Early Pathogen Recognition and Antioxidant System Activation Contributes to Actinidia arguta Tolerance Against Pseudomonas syringae Pathovars actinidiae and actinidifoliorum

Abstract

Actinidia chinensis and A. arguta have distinct tolerances to Pseudomonas syringae pv. actinidiae (Psa), but the reasons underlying the inter-specific variation remain unclear. This study aimed to integrate the metabolic and molecular responses of these two kiwifruit species against the highly pathogenic Psa and the less pathogenic P. syringae pv. actinidifoliorum (Pfm) bacterial strains. Disease development was monitored weekly till 21 days post inoculation (dpi), analysing a broad number and variety of parameters including: colony forming units (CFU), foliar symptoms, total chlorophylls, lipid peroxidation, soluble polyphenols, lignin and defense-related gene expression. At the end of the experimental period A. chinensis inoculated with Psa presented the highest endophytic bacterial population, whereas A. arguta inoculated with Pfm showed the lowest values, also resulting in a lower extent of leaf symptoms. Metabolic responses to infection were also more pronounced in A. chinensis with decreased total chlorophylls (up to 55%) and increased lipid peroxidation (up to 53%), compared with non-inoculated plants. Moreover, at 14 dpi soluble polyphenols and lignin concentrations were significantly higher (112 and 26%, respectively) in Psa-inoculated plants than in controls, while in A. arguta no significant changes were observed in those metabolic responses, except for lignin concentration which was, in general, significantly higher in Psa-inoculated plants (by at least 22%), comparing with control and Pfm-inoculated plants. Genes encoding antioxidant enzymes (SOD, APX and CAT) were upregulated at an earlier stage in Psainoculated A. arguta than in A. chinensis. In contrast, genes related with phenylpropanoids (LOX1) and ethylene (SAM) pathways were downregulated in A. arguta, but upregulated in A. chinensis in the later phases of infection. Expression of Pto3, responsible for pathogen recognition, occurred 2 dpi in A. arguta, but only 14 dpi in A. chinensis. In conclusion, we

Defence‐related pathways, phytohormones and primary metabolism are key players in kiwifruit plant tolerance to Pseudomonas syringae pv. actinidiae

“…‘Ken's Red’ plants (with a single shoot, 5–6 cm height, and 5–10 leaves) were purchased from QualityPlant—Investigação e Produção em Biotecnologia Vegetal, Lda. Plants were maintained in a modified full‐strength Murashige and Skoog (MS) agarized (0.7%) medium in a climate chamber (Aralab Fitoclima 10000EH; Aralab) as described elsewhere (Nunes da Silva et al, 2020). On the day of plant inoculation, a fresh inoculum with 1–2 × 10 7 colony forming units (CFU) ml −1 of a virulent Psa strain (CFBP 7286, isolated in Italy) was prepared in sterile Ringer's solution (NaCl 0.72%, CaCl 2 0.017% and KCl 0.037%, pH 7.4).…”

“…All species of the genus Actinidia can be infected by Psa, but there seems to be considerable variation in their tolerance to the infection. For example, it has been recently demonstrated that species such as Actinidia arguta and Actinidia eriantha are more tolerant to Psa infection than Actinidia chinensis (Nunes da Silva et al, 2020; Song et al, 2019; Vanneste et al, 2014), but the metabolic and transcriptomic features underpinning plant tolerance to Psa are not yet understood.…”

“…PTI and ETI can consequently trigger a hypersensitive response (HR) at the infection site, further leading to changes in structural defences and induction of systemic‐acquired resistance (SAR). These processes may involve strengthening of plant cell walls through the deposition of lignin, impairments in phytohormone regulation and overexpression of several defence‐related genes (Li et al, 2020; Michelotti et al, 2018; Nunes da Silva et al, 2020; Petriccione et al, 2015; Wurms et al, 2017a, 2017b). Recent studies have revealed that Psa Rpm1 effectors trigger immune responses through the gene Rpa1 , which encodes a plant NB‐LRR protein with an N‐terminal coiled‐coil domain (Yoon & Rikkerink, 2020).…”

“…For instance, the expression of the Pto3 gene, which also interacts with pathogen virulence effectors and activates plant defences, was upregulated sooner in A. arguta , as compared with A. chinensis var. deliciosa (Nunes da Silva et al, 2020). Comparative transcriptome analysis also revealed a higher number of differentially expressed genes (DEGs) involved in early disease resistance and rapid response to the pathogen in the tolerant species A. eriantha , when compared to A. chinensis .…”

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

“…Additionally, limited infection is observed in A. arguta seedlings stab-inoculated with Psa, with infection limited to the tissue immediately surrounding the inoculation site (Hoyte et al, 2013). This appears to be related to earlier recognition of Psa3 in A. arguta than in A. chinensis (Nunes da Silva et al, 2020), suggesting that A. arguta has a degree of resistance to Psa, which may be conferred by undiscovered resistance genes recognizing Psa3 effectors.…”