Regulatory Roles of CORONATINE INSENSITIVE 1b (OsCOI1b) in Rice Leaf Senescence

Abstract

Jasmonic acid (JA) is an important phytohormone involved in several developmental processes, including senescence and plant immunity. CORONATINE INSENSITIVE 1 (COI1), encoding a JA receptor, is one of the central factors in JA-responsive signaling pathway in Arabidopsis (Arabidopsis thaliana). Arabidopsis harbors a single COI gene, termed COI1, and its molecular-genetic and physiological functions in JA signaling have been extensively studied using coi1 mutants. However, rice (Oryza sativa) has three COI homologs, termed OsCOI1a, OsCOI1b, and OsCOI2. The phylogenetic analysis suggests that the three OsCOI genes arose from two different duplication events after monocot-dicot divergence. However, it still remains elusive whether each OsCOI has distinct, synergistic, or redundant functions in rice development. Here, we show that using the T-DNA insertion oscoi1b-1 knockout mutant, OsCOI1b is mainly associated with leaf senescence under senescence-promoting conditions. The detached leaves of oscoi1b-1 mutant exhibited a strong stay-green phenotype in dark-induced senescence conditions and delayed leaf senescence in natural field conditions, with substantial retention of chlorophylls and photosynthetic capacity. Furthermore, several senescence-associated genes, including senescence-inducing hormone-related genes were down-regulated in oscoi1b-1 mutants, including ETHYLENE INSENSITIVE 3 (EIN3) and ORESARA1 (ORE1), one of the important cascade of leaf senescence in Arabidopsis. These results strongly suggest that JA-responsive signaling crosstalk with ethylene-responsive signaling to promote leaf senescence. Among the agronomic traits, seed weight and spikelet fertility were significantly decreased in the stay-green oscoi1b-1 mutant, leading to severe reduction of grain yield, indicating that OsCOI1b function or JA signaling is involved in the spikelet fertility and seed filling. Possible divergent roles of the three OsCOI homologs are discussed.