Plant responses upon infection with Verticillium longisporum O1 and Verticillium isaacii Vt305: a histochemical study in cauliflower and broccoli

Variation in life styles occurs in the fungal genus Verticillium. Verticillium longisporum causes diseases on several brassicaceous crops, including cauliflower, but has not been observed causing damage in broccoli. Verticillium isaacii has been frequently isolated from fields of horticultural crops, but the majority of the isolates has exhibited little or no pathogenicity to most of these crops. To broaden the knowledge on plant responses to Verticillium isolates with different life styles on different hosts, we conducted a histochemical study in the stems of cauliflower and broccoli upon infection with the isolates V. longisporum O1 and V. isaacii Vt305, a pathogen and an endophyte in cauliflower, respectively. Vascular occlusions and accumulation of phenolic compounds occurred in stems of both cauliflower (‘susceptible’ host) and broccoli (‘resistant’ host) plants upon infection with the isolate V. longisporum O1. However, such responses were clearly more abundant in cauliflower than in broccoli. Furthermore, in cauliflower plants, V. longisporum O1 altered the xylem structure: proportionally to other cell types, more xylem parenchyma cells were observed in infected plants than in control plants. None of these responses or any other response occurred in cauliflower and broccoli plants inoculated with the isolate V. isaacii Vt305, suggesting a completely different type of interaction compared to the interaction between V. longisporum O1 and the same hosts.