, most of which were also incduced by the thylene-releasing chemical ethephon, suggesting a partal overlap of the salinity and ethylene responses. Silencing of 4 of the 19 CPKs (GhCPK8, GhCPK38, GhCPK54, and GhCPK55) severely compromised the basal cotton resistance to salt stress.

CONCLUSIONS

Our genome-wide expression analysis of CPK genes from up-land cotton suggests that CPKs are involved in multiple developmental responses as well as the response to different abiotic stresses. A cluster of the cotton CPKs was shown to participate in the early signaling events in cotton responses to salt stress. Our results provide significant insights on functional analysis of CPKs in cotton, especially in the context of cotton adaptions to salt stress.

CDPK1 from ginger promotes salinity and drought stress tolerance without yield penalty by improving growth and photosynthesis in Nicotiana tabacum

In plants, transient changes in calcium concentrations of cytosol have been observed during stress conditions like high salt, drought, extreme temperature and mechanical disturbances. Calcium-dependent protein kinases (CDPKs) play important roles in relaying these calcium signatures into downstream effects. In this study, a stress-responsive CDPK gene, ZoCDPK1 was isolated from a stress cDNA generated from ginger using rapid amplification of cDNA ends (RLM-RACE) - PCR technique and characterized its role in stress tolerance. An important aspect seen during the analysis of the deduced protein is a rare coupling between the presence of a nuclear localization sequence in the junction domain and consensus sequence in the EF-hand loops of calmodulin-like domain. ZoCDPK1 is abundantly expressed in rhizome and is rapidly induced by high-salt stress, drought, and jasmonic acid treatment but not by low temperature stress or abscissic acid treatment. The sub-cellular localization of ZoCDPK1-GFP fusion protein was studied in transgenic tobacco epidermal cells using confocal laser scanning microscopy. Over-expression of ginger CDPK1 gene in tobacco conferred tolerance to salinity and drought stress as reflected by the high percentage of seed germination, higher relative water content, expression of stress responsive genes, higher leaf chlorophyll content, increased photosynthetic efficiency and other photosynthetic parameters. In addition, transgenic tobacco subjected to salinity/drought stress exhibited 50% more growth during stress conditions as compared to wild type plant during normal conditions. T3 transgenic plants are able to grow to maturity, flowers early and set viable seeds under continuous salinity or drought stress without yield penalty. The ZoCDPK1 up-regulated the expression levels of stress-related genes RD21A and ERD1 in tobacco plants. These results suggest that ZoCDPK1 functions in the positive regulation of the signaling pathways that are involved in the response to salinity and drought stress in ginger and it is likely operating in a DRE/CRT independent manner.

PROTEIN L-ISOASPARTYL METHYLTRANSFERASE2 is differentially expressed in chickpea and enhances seed vigor and longevity by reducing abnormal isoaspartyl accumulation predominantly in seed nuclear proteins

PROTEIN l-ISOASPARTYL METHYLTRANSFERASE (PIMT) is a widely distributed protein-repairing enzyme that catalyzes the conversion of abnormal l-isoaspartyl residues in spontaneously damaged proteins to normal aspartyl residues. This enzyme is encoded by two divergent genes (PIMT1 and PIMT2) in plants, unlike many other organisms. While the biological role of PIMT1 has been elucidated, the role and significance of the PIMT2 gene in plants is not well defined. Here, we isolated the PIMT2 gene (CaPIMT2) from chickpea (Cicer arietinum), which exhibits a significant increase in isoaspartyl residues in seed proteins coupled with reduced germination vigor under artificial aging conditions. The CaPIMT2 gene is found to be highly divergent and encodes two possible isoforms (CaPIMT2 and CaPIMT2') differing by two amino acids in the region I catalytic domain through alternative splicing. Unlike CaPIMT1, both isoforms possess a unique 56-amino acid amino terminus and exhibit similar yet distinct enzymatic properties. Expression analysis revealed that CaPIMT2 is differentially regulated by stresses and abscisic acid. Confocal visualization of stably expressed green fluorescent protein-fused PIMT proteins and cell fractionation-immunoblot analysis revealed that apart from the plasma membrane, both CaPIMT2 isoforms localize predominantly in the nucleus, while CaPIMT1 localizes in the cytosol. Remarkably, CaPIMT2 enhances seed vigor and longevity by repairing abnormal isoaspartyl residues predominantly in nuclear proteins upon seed-specific expression in Arabidopsis (Arabidopsis thaliana), while CaPIMT1 enhances seed vigor and longevity by repairing such abnormal proteins mainly in the cytosolic fraction. Together, our data suggest that CaPIMT2 has most likely evolved through gene duplication, followed by subfunctionalization to specialize in repairing the nuclear proteome.

