Expression and Functional Roles of Cytokinin Response Factor Proteins in Arabidopsis thaliana

Abstract

Abstract

Cytokinin response factors (CRFs) are transcription factor proteins from the model plant Arabidopsis thaliana (Arabidopsis). CRFs comprise a small, highly conserved subgroup of the AP2/ERF family of plant specific transcription factors. CRF6 is among the first identified members of this group, which were found to be transcriptionally regulated by the plant hormone cytokinin. Cytokinins play key regulatory roles in a wide range of growth and developmental processes from embryogenesis to senescence. Examination of the evolutionary relationships among CRF proteins in Angiosperms reveals a high degree of conservation within five distinct CRF lineages. Expression analyses of the 12 Arabidopsis CRFs demonstrate that all are expressed predominantly in vascular tissues with differences in organ level and developmental patterns. Functional characterization of CRF6 revealed that this gene acts downstream of cytokinin signaling to delay leaf senescence, a process classically linked to cytokinin application. Specifically, the inhibition of leaf senescence known to occur as a result of treatment with exogenous cytokinin is greatly decreased in mutant plants lacking a functional CRF6 gene. In contrast, over-expression of CRF6 leads to delayed leaf senescence without cytokinin treatment. Finally, transcriptome wide analyses indicate that CRF6 acts to repress a set of cytokinin related genes in response to oxidative stress possibly as an adaptive response to such stress. This proposed function is supported by an enhanced stress tolerance phenotype of transgenic plants over-expressing CRF6. Additionally, literature concerning cytokinin regulation of leaf senescence and interactions between cytokinin and stress response will be reviewed and discussed.