The plasma membrane proton ATPase is an important determinant of salt tolerance in plants

Maintaining ion homeostasis is critical for adaptation in a NaCl stress environment. Induced activities of secondary active ion transporters for extracellular evacuation and vacuolar compartmentation of Na\sp{+} are crucial for lowering cytosolic Na\sp{+} levels. The plasma membrane (PM) proton (H\sp{+})-ATPase generates the H\sp{+} electrochemical potential required by proton-coupled secondary active transporters. Thus, modulation of this pump can exert tight control over Na\sp{+} homeostasis regulation in cells. The transcript level of PM H\sp{+}-ATPase is up-regulated in response to NaCl in suspension cultures of glycophyte tobacco and halophyte Atriplex nummularia. Message accumulation was induced by NaCl in roots of both plants and in expanded leaves of A. nummularia. The gene expression was more responsive to NaCl in the halophyte than in the glycophyte. The high level root-specific induction of message is localized predominantly in the elongation zone. In response to NaCl, the mRNA accumulates more in the protoderm (pre-epidermis) of root apical meristem where vascular tissues are not fully differentiated. Serving as one important barrier of roots for ion uptake, the epidermal cells may function actively in regulation of Na\sp{+} uptake, as demonstrated by the increased PM H\sp{+}-ATPase message. When roots have developed vascular tissues, the induction of the mRNA accumulation is localized predominantly in the endodermis. The endodermis has the important role of restricting apoplastic ion movements in mature roots, and this restriction may be accomplished by secondary active transporters supported by the PM H\sp{+}-ATPase. In expanded A. nummularia leaves, NaCl induction of the mRNA level is localized primarily in the bundle sheath cells, where regulation of Na\sp{+} flux must occur as the xylem unloads Na\sp{+}. The fact that induction of the mRNA accumulation correlates with the enhanced pump activity (Braun et al., 1986) indicates an important role of its gene regulation in plant adaptation to salt stress. The PM H\sp{+}-ATPase is actively involved in cells where regulation of Na\sp{+} homeostais occurs, and the capacity to regulate this gene in response to NaCl may be a determinant of salt tolerance

lon Homeostasis in NaCl Stress Environments.

8 páginas, 1 tabla, 1 figura, 61 referencias. This is journal article No. 14,705 from the Purdue Agricultura1 Experiment Station.Homeostasis can be defined as the tendency of a cell or an organism to maintain interna1 steady state, even in response to any environmental perturbation or stimulus tending to disturb normality, because of the coordinate responses of its constituent components. Typically, ions constantly flux in and out of cells in a controlled fashion with net flux adjusted to accommodate cellular requirements, thus creating an ionic homeostasis. When plant cells are exposed to salinity, mediated by high NaCl concentrations, kinetic steady states of ion transport for Na+ and C1- and other ions, such as Kt and Ca2+, are disturbed (Binzel et al., 1988). High apoplastic levels of Na+ and C1- alter aqueous and ionic thermodynamic equilibria, resulting in hyperosmotic stress, ionic imbalance, and toxicity. Thus, it is vital for the plant to re-establish cellular ion homeostasis for metabolic functioning and growth, that is, to adapt to the saline environment.This research was supported in part by U.S. Department of Agriculture/National Research Initiative Competitive Grants Program grant No. 92-37100 -7738, and by Comision Interministerial de Ciencia y Tecnología grant No. BI094-0622.Peer reviewe

Shared and genetically distinct Zea mays transcriptome responses to ongoing and past low temperature exposure

