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

• Rhizobia symbiosis increased lignin content and reduced digestibility of alfalfa.
• The symbiosis enhanced transcript levels of PAL, C4H, C3H, HCT and CCR genes.
• The symbiosis increased Ca levels and decreased K levels in alfalfa.

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.

Figure optionsDownload as PowerPoint slideAn effect of rhizobia symbiosis on lignin level and forage quality in alfalfa. Rhizobia symbiosis increases lignin content and decreases forage digestibility. An increase in lignin content in nodulated alfalfa was associated with an upregulation of transcript levels of several lignin biosynthesis genes such as PAL, C4H, C3H, HCT and CCR. The upregulation of PAL gene may be linked to the increase of Ca level in alfalfa after nodulation. In addition, rhizobia symbiosis could also increase the levels of ADF, NDF and hemicellulose, resulting in a reduction in IVTD of alfafla. Therefore, forage quality was decreased due to the increase of lignin and other cell wall components (ADF, NDF and hemicellulose) induced by rhizobia symbiosis.