Exogenous Jasmonic Acid and Cytokinin Antagonistically Regulate Rice Flag Leaf Senescence by Mediating Chlorophyll Degradation, Membrane Deterioration, and Senescence-Associated Genes Expression

Although it is well known that jasmonic acid (JA) and cytokinin (CK) are involved in regulating leaf senescence, the antagonistic mechanisms of JA and CK on leaf senescence are still unknown. To explore the antagonistic effects of JA and CK on leaf senescence, we treated detached rice flag leaves with JA and CK under dark conditions, and evaluated their chlorophyll contents, membrane deterioration, and expression levels of chlorophyll-degradation-related genes (CDRGs) and senescence-associated genes (SAGs). Our results demonstrated that exogenous application of JA promoted chlorophyll degradation by enhancing the expression levels of CDRGs, promoted membrane deterioration by accelerating the increases in lipid peroxidation and membrane permeability, enhanced the expression levels of SAGs, and consequently accelerated rice flag leaf senescence. On the other hand, exogenous application of CK retarded chlorophyll degradation by down-regulating the expression levels of CDRGs, retarded membrane deterioration by retarding the increases in lipid peroxidation and membrane permeability, down-regulated the expression levels of SAGs, and consequently delayed rice flag leaf senescence. Furthermore, the senescence-accelerating effect of a certain concentration of JA was nullified by the senescence-retarding effect of a certain concentration of CK. These results suggested that exogenous applications of JA and CK were able to antagonistically regulate flag leaf senescence by mediating chlorophyll degradation, membrane deterioration, and SAGs expression. In addition, our results suggested that the progression of flag leaf senescence might not only depend on the level of JA or CK but also depend on the balance between JA and CK.