Incorporation of [14C]Phenylalanine and [14C] Cinnamic Acid into Leaf Pieces and Mesophyll Protoplasts from Oat Primary Leaves for Studies on Flavonoid Metabolism at the Tissue and Cell Level
When the abaxial epidermis was peeled from 5 to 6 day old oat primary leaves, and 3 cm segments were floated on radioactive phenylalanine or cinnamic acid solutions, more than 90 per cent of the radioactivity was incorporated within 3 to 7 h depending on the developmental stage of the leaf. C-glycosylflavones were labelled within 15 min and radioactivity in these compounds increased for several hours. Pulse labelling and pulse chase experiments with either phenylalanine or cinnamic acid, unequivocally demonstrate that oat flavones are stable end products of metabolism. However, this procedure does not distinguish between sequential biosynthesis of various flavones and their interconversion. Cinnamic acid was more efficiently (ca. 20 x) converted into oat leaf flavones than was phenylalanine, when the precursor was fed to leaf pieces, and flavones recovered from mesophyll protoplasts. Different labelling patterns were obtained with whole leaf segments and protoplasts which apparently reflect differences in tissue specific flavone biosynthesis of mesophyll and epidermis. Isolated mesophyll protoplasts incubated with [14C]cinnamic acid synthesize 14C-labelled flavones characteristic of the mesophyll, as well as several unidentified phenylpropanoid derivatives not found in the intact tissue. Data suggest that photosynthetically active mesophyll cells are a main site of tissue specific flavone biosynthesis