Fig. 4. Proposed model for the function of rhizobial GA in symbiosis.

Produce either non-bioactive GA₉ or bioactive GA₄ during symbiosis, depending on whether the rhizobial species has cyp115. If GA₉ is the final product, we propose that host plant GA 3-oxidases convert GA₉ into bioactive GA₄. GA signaling then leads to an increase in nodule size, which increases bacteroid numbers within that nodule and more rhizobia released into the soil upon nodule senescence. Conversely, an absence of rhizobial GA results in decreased nodule size and thus fewer bacteria being released per nodule.