The Lichen Symbiosis—What is so Spectacular about it?

Abstract

Lichen mycobionts are typical representatives of their fungal classes but differ from non-lichenized taxa by their manifold adaptations to symbiosis with a population of minute photobiont cells. Most interesting are the morphologically complex macrolichens, the fungal partner of which competes for space above ground and contains photobiont cells optimally positioned for gas exchange and illumination. Such thalli are the product of an amazing hyphal polymorphism, with multiple switches between polar and apolar growth and hydrophilic or hydrophobic cell wall surfaces. Hydrophobic sealing of the apoplastic continuum between the partners by means of mycobiont-derived hydrophobic compounds canalizes the fluxes of solutes during the often quite dramatic de- and rehydration processes and keeps the algal layer gas-filled at any level of hydration. The impressive tolerance of drought, heat and cold stress of most lichen-forming fungi and their photobionts is due to a very interesting combination of protective and repair mechanisms at the cellular level, the molecular bases of which remain to be explored. Contemporary experimental lichenology is analysed and strategies are proposed aimed at better integration into mainstream biology.

A molecular investigation of free-living and lichenized Nostoc sp. and symbiotic lifestyle determination

“…The molecular mechanisms for these determinations are poorly understood in Nostoc (Adams & Dugan 2008;Honegger 1998). Further studies can help clarify this by investigating differences in genomics and gene-expression profiles in lifestyle-specific lineages across a broad geographic and environmental landscape.…”

Recognition- and defense-related gene expression at 3 resynthesis stages in lichen symbionts

“…In the controlled parasitism of lichens, the fungus is a biotrophic parasite on the alga, and the recognition between partners allows the alga to turn on defensive strategies that limit fungal growth (Ahmadjian and Jacobs 1983;Molina et al 1993;Honegger 1998), which was biochemically examined by Legaz et al (2004). While knowledge of controlled parasitism in lichens is in its infancy, the interaction between lichen symbionts seems to resemble early events that occur in plant-fungus pathogen systems, where recognition of the fungal pathogen by elicitors triggers plant defense mechanisms to control the fungus (Dixon et al 1994;Benhamou 1996;reviews: Crute 1994;Odjakova and Hadjiivanova 2001;Staskawicz 2001;Bogdanove 2002;Dodds and Rathjen 2010).…”

“…Lichens, which are associations between fungi and algae, may be considered a "controlled" parasitism (Ahmadjian and Jacobs 1983;Honegger 1998) rather than a mutualism (Smith 1980). In the controlled parasitism of lichens, the fungus is a biotrophic parasite on the alga, and the recognition between partners allows the alga to turn on defensive strategies that limit fungal growth (Ahmadjian and Jacobs 1983;Molina et al 1993;Honegger 1998), which was biochemically examined by Legaz et al (2004).…”

Evaluation of the Antioxidant Capacities and Cytotoxic Effects of Ten Parmeliaceae Lichen Species

“…The number of lichen species described so far (more than 27000) varies depending on authors criteria and is in continuous change due to the inclusion of molecular data [2]. They are all characterized by their capacity to survive in the most adverse and diverse geoclimatic circumstances; actually, lichens are found disseminated from the poles to the tropics and from the highest mountains to the plains on earth and substrates, which is mainly favored by their evolved strategy of poikilohydry [3]. …”