Arbuscular mycorrhizal (AM) fungi facilitate plant uptake of mineral nutrients and draw organic nutrients from the plant. Organic nutrients are thought to be supplied primarily in the form of sugars. Here we show that the AM fungus Rhizophagus irregularis is a fatty acid auxotroph and that fatty acids synthesized in the host plants are transferred to the fungus to sustain mycorrhizal colonization. The transfer is dependent on RAM2 (REQUIRED FOR ARBUSCULAR MYCORRHIZATION 2) and the ATP binding cassette transporter–mediated plant lipid export pathway. We further show that plant fatty acids can be transferred to the pathogenic fungus Golovinomyces cichoracerum and are required for colonization by pathogens. We suggest that the mutualistic mycorrhizal and pathogenic fungi similarly recruit the fatty acid biosynthesis program to facilitate host invasion.

Widespread resistance, localized relief: Rice blast fungus can devastate a rice harvest. Genes that provide resistance to the fungus usually depress rice yield. Deng et al. analyzed the molecular underpinnings of a rice variant that is resistant to rice blast but still high-yielding (see the Perspective by Wang and Valent). The key locus encodes several R (resistance) genes. One gene confers resistance and is expressed throughout the plant. Another gene fails to confer resistance and is expressed only in pollen and panicles (the rice-producing flower clusters). Because the R proteins function as dimers, heterodimerization in pollen and panicles disables resistance. The plants thus produce smaller but more numerous rice grains, which sustains yield, while the body of the plant resists fungal infection. Science , this issue p. 962 ; see also p. 906

Plant roots associate with diverse microbes (including bacteria, fungi, archaea, protists, and viruses) collectively called the root-associated microbiome. Among them, mycorrhizal fungi colonize host roots and improve their access to nutrients, usually phosphorus and nitrogen. In exchange, plants deliver photosynthetic carbon to the colonizing fungi. This nutrient exchange affects key soil processes, the carbon cycle, and plant health and therefore has a strong influence on the plant and microbe ecosystems. The framework of nutrient exchange and regulation between host plant and arbuscular mycorrhizal fungi has recently been established. The local and systemic regulation of mycorrhizal symbiosis by plant nutrient status and the autoregulation of mycorrhizae are strategies by which plants maintain a stabilizing free-market symbiosis. A better understanding of the synergistic effects between mycorrhizal fungi and mycorrhizosphere microorganisms is an essential precondition for their use as biofertilizers and bioprotectors for sustainable agriculture and forestry management.