Cerato-platanin family proteins: one function for multiple biological roles?

INTRODUCTION Despite the increasing number of sequenced genomes, cerato-platanin family proteins (CPPs) seem to exist only in fungi. Bacteria, oomycota, plants, and animals do not in fact possess CP homologs, whereas genes codifying for CPPs have been found, so far, in more than 50 fungal genomes (Chen et al., 2013). Among these fungi there are plant pathogens, such as Botrytis cinerea and Magnaporthe grisea (Ascomycota), Heterobasidion irregulare and Moniliophthora perniciosa (Basidiomycota), but also biocontrol agents (e.g., Trichoderma spp.), mycorrhizal fungi, saprotrophs, and human pathogens. Experimental data suggest that CPPs play a role during fungus-plant interactions. When T. virens and T. harzianum were respectively co-cultured with cotton or tomato roots, the expression of CPPs increased compared to the culture obtained in the absence of the plant (Djonović et al., 2006; Samolski et al., 2009); in B. cinerea and M. grisea, knockout mutants for CPP genes showed reduced virulence on their plant hosts (see Table 1) (Jeong et al., 2007; Frías et al., 2011). Plants have developed the ability to recognize CPPs and to activate defense responses when in contact with them. In fact, CPPs have been reported to act as microbe/pathogen-associated molecular patterns (MAMPs/PAMPs) (for reviews, see Gaderer et al., 2014; Pazzagli et al., 2014). The secretion of these small proteins (120–130 a.a.) seems to be universal in fungi. CPPs are often the most present in the culture filtrates, but in some studies they have been found localized in the fungal cell walls (Boddi et al., 2004; Seidl et al., 2006; Shah et al., 2009; Frías et al., 2014). Gene knockout experiments have not yet permitted the identification of a clear biological function, and to date it is not easy to answer the question of why fungi produce CPPs. Gene expression data and studies on their biochemical properties have led to the formulation of various hypotheses that may appear different for each homolog. In summary, it is generally believed that CPPs play at least two roles in fungi: one in growth and development (generally referred to as the primary role), which should justify their presence in the cell wall, and one, more elusive, that should explain their secretion and the interaction with plants (Gaderer et al., 2014; Pazzagli et al., 2014). But what is their function? This opinion article will attempt to answer these questions: can we hypothesize a basal function which characterizes CPPs and allows them to have multiple biological roles depending on the context (fungal cell wall or extracellular environment)? Can this function imply a role in plant colonization?