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Rice ERF OsEATB restricts GA biosynthesis - Plant Physiology

Rice ERF OsEATB restricts GA biosynthesis - Plant Physiology

down-regulating a key

down-regulating a key enzyme involved in GA biosynthesis, not through up- regulating the important GA sensitivity repressor (Figure 4). The Arabidopsis GA-deficient mutant dwarf and delayed- flowering (ddf1) phenotypes are caused by increased or ectopic expression of a putative AP2 transcription factor DDF1. DDF1 is involved in the down-regulation of GA biosynthesis (Hiroshi et al., 2004). As a potential negative balance regulator of the ethylene-responsive pathway, OsEATB suppresses the internode elongation process through restriction of GA biosynthesis, specifically down-regulating the expression of OsCPS2 (Table Ⅰ,Fig. 3). The mutation line of ent-kaurene synthase A OsCPS1 showed a dwarf phenotype without flower or seed development, which is a typical phenotype of GA-deficient rice dwarf mutants (Tomoaki et al., 2004). Evidence is presented indicating that OsCPS2 is involved in related secondary metabolism producing defensive phytochemicals (Sladjana et al., 2004).The expression of OsCPS2 is sharply negative-related with OsEATB expression, and the endogenous GA level is decreased in OsEATB transgenic plants alongside suppressed expression of OsCPS2 (Table Ⅱ). No significant differences between the OsEATB transgenic seedlings and wild type in expression of OsCPS1, OsKS1, OsKO2, and OsKAO were found, and the expression of GA20ox2 was slightly up-regulated for the feedback of GA deficiency. All these experimental results strongly demonstrated that OsCPS2 is also associated with GA biosynthesis. Promoter regions (1,300 bp upstream of the translation site) of OsCPS2 do not contain the GCC-box motif, suggesting that this gene may not be a direct target of OsEATB. We propose a model for OsEATB-dependent hormonal regulation of internode elongation in rice (Fig. 7). It was suggested that the important function of OsEATB is to negatively regulate ethylene-induced enhancement of GA responsiveness during the internode elongation process by decreasing GA biosynthesis. 18

OsEATB regulates rice yield components through promotion of tillering and panicle branching Food security for the ever-increasing world population largely relies on the grain yield of crop plants (Xue et al., 2008). The critical components to determine rice yield include the grain number and grain weight. Grain number is contingent on the number of spikelets per panicle and the number of panicles per plant. The number of panicles is mainly determined by the plant architecture and the spikelets per panicle by panicle morphology (i.e. the number of primary/secondary/tertiary branches on each panicle) (Zha et al., 2009). Over-expression of OsEATB decreased plant height (Fig. 3), and increased the numbers of panicles per plant and spikelets per panicle (Fig. 6). More panicles and spikelets resulted in a 37.37% increase in grain yield notwithstanding the 1000-grain weight was 15.00% lower in transgenic lines than the control (Fig. 6). The results of this study show that OsEATB, as a member of AP2/ERF family, positively regulates rice yield components through promotion of rice tillering and panicle branching. The basic structure of a rice panicle is determined by the pattern of branch formation. In rice, the MONO CULM 1 (MOC1) and LAX PANICLE (LAX) genes are necessary for branch meristem formation (Furutani et al., 2006). MOC1 positively regulates tillering by promoting axillary meristem outgrowth (Li et al., 2003; Leyser, 2005). Although the molecular mechanisms that underlie the crosstalk between plant height and branching are poorly understood, it is well known that rice plant height is strongly negatively correlated with tiller number (Hong et al., 2003; Booker et al., 2004). Higher yields are typically obtained from dwarf crops (Spielmeyer et al., 2002). The finding that transgenic rice plants harboring the MOC1 gene are dwarf but produce more tillers than wild-type plants (Li et al., 2003) provides a good opportunity to investigate the genetic control network. Our results indicate that over-expression of OsEATB gene (Fig. 2) reduces rice plant height and panicle length at maturity, promoting rice branching potential in both tillers and 19

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