Hormone-mediated nitrogen signaling in plants: implication of participation of abscissic acid in negative regulation of cytokinin-inducible expression of maize response regulator

In addition to nitrate ion itself, plant growth regulators such as cytokinin function to communicate nitrogen availability between organs. The cytokinin-mediated nitrogen signal is transmitted by a His-Asp phosphorelay system in the target cells. Although mutual crosstalk among the growth regulators in various aspects has been reported, the effect of other hormones on the cytokinin-mediated nitrogen signaling has not been characterized well. In maize (Zea mays L.), abscissic acid (ABA) concentration in tissue is affected by nitrogen availability: it is remarkably higher in the aged leaves than younger leaves under a nitrogen-limited condition, whereas the tendency is reversed in a sufficient condition. The delivery rate of ABA from root to shoot via xylem is also increased under a nitrogen-limited cultivation. These profiles of ABA are reciprocal to those of cytokinin. Cytokinin-dependent induction of ZmRR 1 and ZmRR 2, response regulators, is repressed by co-treatment with ABA in a dose-dependent manner. These results imply that ABA participates in cytokinin-mediated nitrogen signaling by means of negative regulation of ZmRRs.