Involvement of cytokinins in adventitious and lateral root formation

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 formation. 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 adventitious 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.