Antioxidant Mechanisms, Phenylpropanoid Pathway and Photosynthetic Responses in Pepper Cultivars: Insights Into Resistance to Xanthomonas euvesicatoria pv. euvesicatoria

Abstract

Changes in environmental conditions, intensified by the progression of global warming, have created favourable conditions for the spread and intensification of diseases in various agricultural crops. Obtaining cultivars resistant to various abiotic and biotic stresses may be one of the solutions. In this study, the resistance of three pepper cultivars to Xanthomonas euvesicatoria pv. euvesicatoria was analysed inside a greenhouse, evaluating the severity of the disease over time. The participation of biochemical and structural mechanisms in the defence of pepper plants against this pathogen was investigated based on the determination of the activity of the antioxidant enzymes such as guaiacol peroxidase (POX), catalase (CAT), polyphenol oxidase (PPO) and lipoxygenase (LPOX). The quantification of components of the phenylpropanoid pathway (phenylalanine ammonia lyase [PAL], phenolic compounds and flavonoids), the observation of the number of stomatal cells with hypersensitivity reaction (HR) and the content of chlorophyll a, b and carotenoids were also evaluated. Plants of the cultivars All Big, Yolo Wonder and Amarelo SF were challenged with X. euvesicatoria pv. euvesicatoria and the severity evaluated at 14, 21 and 28 days after inoculation. Leaf samples were collected 24, 48 and 96 h after inoculation (HAI) for biochemical and photosynthetic analyses. The plants of the All Big cultivar exhibited 50% higher disease severity compared to the Amarelo SF and Yolo Wonder cultivars, which did not differ significantly from each other. In All Big, 48 HAI, POX activity was 37% higher than in Amarelo SF and 27% higher than in Yolo Wonder. Additionally, PPO activity was 46% higher than in Amarelo SF and 55% higher than in Yolo Wonder. After 96 h of inoculation, Yolo Wonder plants showed the highest CAT enzyme activity, 43% higher than Amarelo SF, but with no significant differences compared to All Big. Furthermore, 96 HAI, Amarelo SF stood out for its higher LPOX activity, 14% higher than All Big and 22% higher than Yolo Wonder. On the other hand, Amarelo SF also stood out by exhibiting 31% more cells undergoing a hypersensitive response compared to Yolo Wonder, but with no significant differences relative to All Big. After 96 h of inoculation, All Big plants displayed higher chlorophyll a levels, exceeding those observed in Amarelo SF by 18% and in Yolo Wonder by 17%. Despite peaks in PAL enzyme activity, as well as in the accumulation of phenolic compounds and flavonoids, no statistically significant differences were found among the evaluated cultivars for any of the measured parameters. This article explores and discusses the role of defence mechanisms, such as the antioxidant system, the phenylpropanoid pathway and the accumulation of photosynthetic pigments, in the pathosystem involving bell pepper plants and X. euvesicatoria pv. euvesicatoria.