Mycorrhizal trifoliate orange plants tolerate soil drought by enhancing photosynthetic physiological activities and reducing active GA3 levels

Gibberellins (GAs) are recognized as regulators of plant growth and drought responses, but it remains ambiguous whether these responses contribute to the enhanced drought tolerance of host plants mediated by arbuscular mycorrhizal (AM) fungi. This study aimed to investigate the effects of an AM fungal inoculation with Rhizophagus intraradices on biomass production, leaf gas exchange, chlorophyll fluorescence response, and leaf GA metabolism in trifoliate orange (Poncirus trifoliata) plants under drought stress. R. intraradices exhibited a pronounced affinity for root colonization, achieving rates between 78.52% and 85.45%, although drought treatment led to a decrease in this colonization. Compared with non-AM plants, AM plants exhibited significantly higher biomass production of leaves, stems and roots, irrespective of soil moistures, underscoring their resilience and growth superiority during drought. Arbuscular mycorrhizal trifoliate orange plants displayed lower bioactive GA1 and GA3 levels than non-AM plants under drought. This reduction was associated with the down-regulation of genes involved in GA biosynthesis (PtCPS, PtKAO and PtGA3ox) and the up-regulation of the GA deactivation gene (PtGA2ox) produced by AM fungal inoculation. Arbuscular mycorrhizal plants also displayed higher photosynthetic physiological activities than non-AM plants, as evidenced by significantly higher chlorophyll index, nitrogen balance index, maximum quantum yield of photosystem II, steady-state quantum yield, transpiration rate, net photosynthetic rate, intercellular CO2 concentration and stomatal conductance, as well as lower non-photochemical quenching during drought. Interestingly, bioactive GA3 levels, rather than GA1, showed a significantly negative correlation with leaf gas exchange parameters and light energy conversion efficiency during photosynthesis. In summary, AM fungal inoculation led to a reduction in leaf active GA levels by inhibiting GA biosynthesis and promoting GA deactivation, which contributed to 'drought avoidance' by suppressing biomass production; AM plants recorded superior photosynthetic physiological activities, associated with the regulation of GA3, not GA1.