Changes in xylem morphology and activity of defense-related enzymes are associated with bean resistance during Fusarium oxysporum colonization.

Genetic resistance is the main strategy to control Fusarium wilt in common bean. Despite this, few studies have focused on defense mechanisms involved in bean resistance to Fusarium oxysporum f. sp. phaseoli (Fop). Thus, the present study aimed to investigate the changes in xylem morphology and involvement of phenylpropanoid compounds and their biosynthetic enzymes in bean resistance against Fop. Uirapuru and UFSC-01 genotypes characterized, respectively, as susceptible and resistant were used. In roots and hypocotyls, guaiacol peroxidase (GPX), phenylalanine ammonia-lyase (PAL), and polyphenol oxidase (PPO) activities were determined at 0, 1, 2, 3, 4, 5, and 6 days after inoculation (dai), and flavonoids, total phenolics, and lignin content were quantified at 0, 3, and 6 dai. Cross sections of taproots and hypocotyls were examined under epifluorescence (at 1, 3, and 6 dai) and transmission electron (at 6 dai) microscopic to analyze the morphology of xylem cell walls. Overall, there was an increase in the activity of all studied enzymes in resistant bean plants, mainly during advanced colonization stages. Modifications in xylem morphology were more intense in roots of resistant genotype resulting in an increase of occluded cells, organelles, and cell wall strengthening. This study provides evidence that bean resistance is associated with increased phenylpropanoid enzymatic activity and cell wall reinforcement of some xylem cells.

Redox status regulation and action of extra- and intravascular defense mechanisms are associated with bean resistance against Fusarium oxysporum f. sp. phaseoli.

Genetic resistance is the main strategy to control one of the most destructive diseases of common bean (Phaseolus vulgaris L), i.e., the Fusarium wilt caused by Fusarium oxysporum f. sp. phaseoli (Fop). However, little is known on host defense reactions in Fop-bean interaction. Thus, this work examined the defense mechanisms in root and hypocotyl tissues of common bean against Fop. Resistant and susceptible bean plants were inoculated by dipping their roots in a conidial suspension. Cross sections of roots and hypocotyls were observed in light microscopy at 1, 3, 6, and 9 days after inoculation (dai) to monitor Fop colonization, and at 3 and 9 dai to detect callose, carbohydrates, lipids, phenolics, and protein, and under electronic microscopy at 9 dai to observe ultrastructural changes in xylem cells. The content of hydrogen peroxide (H2O2), lipid peroxidation, and activity of the antioxidant enzymes ascorbate peroxidase (EC 1.11.1.11) and catalase (EC 1.11.1.6) were monitored spectrophotometrically in roots and hypocotyls at 0, 1, 3, 6, and 9 dai. Fop colonized inter- and intracellularly the epidermis and cortex reaching the xylem vessels faster in susceptible genotype. Fop inoculation induced phenolics and carbohydrates accumulation, callose deposition, and formation of occlusion material inside xylem vessels mainly in resistant genotype. Lipid peroxidation occurred mainly in susceptible plants. In contrast, the antioxidant enzymes seem to have contributed to reducing damage caused by H2O2 accumulation in resistant plants. This study gives evidences that inter- and intracellular physicochemical mechanisms can act together to delay Fop colonization in resistant plants.