Involvement of cytokinins in adventitious and lateral root formation

Abstract

The root systems of higher plant consist of primary, lateral, and adventitious roots. There is a current hypothesis, named 'root apical dominance', that root-derived suppressor negatively regulates the formation of adventitious and lateral roots. According to this hypothesis, we have studied the mechanisms involved in the regulation of adventitious and lateral root forma- tion. trans-zeatin riboside (ZR), one of cytokinin species, was identified in squash root xylem sap as a main suppressor of adventitious root formation in cucumber, suggesting that ZR transported from root via xylem sap acts as an endogenous suppressor of hypocotyl adventi- tious root formation in planta. Moreover, we isolated Arabidopsis mutant, wooden leg-3 (wol-3), for stimulated adventitious root formation in intact plants. Analyses of wol-3 mutant have revealed that cytokinin receptors are necessary for the primary and lateral root formation via the formation of auxin-transporting vascular tissues in the hypocotyl, but not for adventitious roots. In this review, we discuss the relationship between cytokinin and adventitious and lateral root formation.