The gibberellins are a large group of complex diterpenoid natural products, among which several have potent biological activity in plants, where they serve as hormones. Intriguingly, these phytohormones are made not only by the plants in which they serve to regulate growth and development, but by certain plant-associated fungal and bacterial microbes as well. While gibberellin phytohormone biosynthesis has been largely elucidated for higher plants and fungi, which seem to have independently evolved/assembled the corresponding metabolic pathway, the basis for such biosynthesis in bacteria remains enigmatic. Even in higher plants, the origins of gibberellin metabolism remains obscure. Further, there have been recent discoveries demonstrating the existence of novel gibberellin metabolism (particularly catabolism) in higher plants, which have unexplored implications in plant biology. Building on previous NSF-supported studies, this project will both further investigate gibberellin metabolism in higher plants and also define the analogous biosynthetic pathway in bacteria. The bacterial pathway appears to represent yet a third example of a convergent evolutionary solution to the puzzle presented by such complex natural products biosynthesis. These studies will also provide insight into the evolutionary origins of gibberellin metabolism in plants. Thus, the planned wide-ranging studies will provide a comprehensive overview of gibberellin phytohormone metabolism in a phylogenetically diverse group organisms. Gibberellins have played an important role in agriculture, as it was alterations in such phytohormone metabolism that led to high yielding semi-dwarf varieties of rice and wheat, which were a critical component of the "Green Revolution". The biosynthesis of gibberellin phytohormones by plant growth promoting bacteria that are commonly applied to legume crop plants offers additional agricultural significance. Further, the absolute requirement for gibberellin production in higher plants has provided a genetic reservoir of biosynthetic genes, duplication of which has led to a vast super-family (~7,000 known) of related diterpenoid natural products, exhibiting various biological activities and physiological roles (e.g., as defensive antibiotics). Hence, this project, while specifically directed at gibberellin metabolism, will offer insights into a much broader group of natural products. Broader Impact On-going broader outreach efforts also will be continued and expanded. Beyond the inclusion of undergraduate, graduate and postdoctoral students in research, work will be continued with high school students and those from under-represented groups. In addition, the project includes presentations to junior and senior high school science teachers to provide concrete examples of how basic research underlies and informs technological advancements that improve the human condition. An important example is the relationship of gibberellin to the "Green Revolution" as noted above.
Gibberellins (GA) are plant hormones required for normal growth and development in all higher plants, and play a central role in plant metabolism. Intriguingly, these phytohormones also are produced by plant associated fungi and bacteria, presumably to favorably alter host plant physiology. This grant supported our investigations of gibberellin biosynthesis, including the biochemical/enzymatic reactions underlying production of these complex molecules, the evolution of GA metabolism in plants, and the previously unknown metabolic pathway by which bacteria produce GA. Our results provided insights into enzymatic mechanisms, as well as the evolution of plant GA metabolism. However, our most striking results were derived from our studies of GA metabolism in bacteria, from which we have been able to demonstrate that bacteria independently evolved a novel biosynthetic pathway to produce these complex phytohormones. Moreover, although we had originally thought that such phytohormone production was found only in symbiotic nitrogen-fixing rhizobacteria, during the course of our studies results from bacterial genome sequencing projects revealed the presence of such biosynthetic machinery in bacterial plant pathogens as well. Building our biochemical analyses of these biosynthetic enzymes, we also were able to carry our genetic studies that demonstrated that these phytopathogens produce GA as a virulence factor that increases their pathogenicity. Thus, our results may have some more practical agricultural relevance. Finally, this project also provided excellent training for students at both the graduate and undergraduate level, and was broadly synergistic with the principal investigator’s teaching efforts in plant and more general biochemistry courses.
