Tissue distribution of phenylpropanoid metabolism in cotyledons of Raphanus sativus L

The tissue distributions of sinapic acid esters (1-sinapoylglucose, sinapolyl-L-malate, 6,3'-disinapoylsucrose), kaempferol glycosides, free malic acid and of the enzyme involved in the synthesis of sinapoyl-L-malate, 1-sinapoylglucose: L-malate sinapoyltransferase (SMT), have been investigated in cotyledons of Raphanus sativus L. seedlings. The kaempferol glycosides were mainly localized in the upper epidermis. The sinapoyl esters were found in all tissues, but differed markedly in their concentrations. While disinapoylsucrose was localized predominantly in the mesophyll, most sinapoylmalate was found in the epidermal layers, as was most SMT activity. Ultraviolet microscopy and microfluorospectrophotometry of isolated epidermal peels indicated that the epidermal sinapoyl esters were restricted to guard cells, guard mother cells and adjacent epidermal cells. Upon excitation by UV light (365 nm) these exhibited strong blue fluorescence with an emission maximum at about 480 nm. Our results indicate a highly tissue-and cell-specific secondary metabolism in Raphanus cotyledons and indicate that the biosynthesis of sinapoylmalate is intimately related to the malic-acid metabolism of the guard cells.