Expression of alkaline phosphatase genes in arbuscular mycorrhizas

We identified a partial cDNA clone showing similarity to the yeast alkaline phosphatase (ALP) gene PHO8 in an EST library constructed from extraradical hyphae of Glomus intraradices. Using this clone and 5'-RACE (rapid amplification of cDNA ends), we cloned a full-length cDNA for ALP from G. intraradices. By using RT (reverse transcription)-PCR and degenerate primers reflecting conserved regions between the putative amino acid sequences of this cDNA and yeast alkaline phosphatases, we obtained a full-length ALP cDNA from Gigaspora margarita. These genes were designated GiALP and GmALP, respectively. Both GiALP and GmALP were constitutively expressed in mycorrhizal roots, irrespective of the growth stage, infection rate, and environmental phosphate concentration. The levels of GiALP and GmALP transcripts were higher in mycorrhizal roots than in germinating spores and extraradical hyphae. These results suggest that arbuscular mycorrhizal ALP is expressed under symbiotic conditions and that it may have a role in nutrient exchange with host plants rather than in nutrient uptake from the rhizosphere.