Physiological and molecular mechanisms of arbuscular mycorrhizal inoculation and soil-available phosphorus in regulating photosynthesis of alfalfa

The mechanisms by which arbuscular mycorrhizal fungi (AMF) and phosphorus (P) influence photosynthesis and growth of alfalfa ( Medicago sativa L.) remain poorly understood. We investigated how AMF inoculation and soil-available P application affect photosynthesis by combining analyses of photosynthetic and chlorophyll fluorescence parameters, photosynthetic enzyme activities, and transcriptomic, proteomic, and metabolomic data from alfalfa leaves and roots. AMF inoculation and P application enhanced the net photosynthetic rate of alfalfa by regulating carbon fixation pathways, but their regulatory mechanisms differed. AMF may primarily optimized photosynthetic carbon metabolism by upregulating the mRNA expression of MDH (malate dehydrogenase, MS.gene061723 ) in the PEPC (phosphoenolpyruvate carboxylase)-mediated anaplerotic carbon fixation pathway and reducing the protein abundance of MDH ( MS.gene004851 ) in the Rubisco–Calvin cycle, whereas P application may reduce alfalfa dependence on the PEPC ( MS.gene63756 )-mediated anaplerotic carbon fixation pathway and enhance carbon assimilation efficiency through the Rubisco–Calvin cycle. AMF specifically influenced N-glycan biosynthesis, whereas P application modulated aminoacyl-tRNA, ubiquinone/terpenoid-quinone, and ether lipid metabolism, which may contribute to alfalfa growth. Genes captured at both the transcript and protein levels were more abundant in leaves than in roots, and within leaves, more dual-capture genes were detected under AMF than P treatment. Indicating leaves as the center of energy metabolism, it exhibited more rapid gene-to-protein conversion compared to roots. These findings provide a deeper understanding of the physiological and molecular mechanisms underlying AMF- and P-mediated regulation of photosynthesis.