Transcriptome responses of grafted Citrus sinensis plants to inoculation with the arbuscular mycorrhizal fungus Glomus versiforme

'Newhall' grafted onto xiangcheng rootstock with Glomus versiforme or without displayed different responses, and genes related to photosystem II and alpha-linolenic acid metabolism pathways were involved in the responses. Previous studies have shown that there are significant differences in the physiological responses of 'Newhall' (Citrus sinensis) scions grafted onto trifoliate orange (Poncirus trifoliata) to arbuscular mycorrhizal (AM) fungi inoculation under normal and stress conditions. However, little is known about the transcriptomic responses of C. sinensis to AM fungi inoculation. In this study, we investigated the effects of inoculating the AM fungus Glomus versiforme on the biomass, pigment content, magnesium (Mg) content and distribution, net photosynthesis rate, and global transcriptome profile of 'Newhall' scions grafted onto xiangcheng (Citrus junos) rootstock. The results showed that AM inoculation significantly increased plant growth, Mg concentration, and photosynthesis, but not pigment contents. More than 68,299,008 transcripts were examined in spring-flush leaves, and 29 genes were identified as being differentially expressed in response to mycorrhizal colonization. The differentially expressed genes encoded proteinase inhibitors, transporters, and products related to chlorophyll and disease resistance. Genes encoding proteins related to chlorophyll and transport were up-regulated by AM inoculation while genes encoding proteinase inhibitors were down-regulated. Gene Ontology and KEGG database analyses revealed that genes related to photosystem II and alpha-linolenic acid metabolism pathways were involved in the response to AM inoculation. Comparative expression profiling revealed that the enhancement of photosynthesis after AM inoculation was due to activation of the light-harvesting complex family of proteins in photosystem II. Our results provide new insights into plant-mycorrhizal fungi interactions and their effects on plant growth.