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The Ethylene Receptor ETR2 Delays Floral Transition and Affects Starch Accumulation in Rice

Ethylene regulates multiple aspects of plant growth and developmentin dicotyledonous plants; however, its roles in monocotyledonousplants are poorly known. Here, we characterized a subfamilyII ethylene receptor, ETHYLENE RESPONSE2 (ETR2), in rice (Oryzasativa). The ETR2 receptor with a diverged His kinase domainis a Ser/Thr kinase, but not a His kinase, and can phosphorylateits receiver domain. Mutation of the N box of the kinase domainabolished the kinase activity of ETR2. Overexpression of ETR2in transgenic rice plants reduced ethylene sensitivity and delayedfloral transition. Conversely, RNA interference (RNAi) plantsexhibited early flowering and the ETR2 T-DNA insertion mutantetr2 showed enhanced ethylene sensitivity and early flowering.The effective panicles and seed-setting rate were reduced inthe ETR2-overexpressing plants, while thousand-seed weight wassubstantially enhanced in both the ETR2-RNAi plants and theetr2 mutant compared with controls. Starch granules accumulatedin the internodes of the ETR2-overexpressing plants, but notin the etr2 mutant. The GIGANTEA and TERMINAL FLOWER1/CENTRORADIALIShomolog (RCN1) that cause delayed flowering were upregulatedin ETR2-overexpressing plants but downregulated in the etr2mutant. Conversely, the {alpha}-amylase gene RAmy3D was suppressedin ETR2-overexpressing plants but enhanced in the etr2 mutant.Thus, ETR2 may delay flowering and cause starch accumulationin stems by regulating downstream genes.