Role of auxin in the symbiotic relationship between Piriformospora indica and rice plants

Piriformospora indica (P. indica) is a beneficial endophytic fungus colonized with roots of a wide range of host plant species; however, the mechanisms of P. indica symbiosis with different plants could vary and need to be further addressed. Our study showed that root architecture and growth behaviors were changed in P. indicacolonized rice plants. The P. indica-induced primary root shorting and coiling phenomena were consistent with the auxin-responsive phenotype, suggesting that the auxin levels of P. indica-colonized roots were modulated. To investigate the changes in auxin levels, auxin-responsive gene expressions and promoter activities were used as indicators to monitor auxin levels in root tissues. The results showed that the auxin levels of primary roots were increased in P. indica-inoculated seedlings. Furthermore, an auxin inhibitor (p-chlorophenoxyisobutyric acid, PCIB) was applied to block auxin action to confirm whether auxin is a critical factor in regulating P. indicainduced root growth. The P. indica-induced root architecture modification was significantly repressed by PCIB which was added to the rice culture solutions at the initiation of P. indica inoculation. A proposed hypothesis was that the PCIB treatment might result in lower colonization and further reduce the influence of P. indica colonization on root architecture. The effect of PCIB on colonization was observed by counting the percent root colonization. The data showed that the colonization levels of P. indica in PCIB-treated seedlings were significantly decreased. Taken together, our data indicated that auxin levels would be increased in P. indica-colonized rice roots. Moreover, it was demonstrated that auxin has dual roles in the symbiotic relationship between rice and P. indica, which is a signal that triggers P. indica-induced root growth and a positive factor involved in the colonization mechanism.