ABA promotes fatty acid biosynthesis and transport to boost arbuscular mycorrhizal symbiosis in apple roots.

The roots of most land plants are in symbiosis with arbuscular mycorrhizal (AM) fungi. The fungus promotes nutrient uptake from the soil while receiving plant photosynthate as lipids and sugars. Nutrient exchange must be regulated by both partners, but the mechanisms underlying the regulation of lipid supplement from the plant to the AM fungus remain elusive. Here, we conducted a molecular study on the role of increased abscisic acid (ABA) levels during AM fungus infection in the roots of apple (Malus spp.). AM fungus induced the expression of two ABA synthesis genes, MdNCED3.1 and 3.2, in apple roots and increased the ABA content, which promoted the growth of the AM fungus. The effect of ABA on symbiosis was confirmed in transgenic apple roots either overexpressing or silencing MdNCED3.1 or MdNCED3.2. Transcriptome analysis and transgenic manipulation revealed that the transcription factor MdABF2 played a key role in the ABA-mediated formation of symbiosis during AM infection and that MdABF2 could regulate the expression levels of genes related to fatty acid (FA) synthesis (e.g., MdKASIII) and translocation (such as MdSTR2) in apple roots. Activation of these genes boosted the levels of available fatty acids in the roots and increased the AM fungal colonization and arbuscule development in the roots. These results reveal a molecular pathway in which ABA signaling positively regulates fatty acid synthesis and transport, thereby enhancing lipid supply to AM fungi and promoting AM symbiosis.