Regulation of mycorrhizal colonization under stress in tomato depends on symbiotic efficiency

The mutualistic symbiosis between plants and arbuscular mycorrhizal (AM) fungi is based on a balanced nutrient exchange between both partners, with the plant achieving improved nutrition and stress tolerance. The symbiosis is finely-tuned according to plant’s needs and surrounding conditions, usually through phytohormonal signaling. Thus, environmental conditions or stress factors modulating phytohormone signaling may influence the symbiosis. This study compares the colonization abilities of 2 AM fungal species, Funneliformis mosseae and Rhizophagus irregularis, independently or in combination, in tomato plants subjected to different stress conditions. These included salt stress and systemic defense activation by aboveground application of the defense-related hormones methyl jasmonate, abscisic acid and salicylic acid. The results show that root colonization by the two fungal species differs depending on the stress treatment. Nutrient and transcriptional analyses revealed that changes in colonization correlated with differential regulation of nutrient exchange, plant defensive responses, and symbiosis regulatory genes. Specifically, under salt stress R. irregularis colonization decreased, while F. mosseae colonization was promoted. These differential regulation of colonization under stress positively correlated with changes in the functionality of the symbiosis. Overall, the results support that the benefits provided by each AM fungi influence carbon reward and determines the control of root colonization by the host plant.