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

Abstract

In this paper we describe the presence of genes encoding close homologues of an endogenous plant peptide, rapid alkalinisation 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 the 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 based on their ability to induce rapid alkalinisation 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 (ROS burst, induced alkalinisation and MAP kinas activation). Gene expression analysis confirmed that a RALF-encoding gene in Fusarium 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 Fusarium 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 the plant-pathogen interactions. This article is protected by copyright. All rights reserved.