SHORT-ROOT Regulates Primary, Lateral, and Adventitious Root Development in Arabidopsis

SHORT-ROOT (SHR) is a well-characterized regulator of radial patterning and indeterminacy of the Arabidopsis (Arabidopsis thaliana) primary root. However, its role during the elaboration of root system architecture remains unclear. We report that the indeterminate wild-type Arabidopsis root system was transformed into a determinate root system in the shr mutant when growing in soil or agar. The root growth behavior of the shr mutant results from its primary root apical meristem failing to initiate cell division following germination. The inability of shr to reactivate mitotic activity in the root apical meristem is associated with the progressive reduction in the abundance of auxin efflux carriers, PIN-FORMED1 (PIN1), PIN2, PIN3, PIN4, and PIN7. The loss of primary root growth in shr is compensated by the activation of anchor root primordia, whose tissues are radially patterned like the wild type. However, SHR function is not restricted to the primary root but is also required for the initiation and patterning of lateral root primordia. In addition, SHR is necessary to maintain the indeterminate growth of lateral and anchor roots. We conclude that SHR regulates a wide array of Arabidopsis root-related developmental processes.