Plants retain rhythmic physiological responses when adapting to environmental challenges. However, possible integrations between drought conditions and those responses have not received much focus, especially regarding crop plants, and the relationship between abiotic stress and the diurnal cycle is generally not considered. Therefore, we conducted a genome-wide analysis to identify genes showing both diurnal regulation and water-deficiency response in rice (Oryza sativa). Among the 712 drought-responsive genes primary identified, 56.6% are diurnally expressed while 47.6% of the 761 that are down-regulated by drought are also diurnal. Using the β-glucuronidase reporter system and qRT-PCR analyses, we validated expression patterns of two candidate genes, thereby supporting the reliability of our transcriptome data. MapMan analysis indicated that diurnal genes up-regulated by drought are closely associated with the starch-sucrose pathway while those that are down-regulated are involved in photosynthesis. We then confirmed that starch-sucrose contents and chlorophyll fluorescence are altered in a diurnal manner under drought stress, suggesting these metabolic diurnal alterations as a novel indicator to evaluate the drought response in rice leaves. We constructed a functional gene network associated with the starch-sucrose KEGG metabolic pathway for further functional studies, and also developed a regulatory pathway model that includes OsbZIP23 transcription factor.
The detrimental effect of high salinity on crop production is a serious problem. However, the number of genes with known functions relating to salinity tolerance is very limited in rice. To effectively address this limitation, selection of useful candidate genes and identification of major regulatory factors through global approaches are necessary. To this end, we used three data series of affymetrix array data produced with salt-treated samples from NCBI Gene Expression Omnibus (http://www.ncbi.nlm. nih.gov/geo/) and identified 653 rice genes commonly differentially expressed under three salt-stress conditions. While evaluating the quality of selected candidate genes for salt-stress responses, Gene ontology enrichment analysis revealed that responses to salt and water stresses of biological process category are highly overrepresented in salt-stress conditions. In addition, the major salt stress-responsive metabolism process and regulatory gene modules are classified through MapMan analysis, and detailed elements for further studies are suggested. Based on this, we proposed a salt stress-responsive signaling pathway in rice. The functional analysis of the main signal transduction and transcription regulation factors identified in this pathway will shed light on a novel regulatory metabolism process that can be manipulated to develop crops with enhanced salinity tolerance.
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