Effect of rhizobia symbiosis on lignin levels and forage quality in alfalfa (Medicago sativa L.)

Abstract Many beneficial effects of symbiotic rhizobia on leguminous plants have been reported. Here we report a novel effect of rhizobia on forage quality in alfalfa. We found that nodulated alfalfa showed an increase in lignin content and a decrease in digestibility in comparison with non-nodulated plants. Detailed studies revealed that nodulation resulted in an increase in monolignol G unit and S unit. An overall increase in lignin content in nodulated alfalfa was associated with more lignified tissues in the stem and an upregulation of transcript levels of several lignin biosynthesis genes. We hypothesize that an increase in lignin content in nodulated alfalfa is a result of defensive response in plants to rhizobial invasion.