Rhizosphere yeasts improve P uptake of a maize arbuscular mycorrhizal association

Plant roots associate with microorganisms to acquire phosphorus (P) from the soil including arbuscular mycorrhizal (AM) fungi and phosphate solubilizers such as yeasts. However, information on interactions between AM fungi and yeasts is limited. The objective of the present study was to examine possible cooperation between the AM fungus Rhizophagus irregularis and the rhizosphere yeasts Cryptococcus flavus and Candida railenensis in terms of maize plant growth and P uptake employing P isotope tracer monitoring. Compartmented growth units with root-hyphal (RHC) and hyphal (HC) mesh bags allowed studying interactions between R. irregularis and yeasts with and without roots. The P isotope dilution method was used to measure possible phosphate solubilization in soil by the rhizosphere yeasts with subsequent P uptake by the AM association. Plants were grown for 8 weeks in low P soil under controlled growth chamber conditions. Main results showed strong plant growth promotion after inoculation with R. irregularis and both yeast species altered the specific root length of maize. The yeasts also improved maize shoot P content, but only in mycorrhizal plants. More specifically yeasts improved AM hyphal P-32 uptake, but not that of root P-33 uptake. On the other hand, yeasts had no effect on extraradical growth of R. irregularis. Inoculation with yeasts did not result in P isotope dilution measured as specific P isotope activity, indicating that the reported improved P uptake from AM associations in combination with yeasts was most likely not related to P solubilization, whereas alteration in root growth and specific root length seems more likely mechanisms of interactions. We conclude that rhizosphere yeasts promote P nutrition of arbuscular mycorrhizal maize linked to improved specific root length and AM hyphal P uptake.