Auxin promotes Arabidopsis root growth by modulating gibberellin response

The growth of plant organs is influenced by a stream of the phytohormone auxin that flows from the shoot apex to the tip of the root 1 . However, until now it has not been known how auxin regulates the cell proliferation and enlargement that characterizes organ growth. Here we show that auxin controls the growth of roots by modulating cellular responses to the phytohormone gibberellin (GA). GA promotes the growth of plants by opposing the effects of nuclear DELLA protein growth repressors 2‐8 , one of which is Arabidopsis RGA (for repressor of gal-3) 9,10 . GA opposes the action of several DELLA proteins by destabilizing them, reducing both the concentration of detectable DELLA proteins and their growth-restraining effects 9‐14 . We also show that auxin is necessary for GA-mediated control of root growth, and that attenuation of auxin transport or signalling delays the GA-induced disappearance of RGA from root cell nuclei. Our observations indicate that the shoot apex exerts long-distance control on the growth of plant organs through the effect of auxin on GA-mediated DELLA protein destabilization. Auxin mediates at least some of its effects on plant growth by inducing the proteasome-mediated degradation of AUX/IAA proteins 15,16 . However, the mechanism by which auxin regulates growth remains unclear. Furthermore, it is not known how the auxin and GA signalling pathways interact with one another 17 . Here we demonstrate a molecular link between auxin-mediated and GA-mediated growth regulation: auxin controls the growth of Arabidopsis roots through the modulation of the cellular response to GA. Although GA has long been known to regulate shoot growth, its role in the regulation of root growth was less clear. We showed that primary roots of the GA-deficient Arabidopsis ga1-3 mutant 5 are shorter than those of the wild type, whereas GA-treated ga1-3 roots are of a similar length to those of the wild type (Fig. 1a). Thus, GA regulates root growth. Arabidopsis contains genes encoding five DELLA proteins (GAI, RGA, RGL1, RGL2 and RGL3), and genetic analysis has shown that some of these proteins mediate specific components of the GA response 2,3,5‐8 . We showed that roots lacking GAI (gai-t6 ga1-3) or RGA (rga-24 ga1-3) are longer than ga1-3 controls (Fig. 1b), whereas a lack of both GAI and RGA (gai-t6 rga-24 ga1-3) substantially suppresses the ga1-3 phenotype (Fig. 1b). Thus, GAI and RGA act together in the regulation of root (and shoot 5,6 ) growth, as growth repressors whose function is opposed by