Two differentially regulated phosphate transporters from the symbiotic fungus Hebeloma cylindrosporum and phosphorus acquisition by ectomycorrhizal Pinus pinaster

Abstract

SummaryEctomycorrhizal symbiosis markedly improves plant phosphate uptake, but the molecular mechanisms underlying this benefit are still poorly understood. We identified two ESTs in a cDNA library prepared from the ectomycorrhizal basidiomycete Hebeloma cylindrosporum with significant similarities to phosphate transporters from the endomycorrhizal fungus Glomus versiforme and from non-mycorrhizal fungi. The full-length cDNAs corresponding to these two ESTs complemented a yeast phosphate transport mutant (Dpho84). Measurements of 33 P-phosphate influx into yeast expressing either cDNA demonstrated that the encoded proteins, named HcPT1 and HcPT2, were able to mediate Pi:H + symport with different affinities for Pi (K m values of 55 and 4 lM, respectively). Real-time RT-PCR showed that Pi starvation increased the levels of HcPT1 transcripts in H. cylindrosporum hyphae grown in pure culture. Transcript levels of HcPT2 were less dependent on Pi availability. The two transporters were expressed in H. cylindrosporum associated with its natural host plant, Pinus pinaster, grown under low or high P conditions. The presence of ectomycorrhizae increased net Pi uptake rates into intact Pinus pinaster roots at low or high soil P levels. The expression patterns of HcPT1 and HcPT2 indicate that the two fungal phosphate transporters may be involved in uptake of phosphate from the soil solution under the two soil P availability conditions used.

Diversity in phosphorus mobilisation and uptake in ectomycorrhizal fungi

“…Levels of HcPT1 transcripts were always higher in fungal cells exposed to Pi starvation in solution or to low Pi availability in soil, suggesting that the regulation of this transport system is close to that of PHO84 in yeast. In contrast, transcript levels of HcPT2 were less dependent on Pi availability in fungal cells grown in vitro and were upregulated in ectomycorrhizal roots grown in soil with high P availability (Tatry et al 2009). Taken as a whole, these results indicate that H. cylindrosporum might use HcPT1 to mediate Pi uptake when soil P availability is low and HcPT2 when soil P availability is high.…”

“…It is therefore intriguing to find such rather high apparent affinity value measured for HcPT1 expressed in yeast and its functioning when Pi availability is low. As proposed by Tatry et al (2009), this could be due to heterologous expression that may modify the actual rates of Pi uptake and particularly the apparent K m value. Indeed, one would expect a low value of apparent K m of Pi uptake in ectomycorrhizal fungi (see Table 3), close to that found in HcPT2.…”

“…Furthermore, one partial cDNA (GmosPT) and one full-length cDNA (GiPT) putatively coding for Pi transporters have been identified in two AM species, Glomus mosseae and Glomus intraradices, respectively (Benedetto et al 2005;Maldonado-Mendoza et al 2001). Although the polypeptides encoded by GmosPT and GiPT were not functionally characterised yet, they all clustered with fungal Pi:H + polypeptides (Tatry et al 2009). All these transcripts have been predominantly detected in extraradical hyphae, with their expression level enhanced by low P availability, such as reported in G. intraradices (Maldonado-Mendoza et al 2001;Olsson et al 2006) and G. mosseae (Benedetto et al 2005).…”

“…In L. bicolor, whose the genome has been completely sequenced, five genes possibly coding for Pi transporters belonging to the Mafor Facilitating Superfamily (MFS) of transporters have been identified and named LbPht1;1 to LbPht1;5 (http://genome.jgi-psf.org/Lacbi1/Lacbi1.home.html). Two other genes, HcPT1 and HcPT2, have been identified in H. cylindrosporum (Tatry et al 2009). The peptide sequence of HcPT1 is very close to two L. bicolor predicted polypeptides (LbPht1;4 and LbPht1;5) and HcPT2 is close to the three other ones (LbPht1;1-3).…”

“…However, this was done for HcPT1 and HcPT2 (Tatry et al 2009) which were shown able to mediate Pi:H + symport with different affinities for Pi, the K m values being 55 and 4 μM, respectively, for HcPT1 and HcPT2. The apparent K m of HcPT2 was therefore comparable to that reported for PHO84 and lower than that of the gene GvPT (18 μM), which is the only value for a Pi transporter of AM fungi currently available for comparison (Harrison and van Buuren 1995).…”