Serendipita indica accelerates chlorophyll synthesis pathway and photosynthetic efficiency in trifoliate orange subjected to water deficit

Chlorophylls are pivotal in capturing and converting light energy in leaves during photosynthesis, and changes in their biosynthetic pathway represent an adaptive response of plants to challenging environmental conditions. Serendipita indica, a culturable endophytic fungus, has demonstrated potential in enhancing the host plant's drought tolerance, whereas the underlying mechanisms (e.g., chlorophyll biosynthesis pathways) remain unclear. This study explored the impact of S. indica on the growth, chlorophyll synthesis intermediates and chlorophyll components, chlorophyll fluorescence parameters, photosynthesis, and the expression of chlorophyll synthesis-associated genes in trifoliate orange (Poncirus trifoliata) plants subjected to seven weeks of water deficit (WD). Following WD, the colonization of S. indica in roots significantly increased. Despite the growth-inhibiting effects of WD on trifoliate orange, inoculation with S. indica led to a substantial increase in aboveground growth performance and biomass production, with the magnitude of increase being more pronounced under WD than under no water dificit (NW). The WD treatment led to a reduction in the levels of leaf chlorophyll synthesis intermediates, while the colonization of S. indica significantly elevated the levels of 5-aminolevulimic acid, Mg-protoporphyrin IX, protoporphyrin IX, porphobilinogen, and protochlorophyllide in leaves, thus increasing chlorophyll a, b, and a + b concentrations in S. indica-treated plants, particularly under WD. S. indica inoculationi also up-regulated the expression level of PtHEMG1, PtCHLH, PtHEMA1, and PtHEME2 genes under WD. Interestingly, chlorophyll intermediates and the expression of chlorophyll-associated genes were significantly correlated with chlorophyll a in uninoculated plants and with chlorophyll b in inoculated plants. Under WD, S. indica-inoculated plants represented higher steady-state light quantum efficiency and lower steady-state nonphotochemical fluorescence quenching than uninoculated plants, which protected the photosynthetic apparatus from damage and increased the quantum efficiency of PSII. In addition, leaf gas exchange parameters were distinctly boosted by S. indica, with the increase being more pronounced under WD than under NW. In conclusion, S. indica-inoculated plants have better plant growth, chlorophyll biosynthesis, photosynthetic efficiency and gas exchange to adapt to WD.