Unraveling the Role of <scp>TARGET OF RAPAMYCIN</scp> in the Immune Response of <scp><i>Cucumis sativus</i></scp> to <i>Podosphaera xanthii</i>

Powdery mildew, caused by Podosphaera xanthii, poses a significant threat to cucumber production. Appropriate suppression of TARGET OF RAPAMYCIN (TOR) activity confers beneficial effects on cucumber resistance to P. xanthii. TOR serves as a core modulator essential for plant growth, development, and stress resistance. However, the molecular mechanisms underlying its role in plant immunity remain poorly understood. In this study, cucumber seedlings were treated with AZD8055, a TOR-specific inhibitor, and subsequently challenged with P. xanthii. Transcriptomic analysis revealed that TOR inhibition altered several key pathways, including phenylpropanoid biosynthesis, hormone signal transduction, plant-pathogen interaction, and MAPK signaling. Upon P. xanthii attack, TOR inhibition mobilized unique molecular players associated with cell wall modification, redox state, abscisic acid (ABA) signaling, gene transcription regulation, and respiratory burst. Notably, the expression of basic Helix-Loop-Helix 35 (bHLH35) and FCS-Like Zinc finger 15 (FLZ15) was induced during P. xanthii infection. Moreover, silencing bHLH35/FLZ15 decreased cucumber tolerance to P. xanthii stress. Overall, bHLH35 and FLZ15 likely drive immune function due to the action of TOR. Our findings elucidate the molecular events by which TOR modulates plant disease resistance in P. xanthii-infected cucumbers.