Seasonal dynamics of arbuscular mycorrhizal fungal communities in roots in a seminatural grassland

Abstract

Symbiotic arbuscular mycorrhizal fungi (AMF) have been shown to influence both the diversity and productivity of grassland plant communities. These effects have been postulated to depend on the differential effects of individual mycorrhizal taxa on different plant species; however, so far there are few detailed studies of the dynamics of AMF colonization of different plant species. In this study, we characterized the communities of AMF colonizing the roots of two plant species, Prunella vulgaris and Antennaria dioica, in a Swedish seminatural grassland at different times of the year. The AMF small subunit rRNA genes were subjected to PCR, cloning, sequencing, and phylogenetic analysis. Nineteen discrete sequence types belonging to Glomus groups A and B and to the genus Acaulospora were distinguished. No significant seasonal changes in the species compositions of the AMF communities as a whole were observed. However, the two plant species hosted significantly different AMF communities. P. vulgaris hosted a rich AMF community throughout the entire growing season. The presence of AMF in A. dioica decreased dramatically in autumn, while an increased presence of Ascomycetes species was detected.

FIG. 1.

One of the most parsimonious trees obtained in the MP analysis showing the phylogenetic affiliations of the sequences of AMF obtained from the roots of P. vulgaris and A. dioica at different times of the growing season. E. pisiformis and M. polycephala were used as out-groups. The values above the branches are bootstrap values (1,000 replicates) for an MP analysis; only values of ≥70% are shown. Sequence identifiers from this study are in bold type, indicate the host (Pru, P. vulgaris; Ant, A. dioica) and the time of collection (V, May; VI, June; VII, July; VIII, August; IX, September; X, October), and include a clone identity number. Sequence group identifiers on the right (e.g., Glo2 and Aca2) identify clusters with pairwise sequence similarities of ≥94.3%. Identical sequences are grouped and represented by letters, followed by the number of clones having that particular sequence. See the supplemental material for a detailed description of the clone identifiers included in each group. White boxes to the right indicate different AMF families and groups (58).

FIG. 1.

One of the most parsimonious trees obtained in the MP analysis showing the phylogenetic affiliations of the sequences of AMF obtained from the roots of P. vulgaris and A. dioica at different times of the growing season. E. pisiformis and M. polycephala were used as out-groups. The values above the branches are bootstrap values (1,000 replicates) for an MP analysis; only values of ≥70% are shown. Sequence identifiers from this study are in bold type, indicate the host (Pru, P. vulgaris; Ant, A. dioica) and the time of collection (V, May; VI, June; VII, July; VIII, August; IX, September; X, October), and include a clone identity number. Sequence group identifiers on the right (e.g., Glo2 and Aca2) identify clusters with pairwise sequence similarities of ≥94.3%. Identical sequences are grouped and represented by letters, followed by the number of clones having that particular sequence. See the supplemental material for a detailed description of the clone identifiers included in each group. White boxes to the right indicate different AMF families and groups (58).

FIG. 2.

(A) Rank frequency diagram of AMF sequence types present in the roots of P. vulgaris and A. dioica. (B) Sample-effort curves for the AMF community representing the observed and the estimated richness. The sample order was randomized by 1,000 replications. Both diagrams are based only on presence-absence data for each sequence type in the clone libraries.

FIG. 3.

Proportional distribution of the AMF sequence types in the roots of P. vulgaris (P.vul.) and A. dioica (A.dio.) at different times during the growing season. The y axis indicates the proportion of clones assigned to each particular sequence type. Proportions were calculated by pooling samples from all the plots at the same time points for each host. The bar widths are proportional to the numbers of studied clone libraries. The richness of AMF sequence types is indicated on the tops of the respective bars.

FIG. 4.

DCA diagram. Open symbols represent AMF communities in P. vulgaris and closed symbols represent AMF communities in A. dioica. Sampling months are represented as right-side-up triangles for May, squares for June, rectangles for July, circles for August, diamonds for September, and inverted triangles for October.