The rice root system consists of seminal, crown and lateral roots, which correspond to pole-borne, stem-borne and root-borne roots, respectively (Barlow 1994; Rebouillat et al., 2009; Coudert et al., 2010). Generally, the root system of most dicot plants develops from the radicle formed in the embryo, whereas monocot plants have fibrous root system that is characterized by numerous crown roots. Crown roots differentiate in the stem from a radial ground meristem that has common characteristics with the root pericycle (Coudert et al., 2013; Itoh et al., 2005). The phytohormone auxin is essential for root development. Crown root less 4 (crl4) regulates the intracellular traffic of the PINFORMED1 (PIN1) auxin efflux carrier protein. In turn, PIN1 is necessary to concentrate auxin in lateral root funder cells (Kitomi et al., 2008; Liu et al., 2009; Steinmann et al., 1999). The transmission of auxin signaling is controlled by the interaction between AUX/IAA and ARF proteins. WOX11, an auxin-responsive gene, is part of a positive feedback loop of auxin signaling (Zhao et al., 2009). Expression of WOX11 was induced by cytokinin treatment, and downregulation or gain-of-function of WOX11 alter the expression of type-A RR genes. Type-A OsRR1 and RR2 localizes in both the nucleus and the cytosol and inhibits the cytokinin response genes (Tsai et al., 2012). OsCAND1 is involved in auxin signaling in root tips of crown roots. Loss-of-function of OsCAND1 results in cessation of the G2/M transition of the cell cycle in the meristem of crown roots and, consequently, failure of the emergence of crown roots. Collectively, spatio-temporal sequence of events that act in auxin and cytokinin signaling is responsible for the process of crown root initiation to crown root elongation.
