Effect of P availability on temporal dynamics of carbon allocation and glomus intraradices high-affinity P transporter gene induction in arbuscular mycorrhiza

Abstract

Arbuscular mycorrhizal (AM) fungi depend on a C supply from the plant host and simultaneously provide phosphorus to the colonized plant. We therefore evaluated the influence of external P on C allocation in monoxenic Daucus carota-Glomus intraradices cultures in an AM symbiosis. Fungal hyphae proliferated from a solid minimal medium containing colonized roots into a C-free liquid minimal medium with high or low P availability. Roots and hyphae were harvested periodically, and the flow of C from roots to fungus was measured by isotope labeling. We also measured induction of a G. intraradices high-affinity P transporter to estimate fungal P demand. The prevailing hypothesis is that high P availability reduces mycorrhizal fungal growth, but we found that C flow to the fungus was initially highest at the high P level. Only at later harvests, after 100 days of in vitro culture, were C flow and fungal growth limited at high P availability. Thus, AM fungi can benefit initially from P-enriched environments in terms of plant C allocation. As expected, the P transporter induction was significantly greater at low P availability and greatest in very young mycelia. We found no direct link between C flow to the fungus and the P transporter transcription level, which indicates that a good C supply is not essential for induction of the high-affinity P transporter. We describe a mechanism by which P regulates symbiotic C allocation, and we discuss how this mechanism may have evolved in a competitive environment.

FIG. 1.

Root dry mass (A) and C uptake (B) influenced by P availability and time in monoxenic D. carota cultures colonized with an AM fungus. The error bars indicate standard errors (n = 4). The different types of lines indicate the results of two separate experiments in which similar setups were used. Solid symbols indicate high P availability, and open symbols indicate low P availability. Circles indicate normal C availability, and triangles indicate reduced C availability.

FIG. 2.

C flow to the AM fungal mycelium as indicated by C flow to the signature NLFA 16:1ω5 estimated both in extraradical mycelium and in roots. The error bars indicate standard errors (n = 4). The different types of lines indicate the results of two separate experiments in which similar setups were used. Solid symbols indicate high P availability, and open symbols indicate low P availability. Circles indicate normal C availability, and triangles indicate reduced C availability.

FIG. 3.

Neutral lipid-to-phospholipid ratios in intraradical (A) and extraradical (B) mycelia of G. intraradices in monoxenic cultures as estimated by use of the signature fatty acid 16:1ω5. The error bars indicate standard errors (n = 4). The different types of lines indicate the results of two separate experiments in which similar setups were used. Solid symbols indicate high P availability, and open symbols indicate low P availability. Circles indicate normal C availability, and triangles indicate reduced C availability.

FIG. 4.

(A) GiPT transcription levels as determined by quantitative PCR in the two experiments. The different types of lines indicate the results of two separate experiments in which similar setups were used. Solid symbols indicate high P availability, and open symbols indicate low P availability. The error bars indicate standard errors (n = 3). (B and C) GiPT gene induction expressed relative to induction of the β-tubulin gene, a constituently expressed gene. Dissociation curve profiles were generated (B) and gel electrophoresis of GiPT and β-tubulin PCR products (C) was performed using an Mx3000P real-time PCR system and SYBR green chemistry. The dissociation curves (temperature versus fluorescence) show that there is a single dissociation peak for each gene, which indicates that there was no primer dimer formation in the PCRs. Product quality and size were confirmed by gel electrophoresis.