Identification of candidate Pseudomonas syringae pv. actinidiae effectors that trigger resistance in Actinidia arguta

Abstract

Pseudomonas syringae pv. actinidiae (Psa) is a damaging bacterial disease of susceptible kiwifruit (Actinidia spp.). Characterising Psa effectors that are recognised in the resistant species A. arguta (avirulence effectors) will allow the identification of corresponding resistance genes for deployment in breeding programmes. A high-throughput PDQeX DNA extraction and qPCR detection method to quantify Psa from infected Actinidia leaf material was developed. The assay was validated by a significant correlation with traditional plate count quantification of Psa growth (R2=0.82, p=0.0019). Pathogenicity assays of the five Psa biovars on A. arguta var. ‘AA07_03’ indicated that Psa1 J-35 and Psa2 K-28 had increased virulence compared to Psa3 V-13 which was avirulent. This indicated that there were effectors uniquely present in Psa3 V-13 responsible for triggering resistance. To identify these effectors a library of 25 Psa3 V-13 individual deletion strains were pathogenicity tested on ‘AA07_03’. A portion of the exchangeable effector locus (sEEL), the individual deletions of avrRpm1, hopF2, and hopZ5, resulted in an increase of bacterial growth relative to wild-type Psa3 V-13. To discover which effectors in the sEEL were responsible for triggering resistance, Psa3 V-13 ΔsEEL was complemented with individual plasmid-borne effectors from the sEEL. The results indicated that HopAW1 and possibly HopBB1-2 were candidates for triggering resistance in ‘AA07_03’. The Psa3 V-13 ΔhopAW1 deletion mutant demonstrated increased bacterial growth on ‘AA07_03’, indicating a release from host recognition. Plasmid complementation of candidate avirulence effectors into Psa2 K-28 indicated that HopAW1 and HopBB1-2 triggered resistance in ‘AA07_03’. However, previously identified avirulence effectors such as avrRpm1 unexpectedly did not trigger resistance, highlighting the complex role of biovar-specific effector repertoires in triggering or suppressing resistance. Finally, avirulence effector deletion polymutants were developed to study the effect of cumulative effector deletion on host resistance. While these strains escaped host recognition, a cumulative increase of virulence was not observed on ‘AA07_03’. Furthermore, the deletion of hopZ5 from Psa3 V-13 ΔfEEL/ΔhopF2/ΔavrRpm1/ΔhopZ5 resulted in reduced virulence on ‘AA07_03’. The effectors identified in this research can be used to identify the first cognate resistance genes in Actinidia and thus breed durable Psa resistance into future kiwifruit cultivars.