Cytokinin Response Factor 6 Negatively Regulates Leaf Senescence and is Induced in Response to Cytokinin and Numerous Abiotic Stresses

Cytokinin response factor 6 (CRF6) is an Arabidopsis AP2/ERF transcription factor which is transcriptionally induced by cytokinin. Cytokinin is known to delay leaf senescence in wild-type (WT) plants, for example in dark-incubated detached leaves. This response is mediated by the cytokinin receptor Arabidopsis histidine kinase receptor 3 (AHK3). Similar to ahk3 mutants, crf6 leaves show decreased sensitivity to this cytokinin effect. Leaves overexpressing CRF6 retain more Chl than those of the WT under these conditions without exogenous cytokinin. It therefore appears that an increase in expression of CRF6 downstream of the perception of cytokinin by AHK3 is involved in the delay of leaf senescence. Intact crf6 plants also begin to undergo monocarpic senescence sooner than WT plants. Interestingly, plants overexpressing CRF6 display a more extreme acceleration of development than crf6 mutants, suggesting that a specific expression level or localization of CRF6 is necessary to prevent premature senescence. Expression analyses indicate that CRF6 is highly expressed in the veins of mature leaves and that this expression decreases with age. CRF6 expression is shown to be induced by abiotic stress, in addition to increased cytokinin. Together, these findings suggest that CRF6 functions to regulate developmental senescence negatively and may have a similar role in response to stress. CRF6 may therefore be involved in fine-tuning the timing of developmental and stress-induced senescence. CRF6 functioning in negative regulation of senescence is significant in that it is the first process known to be regulated by cytokinin, in which a CRF can be placed specifically downstream of the cytokinin signaling pathway.