Low Dosage of ABA Enhances Arbuscule Formation and Recovers the Inhibitory Effect of Low pH on This Process

Arbuscular mycorrhizal (AM) fungi can form symbiotic associations with plants and play a significant role in enhancing plant tolerance to acidic stress, wherein arbuscules serve as key structures in this process. However, the response patterns of arbuscule development and function under low pH conditions remain poorly understood. Previous studies have shown that abscisic acid (ABA) can regulate arbuscule development, but whether ABA regulates arbuscule development and function under low pH conditions is unknown. In this study, the model plant tomato (Solanum lycopersicum) was used as the host plant, inoculated with AM fungi to investigate the regulatory effects of low pH and exogenous ABA on arbuscule development and function. The results showed that (1) as time progressed, the mycorrhizal colonization increased, and arbuscules gradually developed from the main trunk to mature and senescent stages; however, low pH values inhibited arbuscule development. (2) High concentrations of ABA inhibited root growth and mycorrhizal colonization, whereas low concentrations promoted mycorrhizal colonization, with 10^-7 M identified as the optimal concentration for maximizing mycorrhizal colonization. (3) Both low pH and ABA-deficient mutants significantly inhibited mycorrhizal colonization, alkaline phosphatase activity, and the expression of genes related to arbuscular development. However, exogenous ABA did not significantly affect the expression of genes associated with arbuscular function. Low concentrations of ABA could restore the inhibition of arbuscule development and function caused by low pH and ABA-deficient mutants. Additionally, low pH significantly inhibited the ABA content in mycorrhizae, while exogenous ABA treatment significantly increased the ABA content in mycorrhizae. Our research results indicate that low dosage of ABA enhances arbuscule formation and function, and recovers the inhibitory effect of low pH on this process. Low pH may regulate arbuscule development and function by modulating ABA in roots, and ABA may regulate mycorrhizal development by affecting lipid synthesis and transport.