The cloning of PDR1 OE, a GFP‐PDR1 protein fusion driven by the 35SCaMV promoter, was previously described by Sasse et al. (2015). Three independent PDR1 OE lines have been used for this and previous research and confirmed for GFP‐PDR1 gene and protein expression. P. hybrida W115 (Mitchell) growth was tested in six specific soil mixtures, which differed in Pi content (see details in Table 1) and in the presence or absence of mycorrhizal fungi, in order to study the effects of increased lateral root density and mycorrhiza on plant biomass development and Pi uptake. P. hybrida W115 (wild‐type background of PDR1 OE), PDR1 OE, pdr1 ko (Kretzschmar et al., 2012) and W115 × W138 (wild‐type background of pdr1 ko) plants were grown under long‐day conditions (16 : 8 h, light : dark regime), at 60% relative humidity and 25°C on different soil mixtures. These were: natural soil (from University of Zurich botanical garden, with naturally occurring mycorrhizal fungi); clay granules (Oil Dri US‐Special, Chicago, IL, USA) and mineral soil (subsoil, also known as B‐soil horizon from University of Zurich botanical garden). Also soil mixes were used, as follows: natural soil mix (70% natural soil and 30% mineral soil) and Claymin (50% clay and 50% mineral soil). For clay+ and Claymin+, respective substrates were supplemented with half a teaspoon in 500 ml substrate of a sand inoculum of Rizophagus irregularis, a common AMF frequently used for inoculation studies (Martin et al., 2008). The inoculum was added 2 wk after plant germination with a low Pi inoculation medium prepared as previously described (Reddy et al., 2007), so that a total Pi amount of 0.112 mg was added to the initial 0.46 mg per pot (i.e. a total of 0.001 g l−1). The pot volume used for all experiments was 500 ml. Alternatively, P. hybrida seeds were plated on 0.85% (w/v) Phyto Agar (Duchefa, Haarlem, the Netherlands) medium containing 2.2 g l−1 Murashige and Skoog (half‐strength MS (§ MS)) medium (Duchefa) at 21°C. Low‐Pi MS medium for root experiments contained 0.25 mM instead of 1.25 mM KH2PO4 as in § MS. Clay was chosen against mineral soil and full soil for analyses on the root system architecture because of easier washing away from roots for quantification of mycorrhization while at the same time keeping soil humidity more constant than mineral soil or sand.