Transcriptome profile reveals heat response mechanism at molecular and metabolic levels in rice flag leaf.

Flag leaf is one of the key photosynthesis organs during rice reproductive stage. A time course microarray analysis of rice flag leaf was done after 40°C treatment for 0 min, 20 min, 60 min, 2h, 4h, and 8h. The identified significant heat responsive genes were mainly involved in transcriptional regulation, transport, protein binding, antioxidant, and stress response. KMC analysis discovered the time-dependent gene expression pattern under heat. MapMan analysis demonstrated that, under heat treatment, Hsp genes and genes involved in glycolysis and ubiquitin-proteasome were enhanced, and genes involved in TCA, carotenoid, dihydroflavonol and anthocyanin metabolisms and light-reaction in the photosynthesis were widely repressed. Meanwhile, some rate-limiting enzyme genes in shikimate, lignin, and mevalonic acid metabolisms were up-regulated, revealing the importance of maintaining specific secondary metabolites under heat stress. The present study increased our understanding of heat response in rice flag leaf and provided good candidate genes for crop improvement.

Expression and promoter analysis of six heat stress-inducible genes in rice.

During the long evolutionary process, plant gradually formed a series of strategies and mechanisms to cope with stress environment such as drought, heat, cold, and high salinity. Six highly heat responsive genes were identified in rice by microarray data analysis. The qRT-PCR analysis confirmed that the expression of these six genes were highly heat inducible and moderately responded to salt stress, polyethylene glycol, and abscisic acid treatment, but little affected by cold treatment. Promoters of the three highly heat-inducible genes (OsHsfB2cp, PM19p, and Hsp90p) were used to drive GUS gene expression in rice. The results of the GUS gene expression, histochemical staining, and GUS activities in panicles and flag leaves of the transgenic rice plants confirmed high heat-induced GUS activities and moderate drought-induced activities. The three promoters exhibited similar high activity lever in rice leaf under heat, but OsHsfB2cp and PM19p showed much higher activities in panicles under heat stress. Our work confirmed that the OsHsfB2c and PM19 promoters were highly heat inducible and further characterization and reconstruction of cis-elements in their promoters could lead to the development of highly effective heat-inducible promoters for plant genetic engineering.

Expression profile in rice panicle: insights into heat response mechanism at reproductive stage.

Rice at reproductive stage is more sensitive to environmental changes, and little is known about the mechanism of heat response in rice panicle. Here, using rice microarray, we provided a time course gene expression profile of rice panicle at anther developmental stage 8 after 40°C treatment for 0 min, 20 min, 60 min, 2 h, 4 h, and 8 h. The identified differentially expressed genes were mainly involved in transcriptional regulation, transport, cellular homeostasis, and stress response. The predominant transcription factor gene families responsive to heat stress were Hsf, NAC, AP2/ERF, WRKY, MYB, and C(2)H(2). KMC analysis discovered the time-dependent gene expression pattern under heat stress. The motif co-occurrence analysis on the promoters of genes from an early up-regulated cluster showed the important roles of GCC box, HSE, ABRE, and CE3 in response to heat stress. The regulation model central to ROS combined with transcriptome and ROS quantification data in rice panicle indicated the great importance to maintain ROS balance and the existence of wide cross-talk in heat response. The present study increased our understanding of the heat response in rice panicle and provided good candidate genes for crop improvement.