Allocation of plant carbon to foraging and storage in arbuscular mycorrhizal fungi

Hansson

Burleigh

2006

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...

Section: Discussionmentioning

confidence: 72%

Effect of P Availability on Temporal Dynamics of Carbon Allocation and Glomus intraradices High-Affinity P Transporter Gene Induction in Arbuscular Mycorrhiza

Hansson

Burleigh

2006

“…Typical for Glomus intraradices is that this fungus can also form intraradical spores, which may have a storage function as well. Lipids may, however, also be transported in lipid bodies to the extraradical mycelium within a few days (3,11).…”

mentioning

confidence: 99%

Fungal Lipid Accumulation and Development of Mycelial Structures by Two Arbuscular Mycorrhizal Fungi

Aarle

2003

128

We monitored the development of intraradical and extraradical mycelia of the arbuscular mycorrhizal (AM) fungi Scutellospora calospora and Glomus intraradices when colonizing Plantago lanceolata. The occurrence of arbuscules (branched hyphal structures) and vesicles (lipid storage organs) was compared with the amounts of signature fatty acids. The fatty acid 16:15 was used as a signature for both AM fungal phospholipids (membrane constituents) and neutral lipids (energy storage) in roots (intraradical mycelium) and in soil (extraradical mycelium). The formation of arbuscules and the accumulation of AM fungal phospholipids in intraradical mycelium followed each other closely in both fungal species. In contrast, the neutral lipids of G. intraradices increased continuously in the intraradical mycelium, while vesicle occurrence decreased after initial rapid root colonization by the fungus. S. calospora does not form vesicles and accumulated more neutral lipids in extraradical than in intraradical mycelium, while the opposite pattern was found for G. intraradices. G. intraradices allocated more of its lipids to storage than did S. calospora. Thus, within a species, the fatty acid 16:15 is a good indicator for AM fungal development. The phospholipid fatty acid 16:15 is especially suitable for indicating the frequency of arbuscules in the symbiosis. We propose that the ratio of neutral lipids to phospholipids is more important than is the presence of vesicles in determining the storage status of AM fungi.Fungi in the Glomeromycota are obligate symbionts, which colonize host plant roots from spores, extraradical hyphae, or previously colonized roots. Hyphae grow from colonized roots into the soil and form the extraradical mycelium. Lipid droplets in the hyphae accumulate in the developing spores (3, 7). The extraradical mycelium may spread along the root to form new entry points, but usually spreads out from the host root to form an extensive extraradical mycelium (9,25). From the point of mycorrhizal colonization, intercellular (Arum-type colonization) or intracellular (Paris-type colonization) hyphae spread into the root and side branches of hyphae and produce arbuscules, finely branched hyphal structures surrounded by the host plasma membrane (30). In this way, a large area of contact with the host is created, increasing the area of the host plasma membrane up to 10-fold in colonized cells (31). Arbuscules are short-lived structures believed to have a turnover rate of 1 to 2 weeks (1) and probably are a critical site for nutrient transfer between the symbionts (30). At a later stage, the fungus may form vesicles, which are lipid-filled storage structures (7, 20) with a low turnover rate, in intercellular spaces. Typical for Glomus intraradices is that this fungus can also form intraradical spores, which may have a storage function as well. Lipids may, however, also be transported in lipid bodies to the extraradical mycelium within a few days (3, 11).The amount of neutral lipids (for energy storage) is usually higher ...

“…Plants were harvested 1 week after labeling, and soil and root samples were collected. 13 Cenrichment was determined in fatty acids extracted from mycelium collected from the sand compartment by wet sieving (18).…”

Section: Methodsmentioning

confidence: 99%

¹³ C Incorporation into Signature Fatty Acids as an Assay for Carbon Allocation in Arbuscular Mycorrhiza

Aarle

Gavito

et al. 2005

The ubiquitous arbuscular mycorrhizal fungi consume significant amounts of plant assimilated C, but this C flow has been difficult to quantify. The neutral lipid fatty acid 16:15 is a quantitative signature for most arbuscular mycorrhizal fungi in roots and soil. We measured carbon transfer from four plant species to the arbuscular mycorrhizal fungus Glomus intraradices by estimating 13 C enrichment of 16:15 and compared it with 13 C enrichment of total root and mycelial C. Carbon allocation to mycelia was detected within 1 day in monoxenic arbuscular mycorrhizal root cultures labeled with [13 C]glucose. The 13 C enrichment of neutral lipid fatty acid 16:15 extracted from roots increased from 0.14% 1 day after labeling to 2.2% 7 days after labeling. The colonized roots usually were more enriched for 13 C in the arbuscular mycorrhizal fungal neutral lipid fatty acid 16:15 than for the root specific neutral lipid fatty acid 18:26,9. We labeled plant assimilates by using 13 CO 2 in whole-plant experiments. The extraradical mycelium often was more enriched for 13 C than was the intraradical mycelium, suggesting rapid translocation of carbon to and more active growth by the extraradical mycelium. Since there was a good correlation between 13 C enrichment in neutral lipid fatty acid 16:15 and total 13 C in extraradical mycelia in different systems (r 2 ‫؍‬ 0.94), we propose that the total amount of labeled C in intraradical and extraradical mycelium can be calculated from the 13 C enrichment of 16:15. The method described enables evaluation of C flow from plants to arbuscular mycorrhizal fungi to be made without extraction, purification and identification of fungal mycelia.Arbuscular mycorrhizal (AM) fungal mycelia acquire hexoses released by the roots of their host (3,39,42,43) and metabolize them to lipids, mainly neutral lipids, such as triacylglycerols (39). Neutral lipids are transported throughout the fungal mycelium (4), are metabolized through the glyoxalate cycle (3,24), and probably provide the major fungal energy source. The mechanisms that regulate C transfer from plant to fungus are not well understood (21). However, AM fungal colonization affects plant C metabolism (13,38,51,52) and the genes that regulate this metabolism (20,40).AM fungal neutral lipids usually are stored in intraradical vesicles or in spores and make up a large proportion of the AM fungal biomass (6,22,36). The fatty acids of these lipids have a characteristic and specific composition (7). In Glomus intraradices, 50 to 70% of the neutral lipids are the fatty acid 16:15 (19, 35), which is uncommon in other groups of fungi (28,32,47) and can be used as an AM fungal signature (31, 37).13 C nuclear magnetic resonance has been used to determine C-metabolic pathways after 13 C labeling of monoxenic root cultures (3). The incorporation and turnover of C in AM fungal mycelium can be difficult to measure because the hyphae are not easily extracted and separated from roots or soil.13 C enrichment or 13 C dilution of signature compounds, such as ...