Polyamine metabolism in 'green-islands' on powdery mildew-infected barley leaves: possible interactions with senescence.

Abstract

Polyamine metabolism, lipoxygenase and catalase activities and ethylene production were examined in green-islands induced to form on detached barley leaves infected with the powdery mildew fungus, Erysiphe graminis f. sp. hordei Marchal. Several-fold increases in free polyamines were detected in green-islands, which could not be accounted for by changes in polyamine biosynthetic and degradative enzymes: a small increase in ornithine decarboxylase (ODC) activity, greatly decreased arginine decarboxylase (ADC) activity, virtually unchanged activity of S-adenosylmethionine decarboxylase (AdoMetDC) and greatly increased polyamine oxidase (PAO) activity. Changes in conjugated forms of polyamines suggest that elevated free polyamine concentrations might result from reductions in this pool. Green-island tissues displayed a reduced activity of lipoxygenase, unchanged catalase activity and a greatly lowered rate of ethylene production. Chlorophyll concentration and rates of photosynthesis and dark respiration were slightly increased in these tissues. In senescing regions of infected leaves, there was a small increase in free putrescine and spermine and a decrease in spermidine concentration, while concentrations of conjugated polyamines were substantially reduced. This was accompanied by an overall decrease in ODC and ADC activities and an increased activity of both AdoMetDC and PAO. These regions exhibited lowered lipoxygenase activity, greatly enhanced catalase activity and reduced rates of ethylene production. In addition, senescing regions exhibited a substantial reduction in chlorophyll concentration and the rate of photosynthesis, although dark respiration was similar to control values. These results support the view that green-islands represent areas of the infected leaf where senescence is retarded, biosynthetic activity is retained and in which polyamines may play an important role. On the other hand, although polyamine concentrations were reduced in senescing areas, other changes occurring there suggest that some functional integrity is maintained in that tissue.