Arbuscular mycorrhizal symbiosis in tomato roots with a diverse range of carotene accumulation

Arbuscular mycorrhizal (AM) symbiosis is regulated by carotenoid-derived molecules, including strigolactones and other apocarotenoids. However, the role of root carotene availability remains poorly understood. Here we evaluated AM performance in tomato using two mutants showing contrasting root carotenoid profiles, i.e. cyc-b7, an EMS-derived TILLING mutant carrying a characterized mutation in Cyc-B, and 7458-Y, an uncharacterized mutant line showing high carotenoid accumulation), compared to their related wild type (Red Setter). Not-inoculated and AM fungal inoculated plants were grown under controlled conditions and root colonization parameters were assessed after two months. Root carotenoids were quantified by high-performance liquid chromatography (HPLC), and RNA-seq analysis was performed on root samples to understand the tomato response to the inoculation. Roots of cyc-b7 accumulated significantly lower total carotenoids than Red Setter, including reduced lutein and β-carotene, whereas 7458-Y showed increased β-carotene, together with higher arbuscule abundance than both Red Setter and cyc-b7. Transcriptome profiling revealed a genotype-dependent response to AMF inoculation, with symbiosis-related genes differentially regulated in the two mutant lines. In cyc-b7, AMF inoculation was associated with reduced expression of genes encoding nutrient transporters, as well as of a gene encoding a symbiosis receptor-like kinase (SYMRK), a component of the common symbiosis signaling pathway. By contrast, in 7458-Y, AMF inoculation was associated with up-regulation of a gene encoding a LysM receptor-like kinase involved in AM establishment, and of a gene, SlD27, related to strigolactone biosynthesis. Overall, our results support a link between root carotenoid metabolism and AMF colonization.