Abstract Background Cold temperatures and their alleviation affect many plant traits including the abundance of protein coding gene transcripts. Transcript level changes that occur in response to cold temperatures and their alleviation are shared or vary across genotypes. In this study we identify individual transcripts and groups of functionally related transcripts that consistently respond to cold and its alleviation. Genes that respond differently to temperature changes across genotypes may have limited functional importance. We investigate if these genes share functions, and if their genotype-specific gene expression levels change in magnitude or rank across temperatures. Results We estimate transcript abundances from over 22,000 genes in two unrelated Zea mays inbred lines during and after cold temperature exposure. Genotype and temperature contribute to many genes’ abundances. Past cold exposure affects many fewer genes. Genes up-regulated in cold encode many cytokinin glucoside biosynthesis enzymes, transcription factors, signalling molecules, and proteins involved in diverse environmental responses. After cold exposure, protease inhibitors and cuticular wax genes are newly up-regulated, and environmentally responsive genes continue to be up-regulated. Genes down-regulated in response to cold include many photosynthesis, translation, and DNA replication associated genes. After cold exposure, DNA replication and translation genes are still preferentially downregulated. Lignin and suberin biosynthesis are newly down-regulated. DNA replication, reactive oxygen species response, and anthocyanin biosynthesis genes have strong, genotype-specific temperature responses. The ranks of genotypes’ transcript abundances often change across temperatures. Conclusions We report a large, core transcriptome response to cold and the alleviation of cold. In cold, many of the core suite of genes are up or downregulated to control plant growth and photosynthesis and limit cellular damage. In recovery, core responses are in part to prepare for future stress. Functionally related genes are consistently and greatly up-regulated in a single genotype in response to cold or its alleviation, suggesting positive selection has driven genotype-specific temperature responses in maize

Cytokinin Oxidase Gene Expression in Maize Is Localized to the Vasculature, and Is Induced by Cytokinins, Abscisic Acid, and Abiotic Stress

Cytokinins are hormones that play an essential role in plant growth and development. The irreversible degradation of cytokinins, catalyzed by cytokinin oxidase, is an important mechanism by which plants modulate their cytokinin levels. Cytokinin oxidase has been well characterized biochemically, but its regulation at the molecular level is not well understood. We isolated a cytokinin oxidase open reading frame from maize (Zea mays), called Ckx1, and we used it as a probe in northern and in situ hybridization experiments. We found that the gene is expressed in a developmental manner in the kernel, which correlates with cytokinin levels and cytokinin oxidase activity. In situ hybridization with Ckx1 and transgenic expression of a transcriptional fusion of the Ckx1 promoter to the Escherichia coli β-glucuronidase reporter gene revealed that the gene is expressed in the vascular bundles of kernels, seedling roots, and coleoptiles. We show that Ckx1 gene expression is inducible in various organs by synthetic and natural cytokinins. Ckx1 is also induced by abscisic acid, which may control cytokinin oxidase expression in the kernel under abiotic stress. We hypothesize that under non-stress conditions, cytokinin oxidase in maize plays a role in controlling growth and development via regulation of cytokinin levels transiting in the xylem. In addition, we suggest that under environmental stress conditions, cytokinin oxidase gene induction by abscisic acid results in aberrant degradation of cytokinins therefore impairing normal development

Diacylglycerol acyltransferase 1 contributes to freezing tolerance

Freezing limits plant growth and crop productivity, and plant species in temperate zones have the capacity to develop freezing tolerance through complex modulation of gene expression affecting various aspects of metabolism and physiology. While many components of freezing tolerance have been identified in model species under controlled laboratory conditions, little is known about the mechanisms that impart freezing tolerance in natural populations of wild species. Here, we performed a quantitative trait locus (QTL) study of acclimated freezing tolerance in seedlings of Boechera stricta, a highly adapted relative of Arabidopsis thaliana native to the Rocky Mountains. A single QTL was identified that contained the gene encoding ACYL-COA:DIACYLGLYCEROL ACYLTRANSFERASE 1 (BstDGAT1), whose expression is highly cold responsive. The primary metabolic enzyme DGAT1 catalyzes the final step in assembly of triacylglycerol (TAG) by acyl transfer from acyl-CoA to diacylglycerol. Freezing tolerant plants showed higher DGAT1 expression during cold acclimation than more sensitive plants and this resulted in increased accumulation of TAG in response to subsequent freezing. Levels of oligogalactolipids which are produced by SFR2 (SENSITIVE TO FREEZING 2), an indispensable element of freezing tolerance in Arabidopsis, were also higher in freezing tolerant plants. Furthermore, overexpression of AtDGAT1 led to increased freezing tolerance. We propose that DGAT1 confers freezing tolerance in plants by supporting SFR2-mediated remodeling of chloroplast membranes