RPW8/HR Repeats Predict NLR-dependent Hybrid Performance

Abstract

Keywords RPW8, NLR, natural variation, autoimmunity, hybrid performance, MLKL, HET-S Barragan et al. RPW8/HR repeats and hybrid necrosis 2 SummaryHybrid offspring can look very different from their parents, including having greatly increased or decreased fitness. In many plant species, conflicts between divergent elements of the immune system can cause hybrids to express autoimmunity, a generally deleterious syndrome known as hybrid necrosis. We are investigating multiple hybrid necrosis cases in Arabidopsis thaliana that are caused by allele-specific interactions between different variants at two unlinked resistance (R) gene clusters. One is the RESISTANCE TO PERONOSPORA PARASITICA 7 (RPP7) cluster, which encodes an intracellular nucleotide binding site-leucine rich repeat (NLR) immune receptors that confer strain-specific resistance to oomycetes. The other is the RESISTANCE TO POWDERY MILDEW 8 (RPW8)/HOMOLOG OF RPW8 (HR) locus, which encodes atypical resistance proteins that can confer broad-spectrum resistance to filamentous pathogens. There is extensive structural variation in the RPW8/HR cluster, both at the level of gene copy number and at the level of C-terminal protein repeats of unknown function. We demonstrate that the number of RPW8/HR repeats correlate, albeit in a complex manner, with the severity of hybrid necrosis when these alleles are combined with specific RPP7 variants. This observation suggests that gross structural differences, rather than individual amino acid polymorphisms, guide the genetic interaction between RPW8/HR and RPP7 alleles. We discuss these findings in light of the similarity of RPW8/HR proteins with pore-forming toxins, MLKL and HET-S, from mammals and fungi.

An rpw8 quadruple mutant of Arabidopsis Col-0 is partially compromised in bacterial and fungal resistance

“…Based on previous characterisation of RPW8 (Xiao et al, 2005;Collier et al, 2011;Barragan et al, 2019;Li et al, 2019), we propose the following two hypotheses that could explain the molecular and biochemical function of RPW8. The first hypothesis posits that RPW8 homologs and the RPW8 domain in RPW8-NLRs participate in activation of immune responses through oligomerization and membrane insertion (pore-forming), thus contributing to HR and pathogen resistance.…”

“…Another interesting feature of the RPW8 locus is the intraspecific gene amplification and diversity within Arabidopsis. RenSeq was conducted on 65 Arabidopsis accessions and revealed two types of sequence variation at the RPW8 locus (Barragan et al, 2019;Van de Weyer et al, 2019). Firstly, there have been a large number of recent duplication events.…”

“…Secondly, some RPW8 genes encode proteins with intragenic C-terminal duplications. For instance, RPW8.1 from some accessions contains one or more duplications of a 21amino acid motif (Orgil et al, 2007), and HR4 from Col-0 contains five duplications of a 14amino acid motif (Xiao et al, 2004), whereas HR4 from ICE106 contains only two, and HR4 from Fei-0 only one such motif (Barragan et al, 2019).…”

“…Particularly, DM6 (RPP7) and DM7 (RPW8) are responsible for incompatibility in three crosses: Mrk-0 x KZ10, Lerik1-3 x Fei-0 and ICE79 x Don-0. In KZ10 and Fei-0, the RPW8 genes responsible for the phenotype are RPW8.1 and HR4 respectively (Barragan et al, 2019). RPW8.1 KZ10 has three C-terminal intragenic repeats, the maximum observed among known RPW8.1 alleles.…”

Oligomerization of NLR immune receptor RPP7 triggered by atypical resistance protein RPW8/HR as ligand

“…Notably, these helper NLRs belong to a subclass of CNLs whose CC domains are similar to parts of the non-NLR RPW8/HR immune proteins (and therefore referred to as CCR domains) (Collier et al, 2011). Similar to many NLR genes, the CCR-only RPW8/HR genes are located in a gene cluster, with extensive copy number and sequence variation between wild strains (Barragan et al, 2019;Jorgensen and Emerson, 2008;Orgil et al, 2007;Xiao et al, 2004). However, different from typical NLRs, which function primarily in racespecific resistance, RPW8/HR genes seem to be mostly involved in broad spectrum resistance Wang et al, 2007;Xiao et al, 2001).…”