Scleroderma meridionale ectomycorrhizae on Halimium halimifolium: expanding the Mediterranean symbiotic repertoire

Scleroderma is a gasteroid genus in the Boletales (Basidiomycota), with a cosmopolitan distribution. Species of Scleroderma establish ectomycorrhizal (ECM) symbiosis with a range of coniferous and non-coniferous trees and shrubs, both in temperate and tropical regions, with little tendency to host specificity, a feature that might have facilitated the wide distribution of the genus. With the contribution of confocal laser scanning microscopy, we describe the morpho-anatomical features of the ectomycorrhizae formed by Scleroderma meridionale on Halimium halimifolium, a cistaceous plant belonging to a small group of woody shrubs occurring in open vegetation types in the Mediterranean region. The mycobiont and host plant identity in ECM was verified through molecular tools. Mycorrhizal system is very small, up to 1.9 mm, mostly coralloid to irregularly pinnate. The mantle surface is felty, whitish with silver patches. Differentiated rhizomorphs occur infrequently. Mantle surface is characterized by a network of branched hyphae organized in hyphal boundles. Hyphae are frequently covered by granules or warts. These characters, except for the presence of granules, are similar to those reported for the only two naturally-occurring Scleroderma ECM described so far, i.e. S. bovista on Populus and S. citrinum on Betula and Pinus. On the other side, the peculiarity of S. meridionale + Halimium ECM is the particularly small dimension of mycorrhizal system, a character shared with ECM formed by Cistaceae. At the best of our knowledge, this is the first description of an ectomycorrhiza on Halimium, a plant whose mycorrhizal biology deserves to be explored in greater detail