Age-dependent action of an ABA-inducible receptor kinase, RPK1, as a positive regulator of senescence in Arabidopsis leaves.

Leaf senescence, which constitutes the final stage of leaf development, involves programmed cell death and is intricately regulated by various internal and environmental signals that are incorporated with age-related information. ABA plays diverse and important physiological roles in plants, and is involved in various developmental events and stress responses. ABA has long been regarded as a positive regulator of leaf senescence. However, the cellular mediators of ABA-induced senescence have not been identified. We sought to understand the ABA-induced senescence signaling process in Arabidopsis by examining the function of an ABA- and age-induced gene, RPK1, which encodes a membrane-bound, leucine-rich repeat-containing receptor kinase (receptor protein kinase 1). Loss-of-function mutants in RPK1 were significantly delayed in age-dependent senescence. Furthermore, rpk1 mutants exhibited reduced sensitivity to ABA-induced senescence but little change to jasmonic acid- or ethylene-induced senescence. RPK1 thus mediates ABA-induced leaf senescence as well as age-induced leaf senescence. Conditional overexpression of RPK1 at the mature stage clearly accelerated senescence and cell death, whereas induction of RPK1 at an early developmental stage retarded growth without triggering senescence symptoms. Therefore, RPK1 plays different roles at different stages of development. Consistently, exogenously applied ABA affected leaf senescence in old leaves but not in young leaves. The results, together, showed that membrane-bound RPK1 functions in ABA-dependent leaf senescence. Furthermore, the effect of ABA and ABA-inducible RPK1 on leaf senescence is dependent on the age of the plant, which in part explains the mechanism of functional diversification of ABA action.