The gibberellin (GA) phytohormones are integral to many aspects of plant growth and

development. Perhaps due to the importance of GA signaling, many plant-associated

microbes, both fungal and bacterial, are also capable of producing GAs, presumably to

manipulate their plant host. The plant and fungal GA biosynthetic pathways have been well

studied and fully characterized, revealing that these organisms have convergently evolved the

ability to produce GA. However, GA biosynthesis in bacteria has remained elusive. Many

rhizobia, the bacterial, nitrogen-fixing symbionts associated with legumes, have been found

to contain a putative GA biosynthetic gene cluster/operon (GA operon). Through

characterization of the genes from this operon, GA biosynthesis in rhizobia has now been

fully elucidated, thereby providing the first GA biosynthetic pathway for bacteria. Analysis

of this pathway revealed not only that bacteria have convergently evolved GA biosynthesis

independently of plants and fungi, but also that some of the key biochemical transformations

are conserved, including mechanisms of the ring contraction reaction that represents a

committed step in GA biosynthesis. It has further been found that some rhizobia contain an

additional gene that is responsible for converting the non-bioactive product of the GA operon

into a common bioactive form, and phylogenetic analyses suggest that this gene has been

obtained independently of the GA operon itself. Ultimately, it appears that rhizobia produce

GA in order to manipulate their host during symbiosis, specifically by increasing the size of

the nodules in which they reside, thereby increasing the number of bacteria within the nodule

that can be released into the soil. Thus, bacteria seem to have acquired GA biosynthesis in

order to gain a selective advantage in their symbiosis with legumes.