Regulation of Root Growth by Plant Hormones—Roles for Auxin and Gibberellin

Abstract

Plant hormones are important biotic factors to regulate root growth. Among the seven kinds of plant hormones, auxin and gibberellin (GA) are strong accelerators of shoot growth, but these are not always accelerators for root growth. The classical views of root-growth regulation by auxin and gibberellin are summarized and current theory of the regulation mechanism is described in this review. The concentration-dependent deceleration of root growth is a key to understanding the auxin action on roots, since the endogenous concentration of indole-3-acetic acid (IAA) is inversely proportional to the growth rate. As massive IAA is transported from shoots to roots by polar transport, the influx speed of IAA mainly controls IAA levels in root cells. The classical view of IAA transport in roots has been supported by recent discoveries of IAA-carrier proteins such as AUX1, PINs and MDRs. The role of plasma membrane-located H+-ATPase and its regulation by IAA has also been described for the acid growth phenomenon caused by the acidification of root cell walls. Compared to auxins, GA functions in roots are less remarkable. Nevertheless, GA also plays an indispensable role in the normal development of roots, since artificial GA-depletion causes abnormal expansion and suppression of root elongation. The GA-requirement for normal root growth was unveiled by the use of chemical inhibitors and mutants of GA biosynthesis. GA function that keeps root morphology long and slender is ascribed to the arrangement of cortical microtubules, cellulose microfibrils and unknown additional factor(s). Cross talks among plant hormones were recently found in the signal transduction pathways mainly in aerial organs. GA and IAA de-repress gene expression by degrading the gene-repressing proteins via the ubiquitin-mediated proteasome system. Another interaction of IAA and GA in growth regulation is the enhancement of GA1 level by IAA. Since the final biochemical steps of growth regulation take place in cell walls, possible cross talks are also conceivable in cell wall formation and modification.