The symbiosis between plants and mycorrhizal fungi is a continuous tradeoff as long as both organisms are beneficial to each other (Ryan et al., 2012). Therefore we tested PDR1 OE and wild‐type plants grown on soils with lower Pi concentrations than clay, where high levels of mycorrhization could lead to carbon exploitation from the host plant rather than to beneficial Pi upload from the fungus. On mineral soil (see Table 1), P. hybrida plants could not grow; therefore, we assayed plant biomass production at 8 wag on a substrate mixture called Claymin (50% clay plus 50% mineral soil ± AMF) (see the Materials and Methods section and Table 1). Under these conditions and in the absence of AMF, shoot and root growth of PDR1 OE plants were significantly higher than that of wild‐type plants (Fig. 4a,b,e,f). However, the addition of AMF did not cause significant increases in biomass production (Fig. 4c–f). In Claymin+ growth conditions, the costs of energy supply to the mycorrhiza seem to exceed the benefit obtained from the fungus. In none of the tested substrates could PDR1 OE plants reach the biomass production in full soil: still, on clay+ and Claymin, PDR1 OE plants could produce more shoot and root biomass than the wild‐type up to 8 wag (Fig. 4e,f). Consistently, an increase of Pi uptake is significant only in PDR1 OE plants grown on clay+ or Claymin (Fig. S2c).