Rhizobial gibberellin negatively regulates host nodule number

Abstract

In legume–rhizobia symbiosis, the nodule number is controlled to ensure optimal growth of the host. In Lotus japonicus, the nodule number has been considered to be tightly regulated by host-derived phytohormones and glycopeptides. However, we have discovered a symbiont-derived phytohormonal regulation of nodule number in Mesorhizobium loti. In this study, we found that M. loti synthesized gibberellic acid (GA) under symbiosis. Hosts inoculated with a GA-synthesis-deficient M. loti mutant formed more nodules than those inoculated with the wild-type form at four weeks post inoculation, indicating that GA from already-incorporated rhizobia prevents new nodule formation. Interestingly, the genes for GA synthesis are only found in rhizobial species that inhabit determinate nodules. Our findings suggest that the already-incorporated rhizobia perform GA-associated negative regulation of nodule number to prevent delayed infection by other rhizobia.

Investigating the Phylogenetic Range of Gibberellin Biosynthesis in Bacteria

“…However, while plant and fungal GA biosynthesis has been extensively characterized (Hedden and Thomas, 2012), the relevant pathway in bacteria has only recently been fully elucidated (Tatsukami and Ueda, 2016; Nett et al, 2017b; Nagel et al, 2017). Bacterial GA biosynthesis is associated with a cytochrome P450 (CYP)-rich operon (Figure 1), as demonstrated by functional characterization of the encoded enzymes.…”

“…oryzicola from the separate γ -proteobacteria class were found to be functionally analogous, and the ultimate product of the operon also demonstrated to act as a virulence factor for this rice pathogen (Lu et al, 2015). Even more recently, the three CYPs (CYP112, CYP114 & CYP117), along with ferredoxin (Fd GA ) and short-chain alcohol dehydrogenase/reductase (SDR GA ), found in all copies of the operon were shown to transform ent -kaurene to GA 9 , although this is not thought to exert hormonal activity (Tatsukami and Ueda, 2016; Nett et al, 2017b; Nagel et al, 2017). Notably, the additional CYP (CYP115) found associated with the operon in the phytopathogen X. oryzae pv.…”

“…In particular, the operon is found in sub-sets of both symbiotic nitrogen-fixing rhizobia and phytopathogens. It has been suggested that the production of GA 9 by rhizobia is followed by conversion to bioactive GA 4 by the host plant, and further proposed that this acts to block any further nodulation (Tatsukami and Ueda, 2016), although the investigated Mesorhizobium loti MAFF303099 is capable of producing GA 4 directly (Nett et al, 2017a). On the other hand, the direct production of GA 4 has been hypothesized to reflect the importance of entry via wounded leaf tissue, with associated JA-dependent defense response, for the relevant phytopathogens (Nagel et al, 2017).…”

Labeling Studies Clarify the Committed Step in Bacterial Gibberellin Biosynthesis

“…While rhizobia only express these enzymes and produce GA after differentiation into their nodule-residing bacteroid form, 17 it was possible to observe activity with the individual enzymes upon recombinant expression. 4,16,18,19 Notably, “B” ring contraction requires not only a CYP (CYP114) but also the ferredoxin (Fd GA ) found within the operon, which presumably acts as an electron donor. 4 This is distinct from plant and fungal KAOs, which simply utilize an archetypical cytochrome P450 reductase for their activity.…”

In vitro growth and indoleacetic acid production by Mesorhizobium loti SEMIA806 and SEMIA816 under the influence of copper ions

“…12 Like the IAA, exopolysaccharides are symbiont derived signals essentials for nodulation in legume-rhizobia symbiosis. 17 A positive effect of ion copper in the production of IAA by Pseudomonas chlororaphis O6 was also demonstrated. 4 With the addition of thryptophan to the culture medium, the levels of accumulated IAA after 48h was higher than 30 µg.mL -1 in cultures with a copper concentration of 43 mg.L -1 .…”