Characterization of Two Key Flavonoid 3-O-Glycosyltransferases Involved in the Formation of Flower Color in Rhododendron Delavayi

Abstract

Flower color, largely determined by anthocyanin, is one of the most important ornamental values of <i>Rhododendron delavayi</i>. However, scant information of anthocyanin biosynthesis has been reported in <i>R. delavayi</i>. We found that anthocyanidin 3-<i>O</i>-glycosides were the predominant anthocyanins detected in <i>R. delavayi</i> flowers accounting for 93.68-96.31% of the total anthocyanins during its development, which indicated the key role of flavonoid 3-<i>O</i>-glycosyltransferase (3GT) on <i>R. delavayi</i> flower color formation. Subsequently, based on correlation analysis between anthocyanins accumulation and <i>Rd3GTs</i> expressions during flower development, <i>Rd3GT1</i> and <i>Rd3GT6</i> were preliminarily identified as the pivotal <i>3GT</i> genes involved in the formation of color of <i>R. delavayi</i> flower. Tissue-specific expressions of <i>Rd3GT1</i> and <i>Rd3GT6</i> were examined, and their function as 3GT <i>in vivo</i> was confirmed through introducing into <i>Arabidopsis UGT78D2</i> mutant and <i>Nicotiana tabacum</i> plants. Furthermore, biochemical characterizations showed that both <i>Rd3GT1</i> and <i>Rd3GT6</i> could catalyze the addition of UDP-sugar to the 3-OH of anthocyanidin, and preferred UDP-Gal as their sugar donor and cyanidin as the most efficient substrate. This study not only provides insights into the biosynthesis of anthocyanin in <i>R. delavayi</i>, but also makes contribution to understand the mechanisms of its flower color formation.