It is well known that abscisic acid (ABA)-induced leaf senescence and premature leaf senescence negatively affect the yield of rice (Oryza sativa). However, the molecular mechanism underlying this relationship, especially the upstream transcriptional network that modulates ABA level during leaf senescence, remains largely unknown. Here, we demonstrate a rice NAC transcription factor, OsNAC2, that participates in ABA-induced leaf senescence. Overexpression of OsNAC2 dramatically accelerated leaf senescence, whereas its knockdown lines showed a delay in leaf senescence. Chromatin immunoprecipitation-quantitative PCR, dual-luciferase, and yeast one-hybrid assays demonstrated that OsNAC2 directly activates expression of chlorophyll degradation genes, OsSGR and OsNYC3. Moreover, ectopic expression of OsNAC2 leads to an increase in ABA levels via directly up-regulating expression of ABA biosynthetic genes (OsNCED3 and OsZEP1) as well as down-regulating the ABA catabolic gene (OsABA8ox1). Interestingly, OsNAC2 is upregulated by a lower level of ABA but downregulated by a higher level of ABA, indicating a feedback repression of OsNAC2 by ABA. Additionally, reduced OsNAC2 expression leads to about 10% increase in the grain yield of RNAi lines. The novel ABA-NAC-SAGs regulatory module might provide a new insight into the molecular action of ABA to enhance leaf senescence and elucidates the transcriptional network of ABA production during leaf senescence in rice.Senescence is the last stage of leaf development. During this period, various changes occur at the physiological, biochemical, and molecular levels. For example, macromolecules including lipids, proteins, and nucleic acids are hydrolyzed, which leads to disassembly of mitochondria and nuclei, and to cell death (Buchanan-Wollaston et al., 2005;Ulker et al., 2007). Although senescence is an active process to salvage nutrients from old tissues, precocious senescence will shorten the growth stage of crops and be unfavorable to agronomic production (Woo et al., 2013).The most distinguishing feature in leaf senescence is the yellowing phenotype, which is a visible marker of the degradation of macromolecules (Kim et al., 2006). The chlorophyll degradation pathway is one of the most characterized ones for macromolecule degradation in plants (Hörtensteiner, 2006). Overexpressing NON-YELLOW COLORING1 (NYC1) or NYC1-like genes in rice (Oryza sativa) can induce degradation of chlorophyll . A pph (encoding pheophytinase) mutant is abnormal in chlorophyll degradation during senescence and therefore exhibits a stay-green phenotype (Schelbert et al., 2009). Mutation of the PAO (Pheophorbide a oxygenase) gene leads to retention of chlorophyll in leaves during dark-induced senescence in Arabidopsis (Arabidopsis thaliana; Pruzinská et al., 2005). Recently, the highly conserved STAY-GREEN (SGR) in higher plants has been identified to be chloroplast-localized dechelatase (Shimoda et al., 2016).The senescence process is highly regulated by a range of important factors. It has been demon...

Plants respond to environmental stresses by altering gene expression, and several genes have been found to mediate stress-induced expression, but many additional factors are yet to be identified. OsNAP is a member of the NAC transcription factor family; it is localized in the nucleus, and shows transcriptional activator activity in yeast. Analysis of the OsNAP transcript levels in rice showed that this gene was significantly induced by ABA and abiotic stresses, including high salinity, drought and low temperature. Rice plants overexpressing OsNAP did not show growth retardation, but showed a significantly reduced rate of water loss, enhanced tolerance to high salinity, drought and low temperature at the vegetative stage, and improved yield under drought stress at the flowering stage. Microarray analysis of transgenic plants overexpressing OsNAP revealed that many stress-related genes were up-regulated, including OsPP2C06/OsABI2, OsPP2C09, OsPP2C68 and OsSalT, and some genes coding for stress-related transcription factors (OsDREB1A, OsMYB2, OsAP37 and OsAP59). Our data suggest that OsNAP functions as a transcriptional activator that plays a role in mediating abiotic stress responses in rice.

The brassinosteroids (BRs) represent a class of phytohormones, which regulate numerous aspects of growth and development. Here, a det2-9 mutant defective in BR synthesis was identified from an EMS mutant screening for defects in root length, and was used to investigate the role of BR in root development in Arabidopsis. The det2-9 mutant displays a short-root phenotype, which is result from the reduced cell number in root meristem and decreased cell size in root maturation zone. Ethylene synthesis is highly increased in the det2-9 mutant compared with the wild type, resulting in the hyper-accumulation of ethylene and the consequent inhibition of root growth. The short-root phenotype of det2-9 was partially recovered in the det2-9/acs9 double mutant and det2-9/ein3/eil1-1 triple mutant which have defects either in ethylene synthesis or ethylene signaling, respectively. Exogenous application of BR showed that BRs either positively or negatively regulate ethylene biosynthesis in a concentration-dependent manner. Different from the BR induced ethylene biosynthesis through stabilizing ACSs stability, we found that the BR signaling transcription factors BES1 and BZR1 directly interacted with the promoters of ACS7, ACS9 and ACS11 to repress their expression, indicating a native regulation mechanism under physiological levels of BR. In addition, the det2-9 mutant displayed over accumulated superoxide anions (O2-) compared with the wild-type control, and the increased O2- level was shown to contribute to the inhibition of root growth. The BR-modulated control over the accumulation of O2- acted via the peroxidase pathway rather than via the NADPH oxidase pathway. This study reveals an important mechanism by which the hormone cross-regulation between BRs and ethylene or/and ROS is involved in controlling root growth and development in Arabidopsis.

These authors contributed equally to the work. SUMMARYPlant height and flowering time are key agronomic traits affecting yield in rice (Oryza sativa). In this study, we investigated the functions in rice growth and development of OsNAC2, encoding a NAC transcription factor in rice. Transgenic plants that constitutively expressed OsNAC2 had shorter internodes, shorter spikelets, and were more insensitive to gibberellic acid (GA 3 ). In addition, the levels of GAs decreased in OsNAC2 overexpression plants, compared with the wild-type. Moreover, flowering was delayed for approximately 5 days in transgenic lines. The transcription of Hd3a, a flowering-time related gene, was suppressed in transgenic lines. In addition, transgenic Arabidopsis plants expressing OsNAC2 were also more insensitive to GA 3 . The expression levels of GA biosynthetic genes OsKO2 and OsKAO were repressed. The expression of OsSLRL, encoding a repressor in the GA signal pathway, and OsEATB, which encodes a repressor of GA biosynthesis, were both enhanced. Western blotting indicated that DELLA also accumulated at the protein level. Dual-luciferase reporter analyses, yeast one-hybrid assays and ChIP-qPCR suggested that OsNAC2 directly interacted with the promoter of OsEATB and OsKO2. Taken together, we proposed that OsNAC2 is a negative regulator of the plant height and flowering time, which acts by directly regulating key genes of the GA pathway in rice.

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