Characterization of two arbuscular mycorrhizal fungi in symbiosis with Allium porrum: inflow and flux of phosphate across the symbiotic interface

Abstract

Individual arbuscular mycorrhizal fungi can differ markedly in their ability to improve the phosphate nutrition and growth of host plants. In particular, Scutellospora calospora is relatively ineffective with some hosts and a growth depression is often observed. We have examined the abilities of S. calospora and Glomus sp. ' City Beach ' strain WUM 16, grown in soils which promote extensive mycorrhizal colonization, to transfer phosphate (P) to Allium porrum. Phosphate uptake from the low-P soils (P ! ) was compared with uptake from soils amended with extra P (P " ). In order to relate P transfer to physiological characteristics of the two fungi, inflow of P via fungus to the plant was combined with the surface areas of intercellular hyphae and arbuscules (symbiotic interfaces) to calculate the amount of P transferred per unit area of interface (P fluxes). ' Hyphal inflows ' and ' hyphal fluxes ' were also calculated on the assumption that P uptake by the plant was the same in mycorrhizal and equivalent nonmycorrhizal (NM) plants (the validity of this assumption is discussed). With both soil P treatments, P was taken up by plants colonized by both mycorrhizal fungi to a greater extent than by the equivalent NM controls. Hyphal inflows to plants in P ! soil that were colonized by Glomus sp. ' City Beach ' were greatest from 14-21 d and decreased at later harvests. With P " soil the inflow via Glomus sp. ' City Beach ' peaked at a very high value at 21-28 d. Hyphal inflows into plants in P ! soil that were colonized by S. calospora increased throughout the experiment, and with P " soil they remained steady at high values. With both fungi, the surface areas of the symbiotic interfaces increased greatly after 14 d, and generally there was little effect of higher soil P. With P ! soil, Glomus sp. ' City Beach ' showed no significant differences in the ratio of surface areas of the two interfaces over the course of the experiment. Scutellospora calospora tended to produce a lower percentage of interfacial area contributed by arbuscules. With the mycorrhizal plants growing in their respective soils, there appeared no consistent differences between the two fungi with respect to fluxes of P across the interfaces. With P ! soil, fluxes via Glomus sp. ' City Beach ' were initially higher than those via S. calospora, but later they were higher with S. calospora. With P " soil the only difference was the 28-42-d period, when Glomus sp. ' City Beach ' produced the higher flux (reflecting the low surface area at the time). The results show that relative inefficiency of S. calospora in its ability to transfer P, as reported by others, may result from different environmental conditions, use of different hosts, or even of different fungal isolates (strains).

Arbuscular mycorrhizal fungus influence maize root growth and architecture in rock phosphate amended tropical soil

“…The increased total root length in response to AM inoculation in maize is similar to those reported by Dickson et al (1999), where Allium cepa L. colonized by S. calospora and Glomus sp. 'City Beach' (Glomus Tul.…”

A Phosphate Transporter from Medicago truncatula Involved in the Acquisition of Phosphate Released by Arbuscular Mycorrhizal Fungi

“…Various lines of evidence suggest that most of the phosphate moves into mycorrhizal roots via the fungus (Pearson and Jakobsen, 1993;Liu et al, 1998b;Chiou et al, 2001), and estimates of phosphate flow into hyphae correlate well with the root measurements, providing further support for this theory (Jakobsen et al, 1992). Based on the phosphate inflow measurements and estimations of the area of the arbuscules, it is suggested that phosphate fluxes across the arbuscular interface range between 2 and 29 nmol·m Ϫ2 ·s Ϫ1 and are comparable to transport rates measured in plant and fungal cells in highphosphate environments (Cox and Tinker, 1976;Smith et al, 1994;Smith and Read, 1997;Dickson et al, 1999). The apparent K m for MtPT4 is in the range expected for optimal activity in a relatively high-phosphate environment.…”

“…The concentration of phosphate in the periarbuscular space also is unknown, but estimates of the flux across the arbuscule have been made. Phosphate flow into mycorrhizal roots is significantly higher than that into nonmycorrhizal roots, and depending on the phosphate concentration of the external medium, fluxes of between 8 and 100 ϫ 10 Ϫ13 mol·m Ϫ1 (root)·s Ϫ1 have been measured (Cox and Tinker, 1976;Smith et al, 1994;Smith and Read, 1997;Dickson et al, 1999). Various lines of evidence suggest that most of the phosphate moves into mycorrhizal roots via the fungus (Pearson and Jakobsen, 1993;Liu et al, 1998b;Chiou et al, 2001), and estimates of phosphate flow into hyphae correlate well with the root measurements, providing further support for this theory (Jakobsen et al, 1992).…”

Sanctions, Cooperation, and the Stability of Plant-Rhizosphere Mutualisms

“…We now know that AMF strains differ dramatically in metabolic strategies such as amount of C extracted from their hosts (Jakobsen et al 1992), amount of lipids allocated to storage (van Aarle & Olsson 2003), the transfer of P to their host plants (Boddington & Dodd 1999), C transport capacities across symbiotic interfaces (Dickson et al 1999), and colonization and hyphal-length allocation (Smith et al 2000, Hart & Reader 2005. Fungal allocation strategies can determine benefits conferred to the host.…”