Pectin demethylation in pepper leaves infiltrated with Xanthomonas campestris pv. vesicatoria

Findings in resistant pepper plants. Resistance to bacterial spot, caused by Xanthomonas campestris pv. vesicatoria (Doidge) Dye (XCV), may involve alteration of the plant cell wall and plasma membrane. Pectin degradation may produce elicitors of defensive compounds with a concomitant release of methanol. In our studies, temporal analysis of methanol production by 'Early Calwonder' pepper (Capiscum annuum L.) leaves infiltrated with 108 cells/ml of XCV race 1, excised after dissipation of water soaking, and incubated in vials at 24C, revealed that methanol production was greatest during the first 12 h after excision. Increase in the rate of methanol production was greatest when the interval between bacteria infiltration and leaf excision was 18 h. Estimation of the degree of methylation of cell wall pectin revealed that water-infiltrated leaves had a higher degree of methylation (DOM) (48%) than bacteria-infiltrated leaves (31%). Intercellular washing fluid (IWF) of bacteria-infiltrated leaves had a significantly (P ≤ 0.02) higher pH (7.5) compared to water-infiltrated leaves (5.4) indicating the association of methanol production with cell wall pH.;Studies in resistant and susceptible pepper plants. The relationship between methanol release and changes in the degree of methylation of cell wall pectin was determined in resistant and susceptible pepper plants. DOM of resistant leaves infiltrated with 108 cells/ml of XCV race 1 was 18.3% with 9.7 nmol/g/h of headspace methanol, whereas, the DOM of susceptible leaves was 48% with 0.2 nmol/g/h methanol. IWF of resistant pepper leaves had a significantly higher (P ≤ 0.001) pH (6.9) compared to susceptible leaves (pH 5.1) and control leaves infiltrated with water (pH 5.1) indicating a pH dependent production of methanol. Increase in methanol production was observed in 10R and 20R pepper leaves with increasing pH of the buffer solutions from 5.1 to 8.7. Involvement of the cell wall enzyme PME in the pectin demethylation process was determined by injecting 8-aminonaphthalene-1,3,6-trisulfonate (ANTS) labelled pectin into pepper leaves and analyzing the IWF using capillary electrophoresis. Both pepper genotypes exhibited substantial PME activity in the leaves although there was no difference in the activity between susceptible and resistant pepper leaves infiltrated with XCV.