Ultrastructure of Mycorrhizas of Dracophyllum secundum R. Br. (Ericales:Epacridaceae)

Abstract

Dracophyllum secundum R. Br. (Epacridaceae) often possessed ericoid mycorrhizas; fungal endophytes formed coils within cells of the epidermis of hair-roots. The plant plasma membrane extended around the hyphae. In some epidermal cells of hair-roots, both plant and fungal cells retained their structural integrity, both partners showing mitochondrial, vacuolar and lipid droplet profiles, and with much of the plant cytoplasm associated with the hyphal coils. In other epidermal cells of hair-roots, fungal coils were present but cytoplasmic features of both symbionts appeared to have broken down. Some epidermal cells showed no evidence of fungal infection. These three arrangements could occur in root-cells of the same age, and are interpreted as resulting from different stages in the development and degeneration of the infection by the mycorrhizal fungus. Two structural types of fungal endophyte here found in ericoid mycorrhizas in D. secundum: one with simple septa, Woronin bodies and two-layered walls (presumed to be an Ascomycete), and another with dolipore septa with imperforate parenthesomes (presumed to be a Basidiomycete). The possibilities that the mycorrhizas may be seasonal, and that mycorrhizal status varies from place to place, are discussed.

Biology of mycorrhizal associations of epacrids (Ericaceae)

“…, 1996; A. E. Ashford & W. G. Allaway, unpublished) and also of other epacrids, notably Dracophyllum secundum R.Br. (Allen et al. , 1989), Leucopogon parviflorus (Andr.)…”

“…Before penetration, a hypha encounters the surface mucilage and there are many images of hyphal profiles on the root surface completely enveloped in this mucilage both for epacrids and other Ericaceae (Allen et al. , 1989; Steinke et al.…”

“…, 1995). An electron‐lucent gap containing dispersed material separates the fungal wall from the invaginated plant plasma membrane as in most EM images of ericoid mycorrhizas (Allen et al. , 1989; Briggs & Ashford, 2001).…”

Mycorrhizal and Endophytic Fungi of Epacrids (Ericaceae)

“…Certain Thielavia-like and Capronia-like isolates from northern hemisphere Ericaceae hosts also appear to be non-mycorrhizal, suggesting that further work is required to ascertain the extent of mycorrhiza-forming abilities of isolates in these groups. The only basidiomycete so far isolated from an epacrid hair root system failed to form ericoid mycorrhiza-like structures in W. pungens roots (Midgley et al 2004a), suggesting that it, and the basidiomycete hyphae observed in Dracophyllum secundum epidermal cells by Allen et al (1989), was probably a saprotroph. Although they can infect Ericaceae epidermal cells, the coils formed by isolates from epacrids that have close ITS sequence identity to Phialocephala fortinii are typical of those formed by DSE rather than ericoid mycorrhizal fungi (Davies et al 2003;Midgley et al 2004a).…”

“…These features, together with the fact that the fungi are generally ascomycetes, as evidenced by the presence of simple septa and Woronin bodies (Allen et al 1989;Steinke et al 1996;Briggs and Ashford 2001), are shared with ericoid mycorrhizal infection of other Ericaeae, and are generally taken to indicate mycorrhizal infection. Aside from putatively ericoid mycorrhizal ascomycetes, arbuscular mycorrhiza-like infections, suggesting the presence of Glomales taxa, have also been observed as apparent endophytes of field-collected epacrid hair roots (Khan 1978;McGee 1986;Bellgard 1991;McLean and Lawrie 1996;Reed 1996;Davies et al 2003), and there is a single report of basidiomycete hyphae in epidermal cells of an epacrid (Allen et al 1989). As emphasised by Reed (1996), however, the presence of infection does not necessarily indicate a mutualistic association.…”