Suppression or Activation of Immune Responses by Predicted Secreted Proteins of the Soybean Rust Pathogen Phakopsora pachyrhizi

Abstract

Rust fungi, such as the soybean rust pathogen Phakopsora pachyrhizi, are major threats to crop production. They form specialized haustoria that are hyphal structures intimately associated with host-plant cell membranes. These haustoria have roles in acquiring nutrients and secreting effector proteins that manipulate host immune systems. Functional characterization of effector proteins of rust fungi is important for understanding mechanisms that underlie their virulence and pathogenicity. Hundreds of candidate effector proteins have been predicted for rust pathogens, but it is not clear how to prioritize these effector candidates for further characterization. There is a need for high-throughput approaches for screening effector candidates to obtain experimental evidence for effector-like functions, such as the manipulation of host immune systems. We have focused on identifying effector candidates with immune-related functions in the soybean rust fungus P. pachyrhizi. To facilitate the screening of many P. pachyrhizi effector candidates (named PpECs), we used heterologous expression systems, including the bacterial type III secretion system, Agrobacterium infiltration, a plant virus, and a yeast strain, to establish an experimental pipeline for identifying PpECs with immune-related functions and establishing their subcellular localizations. Several PpECs were identified that could suppress or activate immune responses in nonhost Nicotiana benthamiana, N. tabacum, Arabidopsis, tomato, or pepper plants.

Advances in understanding obligate biotrophy in rust fungi

“…One of the most advanced effectoromic screens performed in rust fungi was recently reported for P. pachyrhizi (Qi et al ., ). The selection of 82 candidate effectors (PpECs) was based on haustoria RNAseq (Link T. et al ., ; Link T.I.…”

“…benthamiana ; assay induction of an immune response in pepper and tomato; and assay cell death suppression in N. benthamiana , Arabidopsis and yeast. Among the 82 PpECs, 17 suppress plant immunity and 30 PpECs target different cell compartments, such as nucleus, nucleolus and cytoplasm (Qi et al ., ). The secretion of one candidate, PpEC23, was confirmed in yeast and it was further shown to suppress the immune response in soybean.…”

“…PpEC23 interacts with itself in the cytoplasm and it localizes in the nucleus when it interacts with GmSPL12I (Qi et al ., ). With this large effectoromic screen, the authors were able to propose a shortlist of 17 P. pachyrhizi priority candidates that can now be specifically addressed in dedicated efforts (Qi et al ., ). This clearly illustrates the power of systematic and methodological approaches to large‐scale screening applied to rust fungi.…”

“…Qi et al . () cloned the 82 PpECs in different vectors in order to: scrutinize their subcellular localization in N . benthamiana ; assay induction of an immune response in pepper and tomato; and assay cell death suppression in N. benthamiana , Arabidopsis and yeast.…”

Comparative transcriptomics of Gymnosporangium spp. teliospores reveals a conserved genetic program at this specific stage of the rust fungal life cycle

“…Discrimination of SPs in each Gymnosporangium spp. helps to narrow down the list of specific candidates and will support future effector studies to better understand the biology of infection as demonstrated for other rust species [50][51][52]. The transcriptome of G. japonicum and its comparison to other…”