Salicylic acid signaling controls the colonization behavior of Colletotrichum tofieldiae in Arabidopsis thaliana

Plant–microbe interactions strongly influence plant growth and nutrient acquisition, and their outcomes depend on nutrient availability. The root endophyte Colletotrichum tofieldiae ( Ct ) promotes growth in Arabidopsis thaliana under inorganic phosphate (Pi) limitation, but its effects under Pi sufficiency and the role of salicylic acid (SA) signaling remain unclear. Here, we examined Pi-dependent growth responses, nutrient accumulation, and SA signaling in wild-type (WT) and SA-deficient ics1 mutant plants co-cultivated with Ct under low, moderate, and high Pi conditions (25, 150, and 625 µM). Under low Pi, Ct significantly enhanced WT growth, increasing leaf number and root length by 41.8% and 50.5%, respectively, and promoting biomass accumulation, with fresh and dry weight increases of 104% and 232% relative to uninoculated controls. Growth promotion was reduced at moderate Pi and shifted toward growth suppression under high Pi. Elemental profiling using inductively coupled plasma mass spectrometry (ICP-MS) revealed pronounced Ct -mediated nutrient accumulation under Pi limitation. At low Pi, phosphorus content increased by 281%, accompanied by significant increases in K (70.1%), S (84.5%), and Ca (73.2%). In contrast, at moderate and high Pi, Ct consistently enhanced P accumulation, while changes in K, S, and Ca were not significant. Ct colonization induced expression of the SA-responsive marker gene PR1 , particularly under low Pi. In contrast, ics1 mutants failed to exhibit Ct- induced growth promotion and instead displayed growth suppression across all Pi conditions. Together, these findings demonstrate that Pi availability and ICS1-mediated SA biosynthesis jointly determine the outcome of the Arabidopsis – Ct interaction.