Temporal and spatial assessment of defense responses in resistant and susceptible cabbage varieties during infection with Xanthomonas campestris pv. campestris

Abstract

Five cabbage varieties were characterized in terms of their resistance to disease caused byXanthomonas campestris pv. campestris(XCC) strain FD91L, and of these two were found to be resistant (Hancock and Green Cup), one partially resistant (Cheers) and two susceptible (Strukton and Perfect Ball). These five cabbage varieties were analysed for peroxidase activity, presence of peroxidase isozymes, and lignin deposition. The most resistant variety (Hancock) and the most susceptible variety (Perfect Ball) also analysed for superoxide dismutase and catalase activities, and hydrogen peroxide accumulation following inoculation with XCC strain FD91L. Superoxide dismutase isozyme analyses were also conducted for Hancock and Perfect Ball varieties. Four week old plants were petiole-inoculated with XCC strain FD91L, which contains the lux operon from Vibrio fischerii. Bacterial movementin planta was monitored in a non-destructive manner via the detection of bioluminescence. Upon symptom development, leaf samples from each variety were taken from three zones representing different stages of disease development (1=non-bioluminescent, non-symptomatic, 2=bioluminescent, non-symptomatic, 3=bioluminescent, symptomatic). An increase in total peroxidase and superoxide dismutase activity, as well as the accumulation of peroxidases and lignin deposition, were observed in all tested varieties following inoculation with FD91L. These increases were greater and generally occurred earlier in resistant varieties when compared to susceptible ones. In contrast, catalase activity decreased in both varieties tested, with a greater decrease observed in the resistant variety (Hancock) than the susceptible one (Perfect Ball). Analysis of superoxide dismutase and peroxidase isozymes revealed the presence of unique isozymes in the resistant cabbage varieties that were not found in the susceptible ones. A peroxidase isozyme (pI 3.6) was observed to accumulate in conjunction with lignin deposition. These findings suggest a multilevel, coordinated resistance similar to a hypersensitive response (HR) in cabbage to X. campestris pv.campestris infection, although the onset of this response was delayed when compared to the occurrence of the classical HR.