Leaf transcriptomic responses to arbuscular mycorrhizal symbioses exerting growth depressions in tomato

Arbuscular mycorrhizal (AM) fungi play important roles in sustainable agriculture, given that they provide multiple benefits for numerous crops. Conversely, negative plant growth effects induced by AM fungi are also occasionally observed. However, little information is available regarding the responses of symbiosis. In this study, compared with an absence of AM fungus inoculation, tomato seedlings inoculated with Funneliformis mosseae or Rhizophagus intraradices were characterized by reduced shoot and root growth. The two AM fungi decreased the carbon contents and the carbon-nitrogen ratios in shoots. To gain further insights into the underlying mechanisms, transcriptomic analyses were performed in the study. A total of 190 and 870 differentially expressed genes (DEGs) were identified in the F. mosseae vs. control and R. intraradices vs. control comparisons, respectively. KEGG enrichment analysis of the former 190 DEGs revealed significant enrichment of the "Protein processing in endoplasmic reticulum," "Flavonoid biosynthesis," "Flavone and flavonol biosynthesis," and "Stilbenoid, diarylheptanoid, and gingerol biosynthesis" pathways, whereas "DNA replication," "Photosynthesis - antenna proteins," "Cutin, suberine, and wax biosynthesis," "Protein processing in endoplasmic reticulum," and "Glycerophospholipid metabolism" were identified as pathways significantly enriched with the latter 870 DEGs. GO functional analysis revealed that among both groups of DEGs, large numbers of genes were assigned the "Response to stimulus" term. Moreover, many of the enriched terms were associated with stimulus and stress response processes, including response to salt stress, heat, and reactive oxygen species. Therefore, the findings indicated that AM fungi may trigger defense-related responses in hosts, even though the symbioses performed growth depressions. These findings will contribute to advancing our current understanding of AM fungi.