Mechanisms underlying mycorrhiza-induced resistance to infection by the blast fungus Magnaporthe oryzae in rice

Abstract

Arbuscular mycorrhizal (AM) fungi form symbiotic associations with roots in most land plants. Root colonization by AM fungi improves the uptake of mineral nutrients in the host plant, mainly phosphorus and nitrogen, which generally improves plant growth and ftness, in exchange for photosynthetically fxed carbon. Another beneft conferred by the AM symbiosis in several plant species is improved resistance to biotic and abiotic stress. The legume species Medicago truncatula and Lotus japonicus have been widely used in studies on the AM symbiosis. Although rice is also a host for AM fungi, our current knowledge on the possible benefcial efects of the AM symbiosis in rice is scarce. In this study, we investigated the efect of inoculation with AM fungi in a panel of temperate rice cultivars focusing on plant growth, grain yield and resistance to infection by the fungal pathogen Magnaporthe oryzae, the causal agent of the rice blast disease. Compared to non-inoculated plants, the AM-inoculated plants had higher Pi content in their leaves. Our results revealed diferential responses to AM inoculation which are dependent on host genotype and identity of the AM fungus. In most rice varieties, the AM symbiosis conferred protection to infection by the rice blast fungus. Field trials with selected rice varieties demonstrated a signifcant increase in grain yield in mycorrhizal rice plants compared with non-mycorrhizal plants. Transcriptomic analysis identifed genes whose expression is systemically regulated in leaves of mycorrhizal rice plants. The AM symbiosis triggers coordinated regulations in the expression of genes involved in lipid signaling (phospholipids and inositol polyphosphates), hormone signaling (jasmonic acid, ethylene), as well as genes involved in phosphate signaling and homeostasis. A diferential regulation (from repression to activation) of miR399 family members occurs in leaves of mycorrhizal rice plants. A better knowledge of the molecular mechanisms associated with the AM symbiosis is a requisite for the development of sustainable rice production systems which will allow a reduction on the use of agrochemicals, fertilizers and pesticides.