Auxin regulation of the gibberellin pathway in pea

Abstract

The auxin indole-3-acetic acid (IAA) has been shown to promote the biosynthesis of the active gibberellin (GA1) in shoots of pea (Pisum sativum). We used northern analysis to investigate the timing of IAA-induced changes in transcript levels of PsGA3ox1 (Mendel’s LE), PsGA2ox1, PsGA2ox2, and PsGA20ox1, key genes for the later stages of GA1 biosynthesis and metabolism in pea. Rapid (2–4 h) changes occurred in the transcript levels of PsGA3ox1, PsGA2ox1, and PsGA2ox2 after treatent with IAA. [14C]GA1 metabolism studies in decapitated shoots indicated that IAA inhibits GA1 deactivation, consistent with the suppression of PsGA2ox1 (SLN) transcript levels by IAA. Studies with the sln mutant also indicated that PsGA2ox1 activity is involved in GA1 deactivation in decapitated shoots. Culture of excised internode tissue in the presence of auxin clearly demonstrated that internode tissue is a site of GA1 biosynthesis per se. Excised internode tissue cultured in the presence/absence of cycloheximide showed that de novo protein synthesis is required for IAA-induced increases in PsGA3ox1 transcript levels. Auxin dose response studies indicated that IAA concentration is a critical determinant of GA1 biosynthesis over 1 to 2 orders of magnitude, and a range of auxins was shown to affect the GA pathway.