IGI1 is involved in axillary branching in Arabidopsis thaliana

Abstract

Genetic approach in mutant screening is an important tool for studying gene function in plant. We selected phenotypically distinct plants in the activation tagged lines and chose a mutant, igi1 (Inflorescence Growth Inhibitor 1) for further analysis. Segregation ratio of the F2 generation, TAIL-PCR walking and genotyping PCR showed that a single T-DNA was inserted at the 200bp upstream of the At1g23540 coding region. Real time PCR indicated that the expression level of IGI1 gene was increased approximately by 1,000 to 3,000 fold in igi/igi homozygous mutant. The homozygous mutant displayed abnormal phenotypes during reproductive stgage and was sterile. The heterozygous mutant plants produced primary inflorescence with short internode, and secondary inflorescences begin to emerge straight and axillary branching was dramatically increased. We attempted RNA interference (RNAi) to revert the phenotype by reducing the mRNA levels. IGI1 expression level was decreased in RNAi lines of igi1 heterozygous mutants, and the phenotypes of the RNAi lines were similar to those of wild type plants. The result showed igi1 mutants phenotypes were caused by overexpresssion of the IGI1 gene. Auxin and cytokinin are involved in shoot branching and apical dominance. In the cytokinin and auxin response, igi1 mutants showed opposite phenotypes in the axillary meristem development. The transcript level of auxin biosynthesis component, CYP79B2 was increased in the igi1 mutants. In the cell division and callus greening test for cytokinin response, igi1 mutants showed similar pattern with wild type. Histochemical analysis by GUS staining showed tissue-specific gene expression in the anther. The expression levels of the IGI1 gene in apical part and flower were higher than in other parts. The IGI1 possesses proline rich domain in N-terminal region and kinase domain in C-terminal region. The proline rich domain in N-terminal region interacted with SUB1, which is negative regulator of the photomorphogenesis and regulates positive regulator HY5 in the photomophogenesis. Taken together with the fact that HY5 binds the promoter region of IGI1 gene, my results suggest that there might be a noble pathway for the axillary meristem development that involves IGI1 and is modulated by light regulated pathway.