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    • br Introduction Lichen forming fungi LFFs have evolved in

    2021-11-30


    Introduction Lichen-forming fungi (LFFs) have evolved in various classes, including Arthoniomycetes, Coniocybomycetes, Dothideomycetes, Eurotiomycetes, Lecanoromycetes, Leotiomycetes and Lichinomycetes, in Ascomycota (James et al., 2006, Schoch et al., 2009a, Lumbsch and Rikkinen, 2017, Prieto et al., 2018). Lecanoromycetes is the largest class to which LFFs belong, with over 95 % of the species being lichenized (Miadlikowska et al., 2014). Although Dothideomycetes and Eurotiomycetes include mainly non-lichenized species, some classes, such as Trypetheliales (Dothideomycetes) and Verrucariales (Eurotiomycetes), include LFFs (Spatafora et al., 2006, Gueidan et al., 2007, Schoch et al., 2009b). Only one order in Coniocybomycetes, Coniocybales (Prieto et al., 2013), is entirely lichenized and associates with green algae (Rikkinen et al., 2016). On the basis of recent classifications, Lücking et al. (2016) suggested that 14–23 independent lichenization events occurred during the evolution of Ascomycota. Approximately 85 % of LFFs have green algae as their primary photosynthetic partners, and 3%–4 % are associated with both green algae and Cyanobacteria (Honegger, 2008). Although LFFs belong to many genera in seven Riboflavin sale of Ascomycota, their green algal partners belong to a more limited number of genera, including Trebouxia, Asterochloris and Diplosphaera in the class Trebouxiophyceae and Trentepohlia in the class Ulvophyceae (Friedl and Büdel, 2008). Thus, the fungal partners may diversify if they receive particular advantages from specific green algae, such as the acquisition of carbon sources. A stable supply of carbon from a photosynthetic partner is a benefit of LFFs and probably the main reason why lichens expanded worldwide and retain their symbiotic relationships for long periods of time. Heterotrophs, such as fungi, cannot survive or grow in nature without a carbon source, and, in fact, the green algal partners release acyclic polyols, such as ribitol, sorbitol, and erythritol (Richardson et al., 1968), when they associate with a fungal partner (Smith, 1968, Green and Smith, 1974). For example, ribitol, which is absent in almost all plants (Negm and Marlow, 1985), is released from Trebouxia spp., the most common photosynthetic partners in lichens (Honegger, 2008). Observations of Xanthoria aureola (Richardson and Smith, 1968a, Richardson and Smith, 1968b) and Xanthoria calcicola (Lines et al., 1989), which associate with green algae (Trebouxia), indicated that ribitol is converted to mannitol and arabitol through the pentose phosphate pathway of fungal metabolism. The other algal partners also release polyols, and polyols are present in many lichens (Lindberg et al., 1953, Lewis and Smith, 1967). Therefore, the polyol transport pathway is most likely essential to LFFs associated with green algae. However, the mechanisms of polyol transfer from green algae to fungi have not been elucidated, and, so far, a polyol transporter has not been isolated from LFFs, even though its function is required for symbiotic associations. Polyols are widely present in diverse organisms (Lewis and Smith, 1967, Pfyffer and Rast, 1980, Honegger et al., 1993, Gustavs et al., 2011), and they function to remove reactive oxygen species (Parida and Das, 2005), to adjust the osmotic pressure of compatible solutes (Wegmann, 1986, Reed et al., 1987), and to store the reducing power provided by NADH or NADPH (Jennings, 1985, Williamson et al., 2002). Polyol transporters have been reported in bacteria, such as D-arabinitol and ribitol transporters from Klebsiella pneumoniae (Heuel et al., 1997), in red alga, such as sugar and mannitol transporters from Galdieria sulphuraria (Schilling and Oesterhelt, 2007), in plants, such as the first reported mannitol transporter from celery (Noiraud et al., 2001), and in humans, such as glycerol, mannitol, sorbitol and the other neutral solute channels (Tsukaguchi et al., 1999). The orthologs or paralogs of these proteins have also been characterized as polyol transporters. Recently, some transporters (or H+ symporters), having the capabilities to transport acyclic polyols, were reported in ascomycetous yeast, such as Debaryomyces hansenii (Syl1 and 2; Pereira et al., 2014), Ambrosiozyma monospora (Lat1 and 2; Londesborough et al., 2014) and Saccharomyces cerevisiae (Hxt11, 13, 15, 16 and 17; Jordan et al., 2016). Some of these yeast proteins have no ability to transport sugars, even though their encoding genes were paralogous to monosaccharide transporters. Palma et al. (2009) revealed the lineages of hexose transporters in nine ascomycetous yeast, but the phylogenetic relationship among the characterized monosaccharide and polyol transporters were unclear in the Ascomycetes.