Auxin-gibberellin interactions in pea: Integrating the old with the new

Recent findings on auxin-gibberellin interactions in pea are reviewed, and related to those from studies conducted in the 1950s and 1960s. It is now clear that in elongating internodes, auxin maintains the level of the bioactive gibberellin, GA(1), by promoting GA(1) biosynthesis and by inhibiting GA(1) deactivation. These effects are mediated by changes in expression of key GA(1) biosynthesis and deactivation genes. in particular, auxin promotes the step GA(20) to GA(1), catalyzed by a GA 3-oxidase encoded by Mendel's LE gene. We have used the traditional system of excised stem segments, in which auxin strongly promotes elongation, to investigate the importance for growth of auxin-induced GA(1). After excision, the level of GA(1) in wild-type (LE) stem segments rapidly drops, but the auxin indole-3-acetic acid (IAA) prevents this decrease. The growth response to IAA was greater in internode segments from LE plants than in segments from the le-l mutant, in which the step GA(20) to GA(1) is impaired. These results indicate that, at least in excised segments, auxin partly promotes elongation by increasing the content of GA(1). We also confirm that excised (light-grown) segments require exogenous auxin in order to respond to GA. On the other hand, decapitated internodes typically respond strongly to GA(1) application, despite being auxin-deficient. Finally, unlike the maintenance of GA(1) content by auxin, other known relationships among the growth-promoting hormones auxin, brassinosteroids, and GA do not appear to involve large changes in hormone level.