Drought poses a serious threat to the sustainability of rice (Oryza sativa) yields in rain-fed agriculture. Here, we report the results of a functional genomics approach that identified a rice NAC (an acronym for NAM [No Apical Meristem], ATAF1-2, and CUC2 [Cup-Shaped Cotyledon]) domain gene, OsNAC10, which improved performance of transgenic rice plants under field drought conditions. Of the 140 OsNAC genes predicted in rice, 18 were identified to be induced by stress conditions. Phylogenic analysis of the 18 OsNAC genes revealed the presence of three subgroups with distinct signature motifs. A group of OsNAC genes were prescreened for enhanced stress tolerance when overexpressed in rice. OsNAC10, one of the effective members selected from prescreening, is expressed predominantly in roots and panicles and induced by drought, high salinity, and abscisic acid. Overexpression of OsNAC10 in rice under the control of the constitutive promoter GOS2 and the root-specific promoter RCc3 increased the plant tolerance to drought, high salinity, and low temperature at the vegetative stage. More importantly, the RCc3: OsNAC10 plants showed significantly enhanced drought tolerance at the reproductive stage, increasing grain yield by 25% to 42% and by 5% to 14% over controls in the field under drought and normal conditions, respectively. Grain yield of GOS2: OsNAC10 plants in the field, in contrast, remained similar to that of controls under both normal and drought conditions. These differences in performance under field drought conditions reflect the differences in expression of OsNAC10-dependent target genes in roots as well as in leaves of the two transgenic plants, as revealed by microarray analyses. Root diameter of the RCc3: OsNAC10 plants was thicker by 1.25-fold than that of the GOS2:OsNAC10 and nontransgenic plants due to the enlarged stele, cortex, and epidermis. Overall, our results demonstrated that root-specific overexpression of OsNAC10 enlarges roots, enhancing drought tolerance of transgenic plants, which increases grain yield significantly under field drought conditions. Plants respond and adapt to abiotic stresses to survive under adverse conditions. Upon exposure of plants to such stresses, many genes are induced, and their products are involved in the protection of cellular machinery from stress-induced damage (Bray, 1993;Thomashow, 1999;Shinozaki et al., 2003). The expression of stress-related genes is largely regulated by specific transcription factors. The overexpression of such transcription factor genes often results in activation of many functional genes related to the particular stress conditions, consequently conferring stress tolerance. For example, the DREB1A/CBF3 gene in transgenic Arabidopsis (Arabidopsis thaliana) activates expression of its stress-related downstream genes, thereby enhancing stress tolerance (Liu et al., 1998;Kasuga et al., 1999).The rice (Oryza sativa) and Arabidopsis genomes each encode more than 1,300 transcriptional regulators, which account for 6% of the estimated total nu...
SummaryDrought conditions are among the most serious challenges to crop production worldwide. Here, we report the results of field evaluations of transgenic rice plants overexpressing OsNAC5, under the control of either the root-specific (RCc3) or constitutive (GOS2) promoters. Field evaluations over three growing seasons revealed that the grain yield of the RCc3:OsNAC5 and GOS2: OsNAC5 plants were increased by 9%-23% and 9%-26% under normal conditions, respectively. Under drought conditions, however, RCc3:OsNAC5 plants showed a significantly higher grain yield of 22%-63%, whilst the GOS2:OsNAC5 plants showed a reduced or similar yield to the nontransgenic (NT) controls. Both the RCc3:OsNAC5 and GOS2:OsNAC5 plants were found to have larger roots due to an enlarged stele and aerenchyma at flowering stage. Cell numbers per cortex layer and stele of developing roots were higher in both transgenic plants than NT controls, contributing to the increase in root diameter. The root diameter was enlarged to a greater extent in the RCc3:OsNAC5, suggesting the importance of this phenotype for enhanced drought tolerance. Microarray experiments identified 25 up-regulated genes by more than three-fold (P < 0.01) in the roots of both transgenic lines. Also identified were 19 and 18 up-regulated genes that are specific to the RCc3:OsNAC5 and GOS2:OsNAC5 roots, respectively. Of the genes specifically up-regulated in the RCc3:OsNAC5 roots, GLP, PDX, MERI5 and O-methyltransferase were implicated in root growth and development. Our present findings demonstrate that the root-specific overexpression of OsNAC5 enlarges roots significantly and thereby enhances drought tolerance and grain yield under field conditions.
SummaryDrought conditions limit agricultural production by preventing crops from reaching their genetically predetermined maximum yields. Here, we present the results of field evaluations of rice overexpressing OsNAC9, a member of the rice NAC domain family. Root-specific (RCc3) and constitutive (GOS2) promoters were used to overexpress OsNAC9 and produced the transgenic RCc3:OsNAC9 and GOS2:OsNAC9 plants. Field evaluations over two cultivating seasons showed that grain yields of the RCc3:OsNAC9 and the GOS2:OsNAC9 plants were increased by 13%-18% and 13%-32% under normal conditions, respectively. Under drought conditions, RCc3:OsNAC9 plants showed an increased grain yield of 28%-72%, whilst the GOS2:OsNAC9 plants remained unchanged. Both transgenic lines exhibited altered root architecture involving an enlarged stele and aerenchyma. The aerenchyma of RCc3:OsNAC9 roots was enlarged to a greater extent than those of GOS2:OsNAC9 and non-transgenic (NT) roots, suggesting the importance of this phenotype for enhanced drought resistance. Microarray experiments identified 40 up-regulated genes by more than threefold (P < 0.01) in the roots of both transgenic lines. These included 9-cis-epoxycarotenoid dioxygenase, an ABA biosynthesis gene, calcium-transporting ATPase, a component of the Ca 2+ signalling pathway involved in cortical cell death and aerenchyma formation, cinnamoyl CoA reductase 1, a gene involved in lignin biosynthesis, and wall-associated kinases¸genes involved in cell elongation and morphogenesis. Interestingly, O-methyltransferase, a gene necessary for barrier formation, was specifically up-regulated only in the RCc3:OsNAC9 roots. Such up-regulated genes that are commonly and specifically up-regulated in OsNAC9 transgenic roots may account for the altered root architecture conferring increased drought resistance phenotype.
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