Concomitantly with the long‐lasting, high mycorrhization rate of PDR1 OE plants on clay+ and the transient effect on natural soil mix, on clay+ we observed a significantly faster and long‐lasting increase of shoot biomass compared with the wild‐type (Fig. 1g) and a similar and transient increase on natural soil mix at 6 wag (Fig. 1h). As PDR1 OE shoot biomass on natural soil mix is lower than on clay+, the R. irregularis inoculum seems to induce a faster and more efficient mycorrhization than the naturally occurring mycorrhizal mix. To test if low Pi and AMF are both necessary for the growth advantage of PDR1 OE plants, we compared biomass production on full soil (our Pi richest soil) and on clay (without AMF). On full soil and on clay, no significant differences in biomass production could be observed 8 wag (Fig. 2a,b), while on clay+, used as positive control in this experimental setup, both shoot and root biomass production of PDR1 OE was again significantly higher than in the wild‐type (Fig. 2c). Interestingly, by comparing clay and clay+ results, it turned out that PDR1 OE can cope better with mycorrhization compared with the wild‐type, which is exploited to an extent by AMF in low‐Pi conditions (Fig. 2d–g). These results indicate that the faster mycorrhization on clay+ obtained through PDR1 OE is a major trait providing consistent advantage for plant growth, and that PDR1 OE plants can sustain more advantageous tradeoffs with AMF compared with the wild‐type. In order to assess whether the growth advantage on clay+ might be conferred by an enhanced plant Pi status, we determined the expression levels in roots of P. hybrida PHOSPHATE TRANSPORTER 3 (PhPT3) and P. hybrida PHOSPHATE TRANSPORTER 5 (PhPT5), which are known to be strongly up‐regulated by mycorrhization (Breuillin et al., 2010). At 6 wag, the phosphate transporters were up‐regulated in PDR1 OE roots on both natural soil mix and clay+. At 8 wag, PhPT3 and PhPT5 were strongly induced under clay+ conditions and slightly down‐regulated in natural soil mix, according to the results obtained with the mycorrhization quantification via qPCR (Fig. S1k–n).