Funneliformis mosseae Modulates Root 14-3-3 Gene Expression to Enhance Drought Tolerance in Trifoliate Orange: A Molecular-Physiological Integration.

Abstract

The 14-3-3 family proteins are involved in plant metabolism and stress signal transduction, while whether and how they contribute to the enhancement of host drought tolerance by arbuscular mycorrhizal (AM) fungi remains unclear. This study aimed to identify the 14-3-3 gene family (GRF) in trifoliate orange (Poncirus trifoliata) and analyze their responses in roots to drought and inoculation with the AM fungus Funneliformis mosseae, integrating molecular data with physiological assessments. Despite drought-inhibited AM fungal colonization and soil mycelium length, AM inoculation markedly increased plant biomass, root growth, soluble sugars, antioxidant enzyme activities, and phytohormone levels under drought. Fifteen PtGRF members were identified with variable gene structures and abscisic acid/stress-related promoter elements. Drought stress predominantly suppressed the expression of most PtGRF genes in non-AM plants, while it triggered the upregulation of a specific subset (PtGRF3, 6, 9, 13, 14) in AM plants. Inoculation with AM fungi under drought conditions dramatically modulated the host's transcriptional response, significantly upregulating 10 of the 13 detected PtGRFs, most notably PtGRF13. Principal component and correlation analyses delineated distinct functional associations among PtGRF members: PtGRF1, PtGRF2, PtGRF5, and PtGRF15 were closely linked to soluble sugar accumulation and antioxidant defense, while PtGRF3, PtGRF6, PtGRF8, PtGRF9, and PtGRF13 showed stronger integration with abscisic acid and isopentenyl adenine. In summary, inoculation with AM fungi enhanced drought tolerance in trifoliate orange by modulating the expression of drought-responsive specific PtGRF genes, which functioned as integrated signaling modules to coordinate sugars, antioxidant defense, and abscisic acid levels.