Condensed tannin biosynthesis genes are regulated separately from other flavonoid biosynthesis genes in apple fruit skin

Although the molecular basis of anthocyanin biosynthesis has been studied in several types of fruit, little is known about how structural flavonoid genes are regulated to balance the requirement for condensed tannin (CT) and flavonol synthesis. We cloned three genes encoding enzymes involved in CT synthesis from cDNA of skin of the red apple variety Malus × domestica Borkh. cv. ‘Cripps Red’; two leucoanthocyanidin reductases (MdLARs) and an anthocyanidin reductase (MdANR). We measured steady-state transcript levels of the CT genes and the other structural genes of the flavonoid pathway by Real Time PCR and correlated this with flavonoid accumulation in skin throughout early fruit growth and during ripening. Transcripts of the MdANR gene were abundant in both green and red skin sectors early in development which correlated with accumulation of CTs in these tissues. The MdLAR1 gene transcripts were detected in green skin early in development but transcripts of MdLAR2 were present only in juvenile fruit. All the CT transcripts were much less abundant during fruit ripening when the majority of anthocyanin accumulated and transcripts of the other pathway genes were at a high level. The results indicated that transcription of the genes for synthesis of CTs in apple fruit skin was differentially regulated from the other flavonoid pathway genes which were regulated in a coordinate fashion to control synthesis of anthocyanins.