Pseudomonas syringae pv. actinidiae (Psa) and Pseudomonas syringae pv. actinidifoliorum (Pfm) are closely related pathovars infecting kiwifruit, but Psa is considered one of the most important global pathogens, whereas Pfm is not. In this study of Actinidia deliciosa ‘Hayward’ responses to the two pathovars, the objective was to test whether differences in plant defense responses mounted against the two pathovars correlated with the contrasting severity of the symptoms caused by them. Results showed that Psa infections were always more severe than Pfm infections, and were associated with highly localized, differential expression of phytohormones and putative defense gene transcripts in stem tissue closest to the inoculation site. Phytohormone concentrations of jasmonic acid (JA), jasmonate isoleucine (JA-Ile), salicylic acid (SA) and abscisic acid were always greater in stem tissue than in leaves, and leaf phytohormones were not affected by pathogen inoculation. Pfm inoculation induced a threefold increase in SA in stems relative to Psa inoculation, and a smaller 1.6-fold induction of JA. Transcript expression showed no effect of inoculation in leaves, but Pfm inoculation resulted in the greatest elevation of the SA marker genes, PR1 and glucan endo-1,3-beta-glucosidase (β-1,3-glucosidase) (32- and 25-fold increases, respectively) in stem tissue surrounding the inoculation site. Pfm inoculation also produced a stronger response than Psa inoculation in localized stem tissue for the SA marker gene PR6, jasmonoyl-isoleucine-12-hydrolase (JIH1), which acts as a negative marker of the JA pathway, and APETALA2/Ethylene response factor 2 transcription factor (AP2 ERF2), which is involved in JA/SA crosstalk. WRKY40 transcription factor (a SA marker) was induced equally in stems by wounding (mock inoculation) and pathovar inoculation. Taken together, these results suggest that the host appears to mount a stronger, localized, SA-based defense response to Pfm than Psa.

The kiwifruit cultivar Actinidia chinensis ‘Hort16A’ is resistant to the polyphagous armoured scale insect pest Hemiberlesia lataniae (Hemiptera: Diaspididae). A cDNA microarray consisting of 17,512 unigenes selected from over 132,000 expressed sequence tags (ESTs) was used to measure the transcriptomic profile of the A. chinensis ‘Hort16A’ canes in response to a controlled infestation of H. lataniae. After 2 days, 272 transcripts were differentially expressed. After 7 days, 5,284 (30%) transcripts were differentially expressed. The transcripts were grouped into 22 major functional categories using MapMan software. After 7 days, transcripts associated with photosynthesis (photosystem II) were significantly down-regulated, while those associated with secondary metabolism were significantly up-regulated. A total of 643 transcripts associated with response to stress were differentially expressed. This included biotic stress-related transcripts orthologous with pathogenesis related proteins, the phenylpropanoid pathway, NBS-LRR (R) genes, and receptor-like kinase–leucine rich repeat signalling proteins. While transcriptional studies are not conclusive in their own right, results were suggestive of a defence response involving both ETI and PTI, with predominance of the SA signalling pathway. Exogenous application of an SA-mimic decreased H. lataniae growth on A. chinensis ‘Hort16A’ plants in two laboratory experiments.

Root drench application of Trichoderma atroviride isolates R32, R33, R40 and R84 promoted the growth of potted radiata pine seedlings. After 6 weeks, seedlings treated with R33 and R84 had thicker stems and greater stem and root biomass (p < 0.05) than untreated controls. Treatment with R32 increased seedling root biomass whilst R40 increased stem diameter. None of the isolates affected seedling height. One isolate, R33, induced systemic resistance to stem inoculation with Diplodia pinea and reduced dieback incidence by 20% compared with untreated controls. To our knowledge, this is the first report of systemic induced resistance by Trichoderma in a pine species. Furthermore, seedlings that were treated with R33 (root drench) plus foliar application of methyl jasmonate (MeJA) expressed elevated peroxidase activity in their stems 2 weeks later, compared with seedlings treated only with MeJA. Because R33 itself did not affect peroxidase activity, this may be indicative of treatment synergy or defence potentiation by R33. Curiously, R33 + MeJA induced terpenoids but suppressed phenylalanine ammonia-lyase activity suggesting possible trade-offs between phenolic and terpenoid defence pathways in the treated seedlings.

Acibenzolar-S-methyl (ASM), the active ingredient in Actigard® (Syngenta), is a plant defence elicitor used for the management of bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) in kiwifruit. We examined changes in gene expression and phytohormone content in two kiwifruit cultivars, ‘Hayward’ (Actinidia chinensis var. deliciosa) and ‘Zesy002’ (A. chinensis var. chinensis), following ASM application. In glasshouse studies, ASM-induced resistance to stem inoculation with Psa was accompanied by upregulation of salicylic acid (SA) defence pathway genes (PR1, PR2, and PR5) in stems and leaves. The expression of PR1 in ASM-treated plants increased by c. 5-fold in ‘Hayward’ and by over 15-fold in ‘Zesy002,’ relative to the untreated controls, whereas PR2 and PR5 increased by up to 4-fold in both cultivars. Orchard studies over three seasons confirmed the utility of PR1, PR2, and PR5 for monitoring ASM-induced responses in mature vines. PR1 and PR5 were more strongly induced by ASM than PR2 and gene upregulation in ‘Hayward,’ was accompanied by a reduction in Psa leaf spotting; no such relationship was measurable for ‘Zesy002’ because leaf spot symptoms are rarely expressed. In the third season, six additional gene candidates, BAD, Gluc2, Class IV Chit, EDS1A, NPR3, and NIMIN2, were responsive to ASM in ‘Hayward’ and ‘Zesy002.’ Gene upregulation was transient, with expression levels increasing by 1 d after ASM application and declining to control levels between 7 and 14 days. Moreover, the amplitude of gene upregulation depended on leaf developmental stage and was greater in the first true leaf and the youngest mature leaves than in immature leaves along the same shoot. Phytohormone content did not show a repeatable response pattern to ASM in potted plants or in vines possibly as a consequence of their wider role in regulating plant growth and mediating environmental responses. In conclusion, this study demonstrates that defence gene expression can be used to monitor responsiveness to ASM in two genetically distinct mature kiwifruit cultivars (‘Hayward’ and ‘Zesy002’) under orchard conditions. The use of defence marker genes could be of broader utility across kiwifruit species and could be used to guide ASM application schedules in the orchard.

Pseudomonas syringae pv. actinidiae (Psa), which causes bacterial canker, is the most serious global pathogen of kiwifruit. Like most bacterial pathogens, control options are limited, but elicitors can reduce disease significantly, particularly those that induce the salicylic acid (SA) pathway. Acibenzolar-S-methyl (ASM), a SA analogue, is one of the most effective elicitors for Psa control. In this study, real-time PCR (qPCR) was used to measure the expression of 18 putative defence genes in Actinidia chinensis var. chinensis 'Hort16A' in response to Psa and ASM. Application of ASM led to up-regulation of RPM1 interacting protein 4 (RIN4), phenylalanine ammonia lyase (PAL), a hypersensitivity-induced response protein (HIRP), and β-1,3-glucosidase. Expression of PAL and HIRP was further enhanced when elicitor application and Psa-inoculation were combined. Elevated gene expression was correlated with decreased disease expression, and supports the hypothesis that elicitor-treated plants are primed to react more rapidly and/or strongly to pathogens.