ABA and Ethylene Mediates Tomato Root Development Modulation During Endophytic Fungal Interaction

The early stages of plant-microbe interaction are critical for establishing beneficial symbioses. We investigated how the endophytic fungus Fusarium solani strain FsK modulates tomato (Solanum lycopersicum) development and hormone pathways during in vitro co-cultivation. Seedlings were sampled at three early interaction stages (pre-contact, T1; initial contact, T2, 3 days post-contact, T3). Root traits and root and leaf transcripts for abscisic acid (ABA) and ethylene (ET) pathways were quantified, alongside fungal ET-biosynthesis genes. FsK altered root system architecture, increasing root area, lateral root number, root-hair length, and fresh biomass. These morphological changes coincided with tissue- and time-specific shifts. In leaves, FsK broadly affected ABA biosynthetic and homeostasis genes (ZEP1, NCED1, ABA2, AAO1, ABA-GT, BG1), indicating reduced de novo synthesis with enhanced deconjugation of stored ABA. ET biosynthesis was curtailed in leaves via down-regulation of ACC oxidase (ACO1-3), with isoform-specific changes in ACC synthase (ACS). The ET receptor ETR1 was transiently expressed early (T1-T2). FsK itself showed staged activation of fungal ET-biosynthesis genes. These results reveal coordinated fungal-plant hormone control at the transcriptional level that promotes root development during early interaction and support FsK's potential as a biostimulant.