Exogenous IBA promotes Castanea henryi seedling growth independently of ectomycorrhizal formation

Chinquapin ( Castanea henryi ) is an important nut-producing tree species in southern China. Symbiosis with ectomycorrhizal fungi (EF) can promote nutrient absorption, which is correlated with colonization rate (CR). Lateral roots (LR) development is a precursor of ectomycorrhizal formation, and exogenous indole-3-butyric acid (IBA) is known to promote LR development. However, it is still not known whether IBA application increases ectomycorrhizal CR and improves nutrient absorption. Also, how do host plants respond to exogenous IBA at the physiological and gene levels is still not clear. To eliminate the influence of fungal growth rate on symbiosis, we selected a strain of EF (LY-9, Scleroderma citrinum 1) whose growth was unaffected by exogenous IBA, and determined the optimal concentration of IBA that promotes LR development. Results indicated that (1) exogenous IBA did not significantly increase CR; (2) both IBA application and EF inoculation enhanced seedling height, root collar diameter, biomass, and root tips. Notably, EF inoculation had a greater promoting effect on seedlings N, P, and K uptake than IBA addition. EF inoculation increased IAA, IBA, and GA 3 content while reducing ABA content in roots and leaves. However, IBA uniquely reduced strigolactone (SLs) content in roots without affecting ABA or GA 3 ; (3) both IBA addition and EF inoculation downregulated the auxin response factor ChARF18 , but only EF induced upregulation of the symbiotic signaling gene ChNORK . These results indicated that exogenous IBA does not enhance EF colonization rate, and that both exogenous IBA and EF improve plant growth, albeit via different physiological and molecular strategies.