Mycorrhiza-mediated manganese dynamics drive photosynthetic adaptation to water deficit in trifoliate orange

Manganese (Mn), a critical component of the photosystem II oxygen-evolving complex, chlorophyll biosynthesis pathway and antioxidant systems, manifests functional mechanisms that remain inadequately elucidated in the context of arbuscular mycorrhizal fungi (AMF)-mediated plant tolerance to water deficit (WD). This study examined how Funneliformis mosseae (T.H. Nicolson & Gerd.) C. Walker & A. Schüßler inoculation enhances WD (55% maximum of the maximum field water capacity for 10 weeks) tolerance in trifoliate orange (Poncirus trifoliata) by modulating Mn chemical forms and key physiological processes. AMF inoculation significantly improved various growth parameters irrespective of soil moisture. AMF inoculation significantly enhanced photosynthetic efficiency, various chlorophyll levels and photosystem stability under WD. In leaves, AMF inoculation significantly increased levels of inorganic, bound and residual Mn fractions under varying moisture conditions, while concurrently reducing oxalate-bound Mn, in addition to an increase in phosphate Mn under WD. AMF colonization upregulated the expression of PtHEMG1 and PtMnSOD under WD, and also modulated the expression of P. trifoliata metal tolerance proteins (PtMTPs), as evidenced by the enhancement of specific PtMTP members (PtMTP4/5/7/9) under normal watered and the suppression of PtMTP3/9 under WD. Correlation analysis demonstrated coordinated regulation among photosynthetic efficiency, Mn levels, PtMTPs and PtHEMG1. In conclusion, the AMF-induced shift in Mn chemical forms (e.g. pectate-/protein-bound Mn) coordinated with enhanced chlorophyll biosynthesis and photosynthetic performance in trifoliate orange plants under WD.