Gibberellin biosynthesis by symbiotic rhizobia and its effect on host legumes
R. NETT (1), X. Lu (1), R. Nagel (1), R. Peters (1) (1) Iowa State University, U.S.A.

Gibberellins (GAs) are complex phytohormones that exert significant effects on plant growth.  GAs can also be produced by various plant-associated fungi and bacteria, and many of these microbes use GA as a virulence factor to enhance infection.  Plant and fungal GA biosynthesis has been comprehensively elucidated, and it is clear that GA production evolved convergently in these organisms.  However, GA biosynthesis in bacteria has remained uncharacterized.  Many rhizobia, the nitrogen-fixing symbionts of legumes (e.g. the soybean symbiont, Bradyrhizobium japonicum) contain an operon predicted to be involved in GA biosynthesis, and previously it has been shown that a subset of genes in this operon can produce ent-kaurene, the precursor to the GAs.  Through heterologous expression, we have identified the function of the remaining genes of this operon, showing that they catalyze GA production, thus representing the first elucidated bacterial GA biosynthetic pathway.  Strikingly, the enzymes within bacterial GA biosynthesis share little homology with their functional analogs in plants and fungi, or differ in enzyme families entirely, strongly suggesting that bacteria have convergently evolved a third distinct means to GA production.  We have also found that knockout of this operon in B. japonicum causes a decrease in soybean root nodule size, and that this can be reversed with exogenous GA application.  Therefore, it appears that rhizobia supply their host with GA in order to increase the size of the nodule they reside in, thereby allowing for an increase in the number of bacteria per nodule and a selective advantage for that lineage upon release back into the soil.

Abstract Number: P2-41
Session Type: Poster