The recognition pattern of green algae by lichenized fungi can be extended to lichens containing a cyanobacterium as photobiont

Lichens are intimate and long-term symbioses of photosynthetic, unicellular algae or cyanobacteria and heterotrophic fungi joined to form a new biological entity different from its individual components. Specificity required for the lichen association can be defined in this context as the preferential, but not exclusive, association of a biont with another. Recognition of compatible algal cells is carried out by specific lectins produced and secreted by the potential mycobiont. However, lichen phenolics are not involved in the recognition process, in contrast to that found for other plant symbioses, such as mycorrhizal or Rhizobium legume associations. Some lectins from phycolichens have been characterized as glycosylated arginases which bind to an algal cell wall receptor identified as an α-1,4-polygalactosylated urease. The binding is improved by Ca and Mn, in a similar way to that described for legume lectins. Two forms of glycosylated arginases seem to be involved in the recognition process, one of them, particulated in the cell wall of fungal hyphae and involved in cell contact between phycoand mycobiont, whereas a second, secreted arginase produces recruitment of compatible algal cells near the surface of fungal hyphae. When glycosylated urease is lacking from the algal cell wall, fungal arginase is internalized, increasing the levels of algal putrescine, which promotes chloroplast disorganization, activation of glucanases and breakdown of the cell wall with loss of the protoplast. Evolution of symbiotic relationships implies then the synchronization of cell division and lectin receptor production, probably as a consequence of the perception of environmental factors (light and temperature).