Ultrastructure of the foliose lichen Myelochroa leucotyliza and its solo fungal and algal (Trebouxia sp.) partners

• Published in: Algal research (Amsterdam) 2022-03, Vol.62, p.102571, Article 102571
• Main Authors: Arakawa, Satoko, Kanaseki, Toku, Wagner, Ralf, Goodenough, Ursula
• Format: Article
• Language: English
• Subjects: algae; cell communication; cell polarity; Cell-wall septum; coculture; culture media; electron microscopy; Endoplasmic reticulum; fungi; Hydrophobin; hyphae; Lichen; lichens; mutualism; Nubbin; plasma membrane; polysaccharides; secondary metabolites; topology; Trebouxia; ultrastructure; Vacuole; vacuoles
• ISSN: 2211-9264; 2211-9264
• DOI: 10.1016/j.algal.2021.102571

Lichens are slow-growing niche-constructing organisms formed via scripted symbiotic/mutualist relationships between fungi, algae, and bacteria. We have used quick-freeze deep-etch electron microscopy (QFDEEM), coupled with several additional microscopic techniques, to analyze the Japanese foliose lichen Myelochroa leucotyliza. We have also examined its constituent fungi and algae grown separately (solo) and in co-culture in the laboratory, as well as solo fungi incubated with conditioned algal media (CM) for 9 days. We report our general findings on M. leucotyliza architecture in a Supplement, and focus this report on three findings of particular interest. 1) Plasma membrane differentiations called nubbins are found adjacent to, and at the center of, the cell-wall septa in both lichenized and solo hyphae, suggesting a role in cell polarity determination and/or cell-cell communication within the hyphae. 2) Nubbins also localize to the tips of hyphal sub-branches produced by the lichenized but not the solo fungi. These in turn form tip-to-tip contacts (bridges) with one another, suggesting a role in inter-hyphal communication. 3) CM treatment of solo fungi induces a) the production of secondary metabolites, polysaccharides, and lipids; b) a transformation of ER topology and its association with the plasma membrane; and c) a dramatic change in vacuole organization, indicating that the algal growth medium contains soluble materials that trigger fungal differentiation. •An circular assembly of large intermembrane particles, called a nubbin, marks the tips of hyphal sub-branches•A nubbin also localizes to the center of hyphal membranes adjacent to cell-wall septa•Soluble components of an algal culture medium (CM) induce changes in hyphal ultrastructure•CM changes affect secondary metabolite and polysaccharide secretion, endoplasmic reticulum, lipid bodies, and vacuoles•Hydrophobin rodlets are absent from the Myelochroa leucotilyza thallus