Cytokinin availability and chloroplast functional lifetime in senescing leaves.

Leaf senescence is often characterized by visible yellowing, reflecting the degradation of photosynthetic pigments and the functional deterioration of photosynthetic complexes, marking a shift from photosynthetic maintenance to nutrient remobilization. Cytokinins are key modulators in this process. This review examines cytokinin action in mature Arabidopsis leaves from the perspective of chloroplast functional lifetime, focusing on how long-distance supply, local metabolism and candidate transport routes may shape cytokinin availability and action during leaf senescence. Cytokinin signaling is further considered in relation to nitrogen status, because chloroplasts are both sites of photosynthetic carbon assimilation and major reservoirs of remobilizable nitrogen. As senescence progresses, chloroplasts are not merely passive targets of degradation. Declining photosynthetic function can generate retrograde signals, alter reactive oxygen species homeostasis, and promote transcriptional programs that accelerate dismantling of the photosynthetic apparatus. Within this framework, cytokinins help preserve chlorophyll content, photosynthetic complexes, Photosystem II function, and chloroplast-related gene expression while buffering redox- and light-dependent feedbacks. Cytokinin action in mature leaves is therefore discussed here mainly in the context of spatiotemporal regulation of chloroplast functional lifetime. Cytokinin availability, nitrogen status, and light-dependent feedback from the photosynthetic apparatus jointly influence when photosynthetic maintenance gives way to chloroplast dismantling and nutrient remobilization. This perspective also reconciles recent evidence from detached Arabidopsis leaves that cytokinins can transiently repress PSII photochemistry under prolonged darkness while delaying subsequent chlorophyll loss, adding another layer to cytokinin-mediated protection during early stages of senescence.