Ectopic expression of the ABA-inducible dehydration-responsive chickpea L-myo-inositol 1-phosphate synthase 2 (CaMIPS2) in Arabidopsis enhances tolerance to salinity and dehydration stress

Myo-inositol participates in many different aspects of plant physiology and myo-inositol 1-phosphate synthase (MIPS; EC 5.5.1.4) catalyzes the rate limiting step of inositol biosynthetic pathway. Chickpea (Cicer arietinum), a drought-tolerant leguminous crop plant, is known to accumulate increased inositol during dehydration stress. Previously, we reported two differentially expressed divergent genes (CaMIPS1 and CaMIPS2) encoding two MIPS isoforms in chickpea. In this communication, we demonstrated that CaMIPS2 is an early dehydration-responsive gene and is also rapidly induced by exogenous ABA application, while CaMIPS1 expression is not much influenced by dehydration or ABA. The regulation of expression of these two genes has been studied by examining their promoter activity through GUS reporter gene and differential promoter activity has been observed. Moreover, unlike CaMIPS1 promoter, CaMIPS2 promoter contains CRT/DRE cis-regulatory element which seems to play a key role in dehydration-induced expression of CaMIPS2. Furthermore, CaMIPS1 and CaMIPS2 have been successfully complemented and shown to repair the defect of seedling growth and altered seed phenotype of Atmips1 mutant. Moreover, Arabidopsis transgenic plants overexpressing CaMIPS1 or CaMIPS2 exhibit improved tolerance to salinity and dehydration stresses and such tolerance of transgenic plants is correlated with their elevated level of inositol. Remarkably, CaMIPS2 transgenic lines perform better in all attributes than CaMIPS1 transformants under such stress conditions, due to comparatively unabated production of inositol by CaMIPS2 enzyme, as this enzyme retains significant activity under stress conditions.

Phospholipid mediated activation of calcium dependent protein kinase 1 (CaCDPK1) from chickpea: a new paradigm of regulation

Phospholipids, the major structural components of membranes, can also have functions in regulating signaling pathways in plants under biotic and abiotic stress. The effects of adding phospholipids on the activity of stress-induced calcium dependent protein kinase (CaCDPK1) from chickpea are reported here. Both autophosphorylation as well as phosphorylation of the added substrate were enhanced specifically by phosphatidylcholine and to a lesser extent by phosphatidic acid, but not by phosphatidylethanolamine. Diacylgylerol, the neutral lipid known to activate mammalian PKC, stimulated CaCDPK1 but at higher concentrations. Increase in V_max of the enzyme activity by these phospholipids significantly decreased the K_m indicating that phospholipids enhance the affinity towards its substrate. In the absence of calcium, addition of phospholipids had no effect on the negligible activity of the enzyme. Intrinsic fluorescence intensity of the CaCDPK1 protein was quenched on adding PA and PC. Higher binding affinity was found with PC (K_½ = 114 nM) compared to PA (K_½ = 335 nM). We also found that the concentration of PA increased in chickpea plants under salt stress. The stimulation by PA and PC suggests regulation of CaCDPK1 by these phospholipids during stress response.

Selective modification of rice (Oryza sativa) gene expression by rice stripe virus infection

Rice stripe disease, caused by rice stripe virus (RSV), is one of the major virus diseases in east Asia. Rice plants infected with RSV usually show symptoms such as chlorosis, weakness, necrosis in newly emerged leaves and stunting. To reveal rice cellular systems influenced by RSV infection, temporal changes in the transcriptome of RSV-infected plants were monitored by a customized rice oligoarray system. The transcriptome changes in RSV-infected plants indicated that protein-synthesis machineries and energy production in the mitochondrion were activated by RSV infection, whereas energy production in the chloroplast and synthesis of cell-structure components were suppressed. The transcription of genes related to host-defence systems under hormone signals and those for gene silencing were not activated at the early infection phase. Together with concurrent observation of virus concentration and symptom development, such transcriptome changes in RSV-infected plants suggest that different sets of various host genes are regulated depending on the development of disease symptoms and the accumulation of RSV.

Two divergent genes encoding L-myo-inositol 1-phosphate synthase1 (CaMIPS1) and 2 (CaMIPS2) are differentially expressed in chickpea

L-myo-inositol 1-phosphate synthase (MIPS; EC5.5.1.4) catalyses the rate-limiting step in inositol biosynthetic pathway, and is extremely widespread in living organisms including plants. Several plants possess multiple copies of MIPS gene(s) indicating a possibility of differential expression of each gene to perform distinct physiological functions. To explore this, two MIPS genes (CaMIPS1 and CaMIPS2) were isolated from a drought-tolerant plant chickpea. Both genes are extremely divergent in respect to their introns, at the same time retaining 85% identity to their exons and functionally complementing inositol auxotroph Schizosaccharomyces pombe. Expression analysis showed both genes were expressed in all organs except seed, where only CaMIPS2 transcript was detected. Under environmental stresses, only CaMIPS2 was induced whereas CaMIPS1 expression remained same, which could be explained by the divergence of their 5' upstream regulatory sequences. Remarkably, both gene products exhibited similar biochemical characteristics; however, CaMIPS2 retained higher activity than CaMIPS1 at a high temperature and salt concentration. Furthermore, functional expression of CaMIPS2 in S. pombe results better growth response than CaMIPS1 under stress environment. Taken together, our results suggest that CaMIPS1 and CaMIPS2 are differentially expressed in chickpea to play discrete though overlapping roles in plant; however CaMIPS2 is likely to be evolved through gene duplication to function under environmental stresses.