In vitro evaluation of Actinidia chinensis cultivars for their resistance to Pseudomonas syringae pv. actinidiae

Kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) has been the most destructive disease to kiwifruit plants since 2010, and so far there has been no way to completely solve it. Breeding and planting Psa-resistant cultivars is considered one of the most effective disease-control strategies, but the resistance of most existing kiwifruit cultivars to Psa is poorly understood. At present, almost all commercially important cultivars are Actinidia chinensis species, which are generally considered to be susceptible to Psa. In the present study, 62 cultivars of A. chinensis were evaluated on the resistance to Psa through an in vitro assay. In order to find early index for these cultivars' resistance to Psa, activities of the defense-related enzymes including peroxidase (POD), polyphenol oxidase (PPO), catalase (CAT), phenylalanine ammonia-lyase (PAL) and superoxide dismutase (SOD) were also analyzed after inoculation of Psa. The results showed that there were a morethan-expected number of A. chinensis cultivars (about 38.71%) classified as Psa-resistant in comparison to Hayward which has been recognized as a Psa-tolerant cultivar. In particular, some red- and yellow-fleshed cultivars were first reported with strong tolerance to Psa. Further study found that these cultivars have an obvious tendency of increasing resistance with ploidy level increasing. A positive correlation was found between the POD and CAT activity to the resistance of the test cultivars after 10 days of inoculation, these two enzyme activities could be used as indicators for assessing kiwifruit resistance to Psa to some extent. Our findings greatly increase the understanding of the resistance of A. chinensis cultivars to Psa and lay a foundation for the use of existing kiwifruit cultivars and the breeding of improved Psa-resistant cultivars.

Automated summary

Kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) has been the most destructive disease for kiwifruit plants. In this study, 62 cultivars of A. chinensis were evaluated for Psa resistance, and the activities of defense-related enzymes were analyzed. The results showed a higher percentage of Psa-resistant A. chinensis cultivars compared to the recognized Psa-tolerant cultivar Hayward. Some red- and yellow-fleshed cultivars were found to have strong tolerance to Psa. The study also revealed a positive correlation between the activities of two enzymes and the resistance of the test cultivars. These findings enhance the understanding of A. chinensis cultivars' resistance to Psa and provide a basis for the use of existing cultivars and breeding improved Psa-resistant ones.