Abstract
The cpt1 mutant of rice was isolated in our laboratory as a mutant showing no coleoptile phototropism and severely impaired root phototropism. Map-based cloning of the CPT1 gene indicated that cpt1 mutation is linked to a deletion of 208 kb at position 85 centimorgan on chromosome 2. This region included a gene homologous to Arabidopsis NPH3 (AtNPH3), known to be essential for hypocotyl phototropism. The cpt1 mutant transformed with genomic DNA of rice NPH3 (OsNPH3) showed normal phototropisms of coleoptiles and roots, demonstrating that OsNPH3 is CPT1. The deduced amino acid sequences of OsNPH3/CPT1 and AtNPH3 showed 58% identity. Database and sequence analysis indicated that rice has at least 36 NPH3 homologs and that OsNPH3 is an ortholog of AtNPH3. The fact that mutation in NPH3 genes results in severely impaired phototropism in both Arabidopsis and rice indicates that the product of NPH3 is essential for phototropism of higher plants.
