Histochemical screening of 30 Rosaceae genera representing all classic subfamilies demonstrated flavan-3-ols (catechins) as general secondary metabolites in roots of Rosaceae. Semi-quantitative LC-MS analyses confirmed the presence of catechin, epicatechin and various dimeric flavan-3-ols (also representing higher polymeric proanthocyanidins) as prominent polyphenols in root tips of Fragaria (strawberry), Malus (apple), Rosa (rose), Pyrus (pear) and Prunus (plum). Distinct patterns of flavan-3-ol distribution at the cellular level were found in strawberry (Fragaria × ananassa) and apple (Malus × domestica) root tips. The calyptras (root caps) showed the most prominent flavan-3-ol staining for these two genera. