Recent advances in understanding the anthocyanin modification steps in carnation flowers

• Wild type carnations accumulate macrocyclic malyl anthocyanidin 3,5-diglucoside.
• All genes and enzymes involved in anthocyanin aglycone synthesis have been identified.
• Malylation and glucosylation at 5-position are catalyzed by novel vacuolar enzymes.
• Defects in malylation cause anthocyanic vacuolar inclusions that alter flower color.

Carnations (Dianthus caryophyllus) are among the most commercially important ornamental flower crops in the world along with roses and chrysanthemums. The variation in carnation flower colors has been established through selective breeding and the genetics of this color variation have been studied for more than half a century. The cyanic coloration of carnation flowers is due to anthocyanins. Recent molecular biology studies have identified the genes that encode the enzymes involved in the biosynthesis of pelargonidin and cyanidin, the anthocyanin backbone (aglycones). The agylcones are converted into anthocyanins by attachment of a glucosyl group at either the 3- or 3,5-positions. A malyl group is then joined to the glucosyl group at the 3-position. Anthocyanidin 3,5-diglucoside can be modified by a malyl group linking the glucosyl groups at the 3- and 5-positions to form a macrocyclic ring in anthocyanidin 3,5-diglucoside. The modification step that attaches a glucosyl group to the 3-position relies on a UDP-glucose dependent glucosyltransferase in the cytosol; however, until recently, the mechanisms for malylation and glucosylation at the 5-position were unclear. It has now been found that novel enzymes, malyl-glucose dependent acyltransferase and acyl-glucose dependent 5-O-glucosyltransferase, are involved in these modifications at the 5-position and that these enzymes function in the vacuole and not the cytosol. Defects in malyltransferase prevent anthocyanin from diffusing in the vacuolar sap and cause formation of anthocyanic vacuolar inclusions that result in a dusky and metallic color in the flower petal. Several factors are now known to influence anthocyanin accumulation in vacuoles and, thereby, to influence the color of the flower.