Acidic soil inhibits the functionality of arbuscular mycorrhizal fungi by reducing arbuscule formation in tomato roots

Acidic soil is widely distributed in terrestrial ecosystems, which causes large challenges to crop production. Arbuscular mycorrhizal fungi (AMF) can increase plant tolerance to acidic soil; however, the effects of acidic soil on the functionality of AMF and arbuscule formation are far from being thoroughly understood. In this study, we inoculated tomato plants with Rhizophagus irregularis at pH 4.5 (original acidic soil) or pH 6.5 (limed soil), and monitored the mycorrhizal colonization, alkaline phosphatase (ALP) activity, and expression of SlPTs (which encode phosphate transporters) and EXO70s (which encode subunits of exocysts) in the roots. We aimed to characterize the arbuscule development in colonized roots in response to acidic soil and to investigate how acidic soil affects the functionality of AMF. Our results revealed that acidic soil sharply reduced arbuscule abundance by approximately 90%, and greatly impeded arbuscule development such that no mature arbuscules were observed. The negative effect of acidic soil on arbuscule formation was supported by EXO70A1-like expression. The functionality of AMF, e.g., ALP activity in arbuscules and the expression of SlPT4 and SlPT5, was simultaneously greatly inhibited in acidic soil in the same manner as that of arbuscule formation. The close relationship between the functionality of AMF and arbuscule abundance in this study indicates that acidic soil strongly inhibits AMF function mainly via a reduction in arbuscule formation. Considering the coupling of arbuscule formation and periarbuscular membrane construction, the fine-tuning of both processes in response to acidic soil merits additional in-depth investigations.