Abstract
When subjected to nutritional stress, bacteria modify their amino acid
metabolism and cell division activities by means of the stringent
response, which is controlled by the Rsh protein in alphaproteobacteria.
Although nutritional stress is common for rhizobia while infecting
legume roots, the stringent response was scarcely studied in this group
of soil bacteria. In this report, we obtained a mutant in the rsh
gene of Bradyrhizobium diazoefficiens, the
N2-fixing symbiont of soybean. This mutant was defective
for type-3-secretion system induction, plant-defense suppression at
early root infection, and competition for nodulation. Furthermore, the
mutant produced smaller nodules, although with normal morphology, which
lead to lower plant biomass production. The autoregulation of nodulation
in N-free condition was compromised in plants inoculated with the
mutant, but it nodulated plants in the presence of 10 mM
NH4NO3, a combined-N
concentration inhibiting nodulation. The rsh mutant released more
auxin to the culture supernatant than the wild-type, which might in part
explain its symbiotic behavior in the presence of combined-N. These
results indicate that B. diazoefficiens stringent response
integrates into the plant defense suppression and autoregulation of
nodulation circuits in soybean, perhaps mediated by the type-3-secretion
system.