![]() ![]() Previous genetic and molecular studies in Arabidopsis have identified the SCF TIR1 ubiquitin ligase as a positive regulator of auxin signaling (Gray et al., 2001). ![]() The role of auxin in maintaining crown root meristem remains to be elucidated. Subsequent patterning of tissues within crown root primordia also requires establishment of an auxin response gradient with a maximum at the tip (Benkova et al., 2003). Based on these findings, an important role for auxin signaling and transport in crown root development is apparent. Moreover, RNA interference in a transgenic plant of rice PIN-formed 1 ( PIN1 ) or overexpression of OsPINOID 1 ( OsPID1 ), which alters polar auxin transport, reduces crown roots (Xu et al., 2005 Morita and Kyozuka, 2007). Recently, it has been reported that loss-of-function of CRL4/OsGNOM1, which encodes a large guanine nucleotide exchange factor for an ADP-ribosylation factor affecting polar auxin transport, yielded a phenotype of no crown root (Kitomi et al., 2008b Liu et al., 2009). Transgene studies have also provided evidence that transgenic rice with overexpression of stabilized Aux/IAA3 ( AUXIN/INDOLE-3-ACETIC ACID 3 ) produces phenotypes with reduced crown roots (Nakamura et al., 2006). Further, ARF is regulated by microRNA to control crown root initiation in Arabidopsis (Gutierrez et al., 2009). This element can bind in vitro with the auxin response factor (ARF) (Inukai et al., 2005). The CRL1 promoter contains two auxin responsive elements, and one is required for the expression of the gene in the stem base. ARL1 / CRL1 expression is induced by auxin and coincides with auxin distribution in the base of the stem where crown root differentiates. The regulatory role of auxin signaling in crown root initiation has been emphasized in studies of the arl1/crl1 mutants, which entirely lacks crown root (Inukai et al., 2005 Liu et al., 2005 Kitomi et al., 2008b Liu et al., 2009). A phytohormone critical for plant root development is auxin (De Smet and Jurgens, 2007). In spite of these findings, however, knowledge about the molecular mechanisms of crown root primordium emergence remains limited. During crown root primordia development, cell elongation and vacuolation of the primordia are suppressed in crl2 and crl3 mutants (Inukai et al., 2001 Kitomi et al., 2008a). Furthermore, initiation of crown root is delayed in wox11 ( WUSCHEL - related Homeobox 11 ) mutants, which produce fewer crown roots (Zhao et al., 2009). CRL4/OsGNOM1 is highly homologous to the GNOM1 protein in Arabidopsis thaliana ( Arabidopsis ). In addition, primordia initiation is impaired in crl4 ( crown rootless 4 )/ Osgnom1 mutants (Kitomi et al., 2008b Liu et al., 2009). The maize-related gene RTCS is also involved in postembryonic shoot-borne root formation (Taramino et al., 2007). CRL1 / ARL1 enc- odes an AS2/LOB-domain protein. The first periclinal division is suppressed in crl1 ( crown rootless 1 )/ arl1 ( adventitious rootless 1 ) (Inukai et al., 2005 Liu et al., 2005). Recently, a few mutants affecting crown root primordia initiation have been identified and characterized in rice, contrib- uting to our understanding of the genetic mechanisms underlying crown root development. Finally, the crown roots emerge from the stem. organization of root is formed with establishment of columella and meta- xylem vessel and elongated and vacuolated cells at the fifth and sixth stages. ![]()
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