Reactive
sulfur species (RSS) are present in root nodules; however, their role in symbiosis and the mechanisms underlying their production remain unclear. We herein investigated whether RSS produced by the
cystathionine γ-
lyase (CSE) of microsymbionts are involved in root nodule symbiosis. A cse mutant of Mesorhizobium loti exhibited the decreased production of
hydrogen sulfide and other RSS. Although the CSE mutation of M. loti did not affect the early stages of symbiosis, i.e.,
infection and nodulation, with Lotus japonicus, it reduced the
nitrogenase activity of nodules and induced their early senescence. Additionally, changes in the production of
sulfur compounds and an increase in
reactive oxygen species (ROS) were observed in the infected cells of nodules induced by the cse mutants. The effects of CSE inhibitors in the L. japonicus rhizosphere on symbiosis with M. loti were also investigated. All three CSE inhibitors suppressed
infection and nodulation by M. loti concomitant with decreased RSS levels and increased ROS and
nitric oxide levels. Therefore, RSS derived from the CSE activity of both the microsymbiont and host plant are required for symbiosis, but function at different stages of symbiosis, possibly with crosstalk with other reactive mole-cular species.