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.