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.Fungi in the phylum Glomeromycota proliferate as part of an arbuscular mycorrhizal (AM) association (5,7,32). Carbon is transferred from colonized plants to AM fungi (14), while the plants usually receive much of their P through hyphal uptake and fungal transfer to the host roots (26, 33). Colonization by AM fungi increases the size of the C sink in the roots (9) and may be a significant cost to the host plant that results in reduced growth at high P levels (1,2,15,18,19,27). The adverse effect of high soil P levels on AM formation is primarily due to high P concentrations in the roots (31), which results in reduced C allocation to the AM fungus (24). Thus, there is an important connection between external P supply and C allocation to the fungal partner in the symbiosis. Moderate levels of P fertilization seem to be optimal for AM colonization (21). While the general effect of high P availability on C flow to the fungus has been well described, the effect of high P availability on C flow to the fungus within a few days is not known.A monoxenic system with carrot root organ cultures in symbiosis with the AM fungus Glomus intraradices can be used to study fungal growth (3) and is a well-established model system for studying metabolism and transport in the AM symbiosis (5, 10). We used a system in which the level of P was varied in a compartment that was mainly available to extraradical hyphae (17), which delayed the time until the root P levels were affected by the P treatment. C allocation was measured by compound-specific...

Responses of soil nitrogen-fixing, ammonia-oxidizing, and denitrifying bacterial communities to long-term chlorimuron-ethyl stress in a continuously cropped soybean field in Northeast China

“…Nitrosospira or Nitrosospira-like AOB dominated in our soybean field, which is in agreement with previous observations (He et al 2007;Mendum et al 1999;Phillips et al 2000;Olsson et al 2006). However, an interesting observation was that chlorimuron-ethyl had chronic toxic effects on Nitrosospira cluster 3a and 3d groups but had few sideeffects on Nitrosospira cluster 3b and 3c.…”

Response of Two Maize Inbred Lines with Contrasting Phosphorus Efficiency and Root Morphology to Mycorrhizal Colonization at Different Soil Phosphorus Supply Levels

“…The AM fungi act as a carbon drain and the carbon consumption of AM fungi may exceed the nutritional or yield benefit produced . A recent study showed that carbon (C) flow to fungi actually increased during the initial period of infection under high P conditions (Olsson et al, 2006). In the present study, intermediate and high P supply tended to suppress the beneficial effects of the AM fungi.…”

Phosphorus Availability Influences Elemental Uptake in the Mycorrhizal Fungus Glomus intraradices , as Revealed by Particle-Induced X-Ray Emission Analysis

“…However, the nutrients stored in spores may reflect not only the need for formation of new fungal biomass but also that of the plants that are being colonized. The establishment of new symbiotic relations could thus depend on the accumulation of plant-limiting nutrients in spores and their transport into the host roots for recognition of a nonparasitic intruder.The nutrient accumulation and distribution in AM fungal spores have not been studied in as much detail as carbon accumulation has (32), although there are clear links between the nutrient availability and C metabolism (16,17,18). Quantitative analysis of elemental composition in AM fungal mycelia is crucial for our understanding of the distribution of nutritional resources.…”

“…The nutrient accumulation and distribution in AM fungal spores have not been studied in as much detail as carbon accumulation has (32), although there are clear links between the nutrient availability and C metabolism (16,17,18). Quantitative analysis of elemental composition in AM fungal mycelia is crucial for our understanding of the distribution of nutritional resources.…”