Characterisation of and changes to pro and anti-oxidant enzyme activities during the hypersensitive reaction in lettuce ( Lactuca sativa L.)

Abstract

Enzymes regulating aspects of reactive oxygen metabolism were characterised in expanded lettuce leaf tissue. Catalase (CAT) activity was optimal between pH 7.0–8.0, predominantly located within peroxisomes and comprised two isoenzymes (pI 5.8 and 6.2). Of three principal superoxide dismutase (SOD) isoenzymes detected, CuZn SOD, (pI 4.90) represented 51.7%, Mn SOD (pI 4.75) 14.6% and a putative Fe SOD (pI 4.85), 33.7% of total activity. Lipoxygenase (LOX) activity was optimal at pH 5. Activities were assessed following inoculation with Pseudomonas syringae pv. phaseolicola and a related hrpD mutant. The wild-type bacterium induced a rapid hypersensitive reaction (HR), associated with a progressive increase in thiobarbituric acid reactive substances (TBARS), indicative of lipid peroxidation, a sustained increase in LOX activity and a later increase in CAT activity. The hrp mutant induced a transitory increase in TBARS, a transient increase in LOX activity, again a later elevation in CAT activity but no macroscopic tissue collapse. In comparison, water infiltration induced a rapid but transient increase in both SOD and CAT activities. From this and our previous work, it is concluded that lettuce cells undergoing an HR experience a prolonged oxidative stress, primarily through an increase in pro-oxidant activities initially occurring in the absence of enhanced antioxidant activities.