To test if PDR1 expression levels influence mycorrhization, we first quantified it via the grid method on natural soil mix (see the Materials and Methods section). PDR1 OE roots exhibited a mycorrhization rate of 20% compared with 10% for wild‐type plants; pdr1 ko plants scored < 2% (Fig. 1b). As the grid method does not easily allow the quantification of mycorrhization along the whole root, we further scored mycorrhizal rates via qPCR with specific mycorrhizal markers: GiTUB_1 (primers kindly provided by Prof. Didier Reinhardt) and GiTUB_2 (Alkan et al., 2004). On natural soil mix, PDR1 OE plants conserved stronger than the wild‐type GiTUB expression levels until 6 wk after germination (wag) and only on clay+ (100% clay supplied with the AMF Rhizophagus irregularis) up to 8 wag (Fig. 1c–f). In detail, relative to the wild‐type on natural soil mix, on clay+ wild‐type plants showed four‐ to fivefold higher GiTUB_1 expression while PDR1 OE plants showed 60‐ to 150‐fold higher expression at 6 and 8 wag, respectively (Fig. 1c,d). With GiTUB_2 the trend was similar on clay+: wild‐type plants scored 1.3‐ to twofold induction and PDR1 OE plants 22–100 times higher expression levels (Fig. 1e,f), confirming the higher mycorrhization capability of PDR1 OE on low‐Pi soils such as clay+. The different expression levels of GiTUB1_1 and GiTUB_2 are probably a result of the primer specificity. A BLAST analysis with GiTUB_1 hit four different types of the genus Glomus (G. clarum, G. claroideum, G. intraradices, G. diaphanum) while GiTUB_2 are specific for G. intraradices (now Rhizophagus irregularis). With both GiTUB primers, PDR1 OE scored six to 10 times stronger expression on natural soil mix at 6 wag, while at 8 wag the wild‐type scored higher mycorrhization than PDR1 OE, although with low statistical significance (Fig. 1d,f). Via the gridline quantification, we observed partly overlapping but underestimated trends, probably owing to the limited amount of root we could visualize on the total and to mycorrhization spatial heterogeneity (Gamper et al., 2008), both along the root and through the cortex layers of P. hybrida roots. In summary, we observed significantly higher mycorrhization only in PDR1 OE plants compared with the wild‐type at 6 wag on clay+ (Fig. S1a,b). The mycorrhizal structures observed via the grid method at 8 wag comprised mostly arbuscules (Fig. S1c–j) both on natural soil mix (88.6 ± 4.9%) and on clay+ (74.6 ± 8.3%). Of the nonarbuscular structures, no fungal vesicles were present on natural soil mix and 8.06 ± 0.92% (n = 3) of vesicles were present on clay+.