The differential and interactive effects of arbuscular mycorrhizal fungus and phosphorus on the lateral root formation in Poncirus trifoliata (L.)

Both arbuscular mycorrhizal fungi (AMF) and phosphorus (P) can regulate the lateral root (LR) formation of plants; however, the differences in their regulation and whether auxin is involved in the regulation are less understood, especially for woody plants. In this study, trifoliate orange ( Poncirus trifoliata (L.) Raf) plants were inoculated with AMF strain, Rhizophagus irregularis BGC JX04B, under two P levels in pot culture. The influences of AMF and P level on LR formation were investigated, and the expression of LR- and auxin-related genes were quantified. Results showed that the mycorrhizal colonization at low P level was higher than that at high P level. AMF increased shoot biomass and decreased R/S ratio. The numbers of 1st and 2nd LR were significantly increased by AMF, while high P level only increased 2nd LR number. Variation partitioning analysis indicates that AMF was more effective than P level in regulating LR formation. At 2 MAT (month after transplanting), AMF significantly increased the expression of PSK6 and RSI-1 , and high P level significantly increased the expression of KRP6 . At 4 MAT, AMF significantly increased the expression of PSK6 , but decreased the expression of KRP6 . High P level significantly inhibited the expression of NAC1 , while AMF increased the expression of NAC1 at low P level at 2 MAT, promoting the LR formation. AMF also significantly increased the expression of NAC1 at 4 MAT. Besides, the development of 1st and 2nd LR presented a positively significantly relation with the expression of PSK6 and NAC1 , and 2nd LR also with the expression of TIR1 . Taken together, these data suggest that AMF and P level differentially affect the LR formation in P. trifoliata via auxin signaling.