Role of Nitric Oxide and Plant Class 1 Hemoglobin in Legume-Rhizobium Symbiosis

Abstract

Host leguminous plants accept their symbiotic rhizobia through the recognition by flavonoids and Nod factor. However, it is not be well understood whether the plant immune system functions during the symbiotic process. To understand the responses of a host plant against various bacteria, we examined expression of a class 1 hemoglobin gene LjHb1 and production of nitric oxide (NO) in Lotus japonicus after inoculation with rhizobia or plant pathogens. NO induces plant defense system and class 1 hemoglobins will control NO level. The symbiontic rhizobia induced both of the expression of LjHb1 and NO production. In contrast, plant pathogens induced continuous NO production without expression of LjHb1. NO was also detected in nitrogen fixing root nodules. Over-expression of LjHb1 enhanced the symbiotic nitrogen fixation. Class 1 Hb may be involved in the host-symbiont recognition and in support of symbiotic nitrogen fixation by modulating NO.