Effector loss drives adaptation of Pseudomonas syringae pv. actinidiae to Actinidia arguta

Abstract

SummaryA pandemic isolate of Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) has devastated kiwifruit orchards growing cultivars of Actinidia chinensis. In contrast, A. arguta (kiwiberry) is resistant to Psa3. This resistance is mediated via effector-triggered immunity, as demonstrated by induction of the hypersensitive response in infected A. arguta leaves, observed by microscopy and quantified by ion-leakage assays. Isolates of Psa3 that cause disease in A. arguta have been isolated and analyzed, revealing a 49 kb deletion in the exchangeable effector locus (EEL). This natural EEL-mutant isolate and strains with synthetic knockouts of the EEL were more virulent in A. arguta plantlets than wild-type Psa3. Screening of a complete library of Psa3 effector knockout strains identified increased growth in planta for knockouts of four effectors – AvrRpm1a, HopF1c, HopZ5a, and the EEL effector HopAW1a – suggesting a resistance response in A. arguta. Hypersensitive response (HR) assays indicate that three of these effectors trigger a host species-specific HR. A Psa3 strain with all four effectors knocked out escaped host recognition, but a cumulative increase in bacterial pathogenicity and virulence was not observed. These avirulence effectors can be used in turn to identify the first cognate resistance genes in Actinidia for breeding durable resistance into future kiwifruit cultivars.

Identification and characterization of innate immunity inActinidia melanandrain response toPseudomonas syringaepv.actinidiae

“…The diploid and Psa-resistant A. melanandra (ME02_01 accession) is a more suitable model system than the previously studied tetraploid A. arguta AA07_03 (Hemara et al, 2022) for both downstream utility in breeding and functional characterisation of trait genetics. Using Illumina short-read and Oxford Nanopore PromethION long-read platforms, the genome of A. melanandra (accession ME02_01) was sequenced and assembled.…”

“…Pathogenicity assays were conducted as described in Hemara et al (2022). Briefly, Actinidia tissue culture plantlets were flooded with Psa inoculum (10 6 CFU/mL).…”

“…Ion leakage was conducted as described in Hemara et al (2022). Bacterial strains (Table S3) carrying empty vector or effector constructs were streaked from glycerol stocks onto LB agar plates with antibiotic selection, were grown for 2 days at 22°C, and were re-streaked onto fresh agar media, and allowed to grow overnight.…”

“…In Psa-resistant kiwiberry A. arguta versus Psa-susceptible A. chinensis, ETI and PTI responses can be affected by the presence or absence of avirulence and virulence Psa effectors (Hemara et al, 2022;Jayaraman et al, 2021Jayaraman et al, , 2023. The molecular signatures underling these dynamically interconnected modes of host immunity can help identify pathways that are specifically activated and co-evolved in both compatible and incompatible kiwifruit-Psa interactions.…”

“…Kiwifruit germplasm survey & Psa3 isolationSamples were taken from leaf spots on vines in the Actinidia germplasm collections at the Plant & Food Research Te Puke and Kerikeri Research Orchards, as described inHemara et al (2022).…”