The understanding of effector proteins has advanced by leaps and bounds in the last few years. Secreted by microorganisms interacting with plants, these small proteins enter a host cell and modify physiological changes, most notably influencing the suppression or activation of host directed immunity. Fungal effector proteins have been characterized in the pathogen infection process as well as the suppression of host defenses in mutualistic associations such as mycorrhizae. There is a recently published book, edited by Francis Martin & Sophien Kamoun, which addresses the current state of knowledge on the biology of microbial effector proteins.
Published on January 6th in the journal PLOS One, the paper “Using Hierarchical Clustering of Secreted Protein Families to Classify and Rank Candidate Effectors of Rust Fungi” authored by Saunders et al., seeks to describe unknown effector proteins by exploring the diversity of secreted proteins of rust fungi. The Kamoun Lab has been at the forefront of understanding effector biology in the fungi, and this paper is a significant contribution to understanding how rust fungi invoke pathogenicity on their host plants.
Rust fungi are a monophyletic group of pathogens which cause damage on many important economic crop plants. In this study, the authors investigated two pathogens with sequenced genomes, Puccinia graminis f. sp. tritici, the cause of wheat stem rust, and poplar leaf rust, Melampsora larici-populina. By developing an analysis pipeline, the authors inspected the secretome of both fungi to search for putative effector proteins and describe their structure and possible mode of infection into a plant. Few plant defense mechanisms have been identified for rust fungi. This study was considered a preliminary step to identify candidate rust effectors in the eventual selection of new resistance (R) genes in plant breeding programs and genome sequencing initiatives.
Eight families of putative effector proteins were identified in the secretomes of P. graminis f. sp. tritici and M. larici-populina, and a total of 6663 proteins were identified by the pipeline, with 2826 proteins containing secretion signal peptide regions. Analysis of the protein motifs identified several conserved cysteine motifs common to other effector proteins previously characterized from fungal and oomycete plant pathogens. Not surprisingly, the authors identified many previously unrecognized proteins with domains that exhibited similarity to known pathogenicity-related or haustorial-expressed fungal proteins. Both P. graminis f. sp. tritici and M. larici-populina showed differences in the types of effectors secreted and the numbers of each putative effector tribe.
As I mentioned before, this study should be considered a first step in the identification of pathogenicity related effector proteins from rust fungi. Next steps would include wet lab characterization and experimental validation for putative effectors identified in this paper. Additional studies will be needed to address the functional expression of these proteins, as well as the R genes expressed in planta, during the infection process initiated by the rust fungus. This paper provides an interesting priority list for further studies in this rapidly advancing area of understanding the biology of the intimacies of the plant-fungus interaction.