Investigating the Phylogenetic Range of Gibberellin Biosynthesis in Bacteria

Certain plant-associated microbes can produce gibberellin (GA) phytohormones, as first described for the rice fungal pathogen Gibberella fujikuroi and, more recently, for bacteria, including several rhizobia and the rice bacterial pathogen Xanthomonas oryzae pv. oryzicola. The relevant enzymes are encoded by a biosynthetic operon that exhibits both a greater phylogenetic range and scattered distribution among plant-associated bacteria. Here, the phylogenetic distribution of this operon was investigated. To demonstrate conserved functionality, the enzymes encoded by the disparate operon from X. translucens pv. translucens, along with those from the most divergent example, found in Erwinia tracheiphila, were biochemically characterized. In both of these phytopathogens, the operon leads to production of the bioactive GA₄. Based on these results, it seems that this operon is widely dedicated to GA biosynthesis. However, there is intriguing variation in the exact product. In particular, although all plant pathogens seem to produce bioactive GA₄, rhizobia generally only produce the penultimate hormonal precursor GA₉. This is suggested to reflect their distinct interactions with plants, because production of GA₄ counteracts the jasmonic-acid-mediated defense response, reflecting the importance of wounds as the entry point for these phytopathogens, whereas such suppression presumably is detrimental in the rhizobial symbiotic relationship.