Nitric oxide production induced in roots of Lotus japonicus by lipopolysaccharide from Mesorhizobium loti.

Lipopolysaccharide (LPS) is a bacterial molecule that induces nitric oxide (NO) production and triggers defense systems in plant-pathogen interactions. NO production is induced in the roots of Lotus japonicus after inoculation of the roots with its microsymbiont Mesorhizobium loti. However, the rhizobial molecule that induces NO production has not yet been identified. We investigated NO production in the roots of L. japonicus by treatment with LPS of M. loti. LPS was prepared by phenol-hot water extraction and separated into several fractions: polysaccharide, lipooligosaccharide, oligosaccharide and lipid A. In the roots of L. japonicus, NO production was observed with an NO-specific fluorescent dye 4, 10 and 24 h after treatment with each fraction of LPS. NO production was detected 4 h after treatment with all fractions. NO production was also detectable 24 h after treatment, except after treatment with the polysaccharide and oligosaccharide fractions. Expression of a class 1 hemoglobin gene and application of an NO scavenger showed that the treatment with LPS and LOS induced a similar response to inoculation with M. loti. These data suggest that LPS of M. loti induces NO production after inoculation with M. loti.