A study of phytohormone biosynthetic gene expression using a circadian clock-related mutant in rice.

We have recently isolated a rice circadian clock-related mutant carrying a null mutation in Os-GIGANTEA(GI) gene, the solo ortholog of Arabidopsis GI. Time-course global transcriptome analyses of leaves from wild-type and osgi mutant grown in the field have revealed that Os-GI affects gene expression of more than half of genes on rice 44k microarray. To better understand the biological significance of circadian clock function in growth and development of rice, we here investigated the gene expression involved in phytohormone biosynthesis. Here we found that mRNA levels of a few major genes encoding GA2-oxidase which can inactivate bioactive gibberellins (GAs) were remarkably increased in osgi-1 plants. This suggests that Os-GI functions to maintain bioactive GA level through the regulation of the GA-deactivating enzyme genes in rice. Consistently, osgi-1 plants showed semi-dwarf phenotype with reduced internode and leaf sheath elongation.