Convergent evolution of effector protease recognition by Arabidopsis and barley

Authors: CARTER, MORGAN - Cornell University; HELM, MATTHEW - Indiana University; CHAPMAN, ANTONY - Iowa State University; WAN, EMILY - Cornell University; RESTREPO SIERRA, ANA MARIA - National University Of Colombia; INNES, ROGER - Indiana University; BOGDANOVE, ADAM - Cornell University; Wise, Roger

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2018
Publication Date: 4/1/2019
Citation: Carter, M., Helm, M., Chapman, A., Wan, E., Restrepo Sierra, A., Innes, R., Bogdanove, A., Wise, R.P. 2019. Convergent evolution of effector protease recognition by Arabidopsis and barley. Molecular Plant-Microbe Interactions. 32(5):550-565. https://doi.org/10.1094/MPMI-07-18-0202-FI.
DOI: https://doi.org/10.1094/MPMI-07-18-0202-FI

Interpretive Summary: Plant disease resistance is often mediated by intracellular innate immune receptors known as nucleotide-binding leucine-rich repeat proteins (NLRs). The primary function of NLRs is to detect the presence of pathogen-secreted effector proteins, sometimes indirectly through effector-induced modification of other host proteins. A well-studied example of an NLR that indirectly detects its cognate effector is RPS5 from Arabidopsis. RPS5 detects the effector protease AvrPphB from the plant pathogenic bacterium, Pseudomonas syringae pv. phaseolicola, by monitoring a host serine/threonine protein kinase, designated PBS1. PBS1 is one of the most well conserved defense-related genes in flowering plants. Determining the function of PBS1 orthologs in diverse plant species is of particular interest, because it will provide insight into the evolution of disease resistance gene specificity, and could enable engineering of new disease resistance specificities in crop plants. In this report, we show that multiple barley varieties indeed recognize and respond to AvrPphB protease activity and that barley also contains PBLs that are cleaved by AvrPphB. Using newly developed nested association mapping resources, we positioned AvrPphB response to a single segregating locus on chromosome 3HS, and identified an NLR gene that we named AvrPphB Resistance 1 (Pbr1). Phylogenetic analyses indicate that Pbr1 and RPS5 are not orthologous genes, hence the ability to recognize AvrPphB protease activity has evolved independently in monocots and dicots. Lastly, we show that wheat varieties also recognize AvrPphB protease activity and harbor an ortholog of Pbr1 in a syntenous position on chromosome 3B, suggesting that the PBS1-decoy system can be deployed in wheat using native wheat Pbs1 genes. Knowledge from this research will impact how plant breeders engineer and select for disease resistance, one of the most important traits that affect crop yield, and thus food security.

Technical Abstract: The Pseudomonas syringae cysteine protease AvrPphB activates the Arabidopsis resistance protein RPS5 by cleaving a second host protein, PBS1. AvrPphB induces defense responses in other plant species, but the genes and mechanisms mediating AvrPphB recognition in those species have not been defined. Here, we show that AvrPphB induces defense responses in diverse barley cultivars. We show also that barley contains two PBS1 orthologs, that their products are cleaved by AvrPphB, and that the barley AvrPphB response maps to a single locus containing a nucleotide-binding leucine-rich repeat (NLR) gene, which we termed AvrPphB Resistance 1 (Pbr1). Transient co-expression of PBR1 with wild-type AvrPphB, but not a protease inactive mutant, triggered defense responses, indicating that PBR1 detects AvrPphB protease activity. Additionally, PBR1 co-immunoprecipitated with barley and N. benthamiana PBS1 proteins, suggesting mechanistic similarity to detection by RPS5. Lastly, we determined that wheat cultivars also recognize AvrPphB protease activity and contain a Pbr1 ortholog. Phylogenetic analyses showed however that Pbr1 is not orthologous to RPS5. Our results indicate that the ability to recognize AvrPphB evolved convergently, and imply that selection to guard PBS1-like proteins is ancient. Also, the results suggest that PBS1-based decoys may be used to engineer protease effector recognition-based resistance in barley and wheat.