Early Pathogen Recognition and Antioxidant System Activation Contributes toActinidia argutaTolerance AgainstPseudomonas syringaePathovarsactinidiaeandactinidifoliorum

Actinidia chinensisandA. argutahave distinct tolerances toPseudomonas syringaepv.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 pathogenicP. syringaepv.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 periodA. chinensisinoculated with Psa presented the highest endophytic bacterial population, whereasA. argutainoculated with Pfm showed the lowest values, also resulting in a lower extent of leaf symptoms. Metabolic responses to infection were also more pronounced inA. chinensiswith 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 inA. argutano 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,APXandCAT) were upregulated at an earlier stage in Psa-inoculatedA. argutathan inA. chinensis. In contrast, genes related with phenylpropanoids (LOX1) and ethylene (SAM) pathways were downregulated inA. arguta, but upregulated inA. chinensisin the later phases of infection. Expression ofPto3, responsible for pathogen recognition, occurred 2 dpi inA. arguta, but only 14 dpi inA. chinensis. In conclusion, we found thatA. argutais more tolerant to Psa and Pfm infection thanA. chinensisand its primary and secondary metabolism is less impacted.A. argutahigher tolerance seems to be related with early pathogen recognition, the activation of plant antioxidant system, and to the suppression of ET and JA pathways from an earlier moment after infection.