Gene expression in flavonoid biosynthesis: Correlation with flavonoid accumulation in developing citrus fruit

Two genes active in the early steps of flavonoid biosynthesis were isolated from a citrus cDNA library, and their expression patterns were analyzed du ring citrus (Citrus unshiu Marc.) fruit development in relation to flavonoi d contents. The respective cDNA clones from chalcone isomerase (CHI, CitCHI ) and flavanone 3-hydroxylase (F3H, CitF3H) are 931 and 1360 bp long, encod ing 222 and 362 amino acid residues. CitCHI seems to be a single-copy gene, whereas a few CitF3H-related sequences are present in the citrus genome. M essenger RNA levels of the flavonoid biosynthetic genes CitCHI and CitF3II and two previously isolated chalcone synthases (CHS, CitCHS1, and CitCHS2) were analyzed in various tissues during fruit development. Transcript level s for CitCHS1, CitCHS2, CitCHI, and CitF3H were generally high in young tis sues and low in senescent tissues. Flavonoid changes were analyzed in relat ion to gene expression. High flavonoid contents were found in young tissues . The high expression of flavonoid biosynthetic genes and high accumulation of flavonoids in young fruits suggest that flavonoids are synthesized in t he early developmental stage.