CHANGES IN METHANOL EVOLUTION AND PECTIN METHYLESTERIFICATION IN RESISTANT AND SUSCEPTIBLE PEPPER LEAVES INOCULATED WITH XANTHOMONAS-CAMPESTRIS PV. VESICATORIA

The hypersensitive response in resistant plants exposed to incompatibl e pathogens involves structural changes in the plant cell wall and pla sma membrane. Cell wad changes may include pectin deesterification res ulting in release of methanol, The time course of methanol production was characterized from 'Early Calwonder 20R' pepper (Capsicum anuum L, ) leaves infiltrated with the incompatible pathogen, Xanthomonas campe stris pv, vesicatoria (Doidge) Dye race I (XCV), In the first time cou rse experiment, leaves were infiltrated with either 10(8) colony-formi ng units/ml of XCV or water control, Leaf panels (1 x 5 cm) were excis ed after dissipation of water soaking, then incubated in vials at 24 d egrees C. Headspace gas was analyzed at 6-hour intervals up to 24 hour s. The rate of methanol production from resistant pepper leaves infilt rated with XCV was greatest during the first 12 hours after excision, In another experiment, leaf panels were harvested at 6-hour intervals up lo 24 hours after inoculation and incubated for 12 hours at 24 degr ees C to determine the relationship between the interval from inoculat ion to leaf excision and methanol production. The highest rate of meth anol production was obtained when the interval between bacterial infil tration and leaf excision was 18 hours. The relationship between metha nol release and changes in the degree of methylesterification (DOM) of cell wall pectin was determined in near isogenic lines of 'Early Calw onder' pepper plants resistant (20R) and susceptible (10R) to XCV race 1, Cell walls were prepared from resistant and susceptible pepper lea ves infiltrated with XCV or water, XCV-treated resistant leaves had 18 % DOM and 9.7 nmol.g(-1).h(-1) of headspace methanol, and the suscepti ble leaves had 48% DOM with 0.2 nmol.g(-1).h(-1) methanol, Susceptible and resistant control leaves infiltrated with water had 55% and 54% D OM, respectively, with no detectable methanol production, Increased me thanol production in resistant pepper leaves inoculated with XCV coinc ided with an increase in cell wall DH. Intercellular washing fluid of resistant pepper leaves had a significantly higher pH (6.9) compared t o susceptible leaves (pH 5,1) and control leaves infiltrated with wate r (pH 5.1). Both 10R and 20R pepper leaves infiltrated with buffer at increasing pH's of 5.1, 6.9 or 8.7 had increased methanol production. Since deesterified pectin is more susceptible to degradation, demethyl ation may facilitate formation of pectic oligomers with defensive sign alling activity.