Arbuscular mycorrhizal inhibition of growth in barley cannot be attributed to extent of colonization, fungal phosphorus uptake or effects on expression of plant phosphate transporter genes

• Here, we used phosphorus-32 (³²P) labelling in compartmented pots combined with quantitative real-time polymerase chain reaction (PCR) analysis of phosphate (Pi) transporter gene expression to investigate regulation of Pi uptake pathways in barley (Hordeum vulgare), an arbuscular mycorrhizal (AM) plant that does not show strong positive growth responses to colonization. • Barley was colonized well by Glomus intraradices and poorly by Glomus geosporum, but both fungi induced significant and similar growth depressions compared with nonmycorrhizal controls. The lack of correlation between per cent colonization and extent of growth depression suggests that the latter is not related to carbon drain to the fungus • The contribution of the AM Pi uptake pathway for the two fungi was, in general, related to per cent colonization and expression of the AM-inducible Pi transporter gene, HvPT8, but not to plant responsiveness. Glomus intraradices contributed 48% of total plant P whereas G. geosporum contributed very little. • The growth depression in plants where the AM uptake pathway was functional suggests that the contribution of the direct Pi uptake pathway via root hairs and epidermis was decreased. This decrease was not correlated with downregulation of the epidermal-expressed Pi transporter genes, HvPT1 and HvPT2. We hypothesize post-transcriptional or post-translational control of this transport process by AM colonization.