DEVELOPMENTAL REGULATION OF THE GIBBERELLIN BIOSYNTHETIC GENE GA1 IN ARABIDOPSIS-THALIANA

The GA I gene of Arabidopsis thaliana encodes enf-kaurene synthase A ( KSA), which catalyzes the first committed step in the biosynthetic pat hway of the plant hormone gibberellin (GA). Its location in the GA bio synthetic pathway has led to speculation that KSA regulation is one of the controlling steps. However, because KSA activity is so low that i t is only measurable in Arabidopsis siliques, GA1 promoter-GUS reporte r gene fusions and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) were used to examine the expression pattern of GA1. The results from this study indicate that GA1 gene expression is high ly regulated during growth and development, and it is restricted to sp ecific cell types at the sites of expression. GA1 promoter activity is highest in rapidly growing tissues, e.g. shoot apices, root tips, dev eloping flowers and seeds. It is also active in the vascular tissue of some non-growing organs, such as expanded leaves, suggesting that the se leaves may be a site of GA synthesis for transport to other organs. It was also found that the first one or two introns in the GA1 gene a re required for proper expression. Because of the high degree of regul ation, GAT may act as a gatekeeper, controlling the flow of metabolite s into the GA biosynthetic pathway, while the levels of specific bioac tive GAs are controlled by other downstream steps.