A maize calcineurin B‐like interacting protein kinase ZmCIPK42 confers salt stress tolerance

Calcineurin B‐like (CBL) and CBL‐interacting protein kinase (CIPK) play a crucial role in biotic and abiotic stress responses. However, the roles of different CIPKs in biotic and abiotic stress responses are less well characterized. In this study, we identified a mutation leading to an early protein termination of the maize CIPK gene ZmCIPK42 that undergoes a G to A mutation at the coding region via searching for genes involved in salt stress tolerance and ion homeostasis from maize with querying the EMS mutant library of maize B73. The mutant zmcipk42 plants have less branched tassel and impaired salt stress tolerance at the seedling stage. Quantitative real‐time PCR analysis revealed that ZmCIPK42was expressed in diverse tissues and was induced by NaCl stress. A yeast two‐hybrid screen identified a proteinase inhibitor (ZmMPI) as well as calcineurin B‐like protein 1 and protein 4 (ZmCBL1, ZmCBL4) as interaction partners of ZmCIPK42. These interactions were further confirmed by bimolecular fluorescence complementation in plant cells. Moreover, over‐expressing ZmCIPK42 resulted in enhanced tolerance to high salinity in both maize and Arabidopsis. These findings suggest that ZmCIPK42 is a positive regulator of salt stress tolerance and is a promising candidate gene to improve salt stress tolerance in maize through genetic manipulation.     

The Arabidopsis STRESS RESPONSE SUPPRESSOR DEAD‐box RNA helicases are nucleolar‐ and chromocenter‐localized proteins that undergo stress‐mediated relocalization and are involved in epigenetic gene silencing

DEAD‐box RNA helicases are involved in many aspects of RNA metabolism and in diverse biological processes in plants. Arabidopsis thaliana mutants of two DEAD‐box RNA helicases, STRESS RESPONSE SUPPRESSOR1 (STRS1) and STRS2 were previously shown to exhibit tolerance to abiotic stresses and up‐regulated stress‐responsive gene expression. Here, we show that Arabidopsis STRS‐overexpressing lines displayed a less tolerant phenotype and reduced expression of stress‐induced genes confirming the STRSs as attenuators of Arabidopsis stress responses. GFP–STRS fusion proteins exhibited localization to the nucleolus, nucleoplasm and chromocenters and exhibited relocalization in response to abscisic acid (ABA) treatment and various stresses. This relocalization was reversed when stress treatments were removed. The STRS proteins displayed mis‐localization in specific gene‐silencing mutants and exhibited RNA‐dependent ATPase and RNA‐unwinding activities. In particular, STRS2 showed mis‐localization in three out of four mutants of the RNA‐directed DNA methylation (RdDM) pathway while STRS1 was mis‐localized in the hd2c mutant that is defective in histone deacetylase activity. Furthermore, heterochromatic RdDM target loci displayed reduced DNA methylation and increased expression in the strs mutants. Taken together, our findings suggest that the STRS proteins are involved in epigenetic silencing of gene expression to bring about suppression of the Arabidopsis stress response.     

The involvement of wheat U‐box E3 ubiquitin ligase TaPUB1 in salt stress tolerance

U‐box E3 ubiquitin ligases play important roles in the ubiquitin/26S proteasome machinery and in abiotic stress responses. TaPUB1‐overexpressing wheat (Triticum aestivum L.) were generated to evaluate its function in salt tolerance. These plants had more salt stress tolerance during seedling and flowering stages, whereas the TaPUB1‐RNA interference (RNAi)‐mediated knock‐down transgenic wheat showed more salt stress sensitivity than the wild type (WT). TaPUB1 overexpression upregulated the expression of genes related to ion channels and increased the net root Na⁺ efflux, but decreased the net K⁺ efflux and H⁺ influx, thereby maintaining a low cytosolic Na⁺/K⁺ ratio, compared with the WT. However, RNAi‐mediated knock‐down plants showed the opposite response to salt stress. TaPUB1 could induce the expression of some genes that improved the antioxidant capacity of plants under salt stress. TaPUB1 also interacted with TaMP (Triticum aestivum α‐mannosidase protein), a regulator playing an important role in salt response in yeast and in plants. Thus, low cytosolic Na⁺/K⁺ ratios and better antioxidant enzyme activities could be maintained in wheat with overexpression of TaPUB1 under salt stress. Therefore, we conclude that the U‐box E3 ubiquitin ligase TaPUB1 positively regulates salt stress tolerance in wheat.     

Role of Arabidopsis AtPI4Kγ3, a type II phosphoinositide 4‐kinase,in abiotic stress responses and floral transition

Phosphoinositides (PIs) are essential metabolites which are generated by various lipid kinases and rapidly respond to a variety of environmental stimuli in eukaryotes. One of the precursors of important regulatory PIs, phosphatidylinositol (PtdIn) 4‐phosphate, is synthesized by PtdIns 4‐kinases (PI4K). Despite its wide distribution in eukaryotes, its role in plants remains largely unknown. Here, we show that the activity of AtPI4Kγ3 gene, an Arabidopsis (Arabidopsis thaliana) type II PtdIn 4‐kinase, is regulated by DNA demethylation and some abiotic stresses. AtPI4Kγ3 is targeted to the nucleus and selectively bounds to a few PtdIns. It possessed autophosphorylation activity but unexpectedly had no detectable lipid kinase activity. Overexpression of AtPI4Kγ3 revealed enhanced tolerance to high salinity or ABA along with inducible expression of a host of stress‐responsive genes and an optimal accumulation of reactive oxygen species. Furthermore, overexpressed AtPI4Kγ3 augmented the salt tolerance of bzip60 mutants. The ubiquitin‐like domain of AtPI4Kγ3 is demonstrated to be essential for salt stress tolerance. Besides, AtPI4Kγ3‐overexpressed plants showed a late‐flowering phenotype, which was caused by the regulation of some flowering pathway integrators. In all, our results indicate that AtPI4Kγ3 is necessary for reinforcement of plant response to abiotic stresses and delay of the floral transition.