Fungal phytopathogens encode functional homologues of plant rapid alkalinization factor (RALF) peptides

Abstract

In this article, we describe the presence of genes encoding close homologues of an endogenous plant peptide, rapid alkalinization factor (RALF), within the genomes of 26 species of phytopathogenic fungi. Members of the RALF family are key growth factors in plants, and the sequence of the RALF active region is well conserved between plant and fungal proteins. RALF1-like sequences were observed in most cases; however, RALF27-like sequences were present in the Sphaerulina musiva and Septoria populicola genomes. These two species are pathogens of poplar and, interestingly, the closest relative to their respective RALF genes is a poplar RALF27-like sequence. RALF peptides control cellular expansion during plant development, but were originally defined on the basis of their ability to induce rapid alkalinization in tobacco cell cultures. To test whether the fungal RALF peptides were biologically active in plants, we synthesized RALF peptides corresponding to those encoded by two sequenced genomes of the tomato pathogen Fusarium oxysporum f. sp. lycopersici. One of these peptides inhibited the growth of tomato seedlings and elicited responses in tomato and Nicotiana benthamiana typical of endogenous plant RALF peptides (reactive oxygen species burst, induced alkalinization and mitogen-activated protein kinase activation). Gene expression analysis confirmed that a RALF-encoding gene in F. oxysporum f. sp. lycopersici was expressed during infection on tomato. However, a subsequent reverse genetics approach revealed that the RALF peptide was not required by F. oxysporum f. sp. lycopersici for infection on tomato roots. This study has demonstrated the presence of functionally active RALF peptides encoded within phytopathogens that harbour an as yet undetermined role in plant-pathogen interactions.

FERONIA and Her Pals: Functions and Mechanisms

“…Moreover, defense-related responses were elevated in tomato and Arabidopsis inoculated with fungal lines in which the F-RALF gene was deleted relative to wildtype fungal infection. These observations are consistent with F-RALF function as a virulent factor, and, at least under certain growth conditions, F-RALF suppresses host immunity to facilitate pathogenesis (Masachis et al, 2016;Thynne et al, 2016). When infected by F. oxysporum, feronia mutants were more resistant to disease symptoms relative to wild-type plants (Masachis et al, 2016).…”

A Comprehensive Analysis of RALF Proteins in Green Plants Suggests There Are Two Distinct Functional Groups

“…Secreted peptides are commonly associated with the local communication and control of cell proliferation, growth and differentiation (Meng et al, 2012), and the relative simplicity of the chlorophytes seemingly does not require such signaling. The recent identification of RALFs within fungi (Thynne et al, 2016) suggests that RALF pathways can be hijacked for the benefit of pathogens, further demonstrating their general importance within plant development. Whether these fungal RALFs originated through horizontal gene transfer or co-evolution is not yet clear.…”

“…This is further demonstrated by the presence of pairs of RALF proteins exhibiting high homology to one another (Cao and Shi, 2012). Intriguingly, biologically-active RALF homologs, typically of RALF1, have also been identified within numerous fungal phytopathogens, with these potentially acting in plant-pathogen interactions (Thynne et al, 2016). The seemingly ubiquitous presence of RALFs across the plant kingdom is further evidence of their general importance.…”

Receptor Kinases in Plant-Pathogen Interactions: More Than Pattern Recognition

“…A search of fungal genomes identified numerous RALF-like peptides (Masachis et al, 2016;Thynne et al, 2016). Strikingly, all fungi encoding RALF-like sequences are plant pathogens.…”