Cytokinins Interact with Elements of the Circadian Clock
Elements of the two-component cytokinin response pathway also play a role in the circadian clock, which entrains plants to light/dark diurnal cycles. Disruption of two type-A ARRs (ARR3 and ARR4) resulted in a longer circadian period relative to wild-type plants (Salomé et al. 2005). Surprisingly, the altered periodicity of the arr3arr4 double mutant was suppressed by a reduction in ARR8 and ARR9 function, even though no circadian period phenotype was observed in the arr8arr9 double mutant themselves. This suggests that these type-A ARRs can act antagonistically in this response. Intriguingly, the expression of the ARR9 gene (but none of the other two-component genes) displays a strong circadian oscillation, and the timing of this expression is controlled by the major clock components (CCA1, LHY, and TOC1; see textbook Chapter 17) (Ishida et al. 2008). A mechanistic link for this effect is suggested by the observation that cytokinin induces the expression of LHY and CCA1 genes, but represses the expression of TOC1 (Zheng et al. 2006). Thus, there appears to be an interdependent regulatory loop between the clock genes and cytokinin response genes (i.e., ARR9).