Effector loss drives adaptation of Pseudomonas syringae pv. actinidiae biovar 3 to Actinidia arguta.

A 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 not a host of Psa3. 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 51 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.

Postharvest Application of Acibenzolar-S-Methyl Activates Salicylic Acid Pathway Genes in Kiwifruit Vines.

The plant defence inducer Actigard® (acibenzolar-S-methyl [ASM]) is applied before flowering and after fruit harvest to control bacterial canker in kiwifruit caused by Pseudomonas syringae pv. actinidiae. Pre-flowering application of ASM is known to upregulate defence gene expression; however, the effect of postharvest ASM on defence gene expression in the vine is unknown. In this study, the expression of eight "defence marker" genes was measured in the leaves of Actinidia chinensis var. chinensis, "Zesy002," and Actinidia chinensis var. deliciosa, "Hayward," vines after postharvest treatment with ASM and/or copper. There were two orchards per cultivar with harvest dates approximately three weeks apart for investigating potential changes in responsiveness to ASM during the harvest period. In all trials, postharvest ASM induced the expression of salicylic-acid-pathway defence genes PR1, PR2, PR5, BAD, DMR6, NIMIN2, and WRKY70. Gene upregulation was the greatest at 1 day and 7 days after treatment and declined to the control level after 3 weeks. In "Zesy002", the ASM-induced response was greater at the early harvest site than at the late harvest site. This decline was concomitant with leaf yellowing and a reduction in RNA yield. Effects of postharvest ASM on gene expression did not persist into the following spring, nor were vines conditioned to respond more strongly to pre-flowering ASM application.