title: Lysine metabolism in barley leaves and in barley powdery mildew
Creators: name: Jackson, Samantha Angela Lindsay
Publisher: name: University of Glasgow
Year: 1995
Subjects: subject: Plant pathology

description: Lysine levels and enzyme activity: The activity of diaminopimelic acid decarboxylase, an enzyme of the diaminopimelic acid pathway of lysine biosynthesis, and usually associated with plants, bacteria and certain lower fungi, increased early in powdery mildew infection of barley leaves. The level of soluble lysine, the product of this enzyme, also increased early in the infection. Both soluble and bound lysine were reduced by 6 days after inoculation (DAI). At the same time, the activity of diaminopimelic acid decarboxylase was not significantly different in whole infected leaves when compared to healthy leaves, but was slightly reduced in isolated chloroplasts. Later in the infection (9 DAI), the activity of diaminopimelic acid decarboxylase was significantly greater in inter-pustule regions than in pustules. Activity of dihydrodipicolinic acid synthase was reduced in infected leaves and in. chloroplasts throughout the course of infection. This reduced activity was not due to increased levels of lysine, which has been shown to control the activity of this enzyme. It is suggested that changes in chloroplast integrity may have caused the observed reduction in activity of these two chloroplast localised enzymes. During sporulation the levels of both soluble and bound lysine were reduced in infected leaves. Activity of lysine decarboxylase in whole infected barley leaves was reduced (3-9 DAI) compared to controls, and reductions were found to occur in both pustule and inter-pustule regions of the leaf. At the same time, activity was higher in chloroplasts isolated from whole infected leaves than in those isolated from healthy leaves, and a similar situation was found in chloroplasts isolated from pustules and inter-pustule regions 6 DAI. By 9 DAI, however, the activity of the enzyme in chloroplasts isolated from pustules was lower than in inter-pustule regions and healthy leaves. It is suggested that these changes in LDC activity are due to a reduction in activity of cytosolic LDC, while activity of the chloroplastic enzyme is stimulated in the inter-pustule regions of infected leaves. The activities of two enzymes of the aminoadipic acid pathway of lysine biosynthesis were examined. This pathway is normally associated with higher fungi. Activity of saccharopine dehydrogenase was not significantly different in infected leaves until 9 DAI, when activity increased. In contrast to dihydrodipicolinic acid synthase and diaminopimelic acid decarboxylase, this increase was located in the pustules. It is likely that this increase was due to the presence of the mildew. Enzyme activity in isolated mildew mycelium was greater than in infected leaves. No aminoadipic acid reductase activity was found in healthy leaves. Activity of this enzyme and of saccharopine dehydrogenase was similar in infected leaves, but the activity of aminoadipic acid reductase was higher in the isolated mycelium. It is suggested therefore that aminoadipic acid reductase is only found in the fungal mycelium. 14C-Aspartate uptake Infection of barley with powdery mildew altered the uptake and metabolism of 14C-aspartate. Uptake was greater in infected leaves 2 h after feeding, but was reduced thereafter. Aspartate was more rapidly metabolised in infected leaves than in healthy leaves, probably to homoserine and threonine. The fungus took up aspartate, homoserine and threonine from the plant, but these were apparently not further metabolised over the experimental period. This suggests that barley can metabolise aspartate, probably forming homoserine, threonine and lysine, while such metabolism does not occur in the powdery mildew fungus. This would confirm the presence of the diaminopimelic acid pathway for lysine biosynthesis in barley and its absence in Erysiphe graminis. (Abstract shortened by ProQuest.).