Cytoplasmic accumulation of flavonoids in flower petals and its relevance to yellow flower colouration

It is widely accepted that the mix of flavonoids in the cell vacuole is the source of flavonoid based petal colour, and that analysis of the petal ext ract reveals the nature and relative levels of vacuolar flavonoid pigments. However, it has recently been established with lisianthus flowers that som e petal flavonoids can be excluded from the vacuolar mix through deposition in the cell wall or through complexation with proteins inside the vacuole, and that these flavonoids are not readily extractable. The present work de monstrates that flavonoids can also be compartmented within the cell cytopl asm. Using adaxial epidermal peels from the petals of lisianthus (Eustoma g randiflorum), Lathyrus chrysanthus and Dianthus caryophyllus, light and las er scanning confocal microscopy studies revealed a significant concentratio n of petal flavonoids in the cell cytoplasm of some tissues. With lisianthu s, flavonoid analyses of isolated protoplasts and vacuoles were used to est ablish that ca 14% of petal flavonoids are located in the cytoplasm (cf. 30 % in the cell wall and 56% in the vacuole). The cytoplasmic flavonoids are predominantly acylated glycosides (cf. non-acylated in the cell wall). Flav onoid aggregation on a cytoplasmic protein substrate provides a rational me chanism to account for how colourless flavonoid glycosides can produce yell ow colouration in petals, and perhaps also in other plant parts. High vacuo lar concentrations of such flavonoids are shown to be insufficient. (C) 200 1 Elsevier Science Ltd. All rights reserved.