The expression of GintPT, the phosphate transporter of Rhizophagus irregularis, depends on the symbiotic status and phosphate availability

Abstract

The development of mutualistic interactions with arbuscular mycorrhizal (AM) fungi is one of the most important adaptation of terrestrial plants to face mineral nutrition requirements. As an essential plant nutrient, phosphorus uptake is acknowledged as a major benefit of the AM symbiosis, but the molecular mechanisms of its transport as inorganic phosphate (Pi) from the soil to root cells via AM fungi remain poorly known. Here we monitored the expression profile of the high-affinity phosphate transporter (PT) gene (GintPT) of Rhizophagus irregularis (DAOM 197198) in fungal structures (spores, extraradical mycelium and arbuscules), under different Pi availability, and in respect to plant connection. GintPT resulted constitutively expressed along the major steps of the fungal life cycle and the connection with the host plant was crucial to warrant GintPT high expression levels in the extraradical mycelium. The influence of Pi availability on gene expression of the fungal GintPT and the Medicago truncatula symbiosis-specific Pi transporter (MtPT4) was examined by qRT-PCR assay on microdissected arbusculated cells. The expression profiles of both genes revealed that these transporters are sensitive to changing Pi conditions: we observed that MtPT4 mRNA abundance is higher at 320 than at 32 μM suggesting that the flow towards the plant requires high concentrations. Taken on the whole, the findings highlight novel traits for the functioning of the GintPT gene and offer a molecular scenario to the models describing nutrient transfers as a cooperation between the mycorrhizal partners.

Arbuscular Mycorrhizal Symbiosis Requires a Phosphate Transceptor in the Gigaspora margarita Fungal Symbiont

“…GigmPT may mediate phosphate uptake at fungus-soil interface under Pi shortage regarding its transcription in extraradical hyphae (Supplemental Figure 4), and could be functional in AM symbiosis inside the roots regarding transcription of GigmPT during the intraradical phase ( Figure 3A and 3B). We further localized GigmPT expression in the arbuscules and intraradical hyphae ( Figures 3C and 4), and found that the expression profiling of GigmPT is similar to its homologs, as described previously ( Benedetto et al, 2005, Balestrini et al, 2007and Fiorilli et al, 2013. This finding poses a new question regarding the function of GigmPT at the plantfungal interface: whether the fungus reabsorbs the Pi from the apoplast or PAS (Fiorilli et al, 2013).…”

“…We further localized GigmPT expression in the arbuscules and intraradical hyphae ( Figures 3C and 4), and found that the expression profiling of GigmPT is similar to its homologs, as described previously ( Benedetto et al, 2005, Balestrini et al, 2007and Fiorilli et al, 2013. This finding poses a new question regarding the function of GigmPT at the plantfungal interface: whether the fungus reabsorbs the Pi from the apoplast or PAS (Fiorilli et al, 2013). Based on the high-affinity property of GigmPT and the localization of GigmPT protein in the arbuscules, we prefer the reabsorption hypothesis whereby GigmPT is in charge of the Pi influx from the apoplast to the fungus (Supplemental Figure 19).…”

“…In AM fungi, transport of Pi across the PM of extraradical hyphae is mediated by known PTs (Supplemental Figure 19), such as GvPT, GintPT, and GmosPT (Harrison and van Buuren, 1995, Benedetto et al, 2005and Fiorilli et al, 2013. The new results show that G. margarita expresses a phosphate transporter (GigmPT) that is able to transport phosphate ( Figure 2).…”

“…Radiotracer studies demonstrated that the extraradical hyphae acquire phosphate, which is later translocated to the intraradical hyphae and then released to the plant cells (Jakobsen et al, 1992). The activated AM fungal phosphate transporters (PTs) absorb external inorganic phosphate (Pi) into the extraradical hyphae (Harrison and van Buuren, 1995, Maldonado-Mendoza et al, 2001and Fiorilli et al, 2013 and Pi accumulates in the vacuoles of extraradical hyphae as polyphosphate (polyPi) (Ezawa et al, 2003). PolyPi chains are thought to be transferred by a tubular vacuolar network (Timonen et al, 2001 andUetake et al, 2002) into the intraradical hyphae and arbuscules, where the Pi is hydrolyzed from polyPi by a fungal phosphatase and exopolyphosphatase, after which the Pi is released into the plant interfacial apoplast of arbuscules (Javot et al, 2007).…”

“…These fungal PT genes that have been identified so far are located in the extraradical hyphae, thus suggesting their roles in Pi acquisition from the soil. GmosPT and GintPT are also expressed in the intraradical fungal structures ( Balestrini et al, 2007 andFiorilli et al, 2013), thus indicating that these transporters may play potential roles in intraradical hyphae and arbuscules. This finding raises new questions about the function of AM fungal PTs during AM symbiosis.…”