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

The roots of most land plants form symbioses with arbuscular mycorrhizal (AM) fungi. The fungus promotes nutrient uptake from the soil while receiving plant-derived photosynthates as lipids and sugars. Nutrient exchange must be regulated by both partners; however, the mechanisms underlying the regulation of lipid supply from the plant to the AM fungus remain unclear. Here, we performed a molecular study on the role of elevated abscisic acid (ABA) levels during AM fungal infection in apple (Malus spp.) roots. AM fungal colonization induced the expression of two ABA biosynthesis genes, MdNCED3.1 and MdNCED3.2, in apple roots and increased ABA content, which promoted AM fungal growth. The effect of ABA on symbiosis was confirmed in transgenic apple roots overexpressing or silencing MdNCED3.1 or MdNCED3.2. Transcriptome analysis and transgenic experiments revealed that the transcription factor MdABF2 plays a key role in ABA-mediated symbiosis during AM infection and regulates the expression of genes associated with fatty acid (FA) biosynthesis (e.g., MdKASIII) and transport (such as MdSTR2) in apple roots. Activation of these genes increased FA levels in roots and enhanced AM fungal colonization and arbuscule development. These findings identify a molecular pathway in which ABA signaling positively regulates FA biosynthesis and transport, thereby increasing lipid supply to AM fungi and promoting AM symbiosis.