Arbuscular mycorrhizal symbiosis enhances virus accumulation and attenuates resistance-related gene expression in tomato plants infected with Beet curly top Iran virus

Beet curly top Iran virus (BCTIV), a member of the genus Becurtovirus, is one of the causal agents for curly top disease in tomato plants. Arbuscular mycorrhizal (AM) fungi provide nutrients for host plants and are associated with their improved growth. However, the impact of AM on infection by plant viruses is not well understood. In this study, the interaction between Funneliformis mosseae and BCTIV in a susceptible tomato cultivar (Early Eurbana) was investigated. In a completely randomized design experiment, tomato seedlings were inoculated with F. mosseae, and after 4 weeks, they were inoculated with an infectious clone of BCTIV. Four treatments were included: untreated control plants (C), BCTIV-infected plants (V), mycorrhizal plants (M) and BCTIV-infected mycorrhizal plants (MV). Results of symptom evaluation based on a disease severity index showed a higher disease severity in MV plants compared to V plants. Supporting this result, a higher level of virus accumulation was observed in MV plants and this became more significant after long-term infection. The expression of three defense-related genes including HSP90, RLK and PRP1 was attenuated in MV plants compared to V plants, which may explain the enhanced symptom production and viral accumulation in these plants. A similar percentage of root colonization by F. mosseae in M and MV plants indicated that root colonization was not affected by BCTIV infection. These results show that mycorrhizal symbiosis increases the susceptibility of tomato plants to virus infection and favors BCTIV accumulation and symptom production.