The observed diversification of (di)terpenoids in plants suggests that these natural products play important roles in such organisms. Chief among these are the gibberellin phytohormones, produced by both plants and associated microbes, and which have given rise to the particularly extensive super-family of labdane-related diterpenoids. To demonstrate the physiological relevance of these natural products, we have been investigating their role in plant-microbe interactions. Based on our extensive work on rice labdane-related diterpenoid metabolism, we have taken a reverse genetic approach to elucidate their role in the plant. In addition to serving as antibiotic phytoalexins, we have found that the momilactones serve as (other, not rice) plant growth inhibiting allelochemicals. On the other hand, we also have been investigating the production of gibberellins by plant-associated bacteria, both symbiotic (nodulating nitrogen-fixing) rhizobia and phyto-pathogens. Interestingly, we have found that bacterial production of gibberellin serves to suppress the plant microbial defense response, for both the phytopathogens and symbionts (to enable their eventual escape from nodules upon plant senescence). Furthermore, such suppression by nodulating rhizobia seems to have a tritrophic effect, increasing the susceptibility of the nodulated legume plant to infection by microbial pathogens. Particularly given that knocking-out gibberellin production does not otherwise affect the symbiotic relationship (most critically nitrogen-fixation), and that these results were obtained with the important crop soybean, this finding may have some agricultural relevance.
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