Versatile effect of cytokinin on detached senescing leaves of Arabidopsis in the light

The dark-detached leaves are widely used as a model to investigate senescence-delaying mechanisms of cytokinin (CК). The role of CK during senescence of detached Arabidopsis leaves in the light is less examined, although they provide unique opportunities for the study of the interaction of CK and the photosynthetic apparatus. Within the first four days of incubation of the leaves on water photosynthesis activity, chloroplast gene expression and chlorophyll content declined in all genotypes with a functioning CK receptor AHK3, while anthocyanin level and expression of its regulatory and biosynthesis genes increased. Application of exogenous CK visibly retarded the senescence symptoms but decreased photosystem II function, anthocyanin accumulation and promoted ROS production. The hormone stimulated starch accumulation in the chloroplasts and altered the expression of the gene for the extracellular cell wall invertase CWINV1. Although the AHK3 receptor played the most important role in the CK action, we also observed specific effects in loss-of function mutants for phosphortransmitters and type B response regulators. The results suggest that CK action in detached Arabidopsis leaves in the light is superimposed on modified carbon fluxes and starch turnover which results in an intricate combination of stress-related and senescence inhibiting effects.