The molecular biology of cyanogenesis.

The cyanogenic polymorphism in Trifolium repens L. (white clover) has been used as the basis of a study of the genetic control of cyanogenesis. The Ac locus controls the presence of two cyanoglucosides in white clover. Biochemical characterization of cyanoglucoside biosynthesis in plants containing different Ac alleles has shown that this is a complex locus which affects more than one step in the pathway. A study of the in vivo synthesis and processing of the cyanogenic beta-glucosidase (linamarase) of white clover led to the isolation of cDNA clones for this enzyme. The cloning strategy and structure of the cDNA clones is described. Together with biochemical and genetic data, these clones have been used to characterize the Li locus which controls linamarase activity in white clover. It has been shown that 'null' alleles of the Li locus result in very reduced levels of transcription of homologous mRNA sequences. The use of these cDNA clones to investigate the genomic organization of cyanogenesis genes in both white clover and other cyanogenic species is described, and their use in structural analysis of the cyanogenic beta-glucosidase is discussed.