The Rice COLEOPTILE PHOTOTROPISM1 Gene Encoding an Ortholog of Arabidopsis NPH3 Is Required for Phototropism of Coleoptiles and Lateral Translocation of Auxin

Abstract

We isolated a mutant, named coleoptile phototropism1 (cpt1), from g-ray-mutagenized japonica-type rice (Oryza sativa). This mutant showed no coleoptile phototropism and severely reduced root phototropism after continuous stimulation. A map-based cloning strategy and transgenic complementation test were applied to demonstrate that a NPH3-like gene deleted in the mutant corresponds to CPT1. Phylogenetic analysis of putative CPT1 homologs of rice and related proteins indicated that CPT1 has an orthologous relationship with Arabidopsis thaliana NPH3. These results, along with those for Arabidopsis, demonstrate that NPH3/CPT1 is a key signal transduction component of higher plant phototropism. In an extended study with the cpt1 mutant, it was found that phototropic differential growth is accompanied by a CPT1-independent inhibition of net growth. Kinetic investigation further indicated that a small phototropism occurs in cpt1 coleoptiles. This response, induced only transiently, was thought to be caused by the CPT1-independent growth inhibition. The 3 H-indole-3-acetic acid applied to the coleoptile tip was asymmetrically distributed between the two sides of phototropically responding coleoptiles. However, no asymmetry was induced in cpt1 coleoptiles, indicating that lateral translocation of auxin occurs downstream of CPT1. It is concluded that the CPT1-dependent major phototropism of coleoptiles is achieved by lateral auxin translocation and subsequent growth redistribution.

Comparative gene expression analysis in stems of <i>Dolichos purpureus</i> and <i>Arabidopsis thaliana</i>

“…The NPH1 level in the cells was down-regulated by 24 hours light exposure [65]. NPH3 interacts with NPH1 and is involved in auxin-regulated organogenesis [66][67][68]. NPH3 encoding genes At1g03010, At1g30440 was upregulated in epidermis at base stem and inner parts of the top stem, similar to NPH1 (Figure 3), in agreement with the optical observations of phototropin1 location in A. thaliana [65].…”

The geneMACCHI-BOU 4/ENHANCER OF PINOIDencodes a NPH3-like protein and reveals similarities between organogenesis and phototropism at the molecular level

“…The rice genome consists of at least 26 members of the NPH3 family and two members (Os06g0184500, Os09g0420900) exhibit high similarity to MAB4/ ENP and are less similar to COLEOPTILE PHOTOTROPISM 1 (CPT1), the rice NPH3 ortholog, the mutation of which caused a lack of phototropic response in the coleoptile (Fig. 2C) (Haga et al, 2005). Whereas NPH3, RPT2 and CPT1 contain a BTB/POZ (broad complex, tramtrack, and bric à brac/pox virus and zinc finger) domain at the N-terminal region and a coiled-coil domain at the C terminus, MAB4/ENP contains a BTB/POZ domain at the Nterminal region, but no distinct coiled-coil domain.…”

“…The rpt2 mutant has defects in phototropism of roots, and blue light-induced stomatal opening (Sakai et al, 2000;Inada et al, 2004). Mutation of rice CPT1, an ortholog of NPH3, results in a lack of coleoptile phototropism and severely reduced root phototropism (Haga et al, 2005). In the cpt1 coleoptiles, asymmetrical auxin distribution was not established in response to blue light, suggesting that CPT1 regulates auxin transport in phototropism.…”

Circumnutation of rice coleoptiles: its relationships with gravitropism and absence in lazy mutants

“…In contrast, phototropically stimulated coleoptiles stopped nutating as the extent of phototropic curvature became large. When the coleoptiles of the cpt1 mutant -which is specifically impaired in phototropism (Haga et al 2005) -were stimulated with unilateral blue light, they nutated without developing a phototropic curvature (Fig. 7f & g).…”