Abstract: We report an analysis of salt-stress responses in the monocotyledonous halophyte Festuca rubra ssp. litoralis. Salt-dependent expression of transcripts encoding a PIP2;1 aquaporin, V-ATPase subunit B, and the Na+/H+ antiporter NHX was characterized. Transcription of FrPIP2;1, FrVHA-B, and FrNHX1 was induced in root tissue of F. rubra ssp. litoralis by salt treatment, and during salt-stress F. rubra ssp. litoralis accumulated sodium in leaves and roots. Cell specificity of FrPIP2;1, FrVHA-B, and FrNHX1 transcription was analyzed by in situ PCR in roots of F. rubra ssp. litoralis. Expression of the genes was localized to the root epidermis, cortex cells, endodermis, and the vascular tissue. In plants treated with 500 mM NaCl, transcripts were repressed in the epidermis and the outer cortex cells, whereas endodermis and vasculature showed strong signals. These data demonstrate that transcriptional regulation of the aquaporin PIP2;1, V-ATPase, and the Na+/H+ antiporter NHX is correlated with salt tolerance in F. rubra ssp. litoralis and suggests coordinated control of ion homeostasis and water status at high salinity in plants. Salt-induced transcript accumulation in F. rubra ssp. litoralis was further monitored by cDNA-arrays with expressed sequence tags derived from a cDNA subtraction library. The salt-regulated transcripts included those involved in the control of gene expression and signal transduction elements such as a serine/threonine protein kinase, an SNF1-related protein kinase, and a WRKY-type transcription factor. Other ESTs with salt-dependent regulation included transcripts encoding proteins that function in metabolism, general stress responses, and defense and transport proteins.
Abstract: Halophytes survive high salinity by using complex adaptive mechanisms. In a search for novel molecular mechanisms involved in salt acclimation, transcript analyses revealed increased expression of a SUI-homologous translation initiation factor eIF-1 in the salt-tolerant grass species Festuca rubra ssp. littoralis but not in rice. Upon analysis of the cell specificity of eIF-1 transcription by in situ polymerase chain reaction (PCR), predominant signals were detected in rice leaf mesophyll. To further examine the role of eIF-1 in salt tolerance, transgenic rice plants were generated that over-express this factor under the control of the CaMV-35S promoter. The eIF-1 over-expressing lines showed improved growth under salt stress that was correlated with maintenance of photosynthetic activity and reduced Na(+) and Cl(-) accumulation in leaves. The transgenic rice lines also activated expression of the vacuolar H(+)-ATPase. In addition, an oxidoreductase that belongs to the aldo/keto reductase family was identified as a gene with modified expression in the eIF-1 over-expressing lines, compared with wild-type rice. Our data suggest that eIF-1 has a central function in salt-stress adaptation in rice by regulating ion accumulation and the intracellular redox status.
Abstract: BACKGROUND: Plants respond to extracellularly perceived abiotic stresses such as low temperature, drought, and salinity by activation of complex intracellular signaling cascades that regulate acclimatory biochemical and physiological changes. Protein kinases are major signal transduction factors that have a central role in mediating acclimation to environmental changes in eukaryotic organisms. In this study, we characterized the function of the sucrose nonfermenting 1-related protein kinase2 (SnRK2) SAPK4 in the salt stress response of rice. RESULTS: Translational fusion of SAPK4 with the green fluorescent protein (GFP) showed subcellular localization in cytoplasm and nucleus. To examine the role of SAPK4 in salt tolerance we generated transgenic rice plants with over-expression of rice SAPK4 under control of the CaMV-35S promoter. Induced expression of SAPK4 resulted in improved germination, growth and development under salt stress both in seedlings and mature plants. In response to salt stress, the SAPK4-overexpressing rice accumulated less Na+ and Cl- and showed improved photosynthesis. SAPK4-regulated genes with functions in ion homeostasis and oxidative stress response were identified: the vacuolar H+-ATPase, the Na+/H+ antiporter NHX1, the Cl- channel OsCLC1 and a catalase. CONCLUSION: Our results show that SAPK4 regulates ion homeostasis and growth and development under salinity and suggest function of SAPK4 as a regulatory factor in plant salt stress acclimation. Identification of signaling elements involved in stress adaptation in plants presents a powerful approach to identify transcriptional activators of adaptive mechanisms to environmental changes that have the potential to improve tolerance in crop plants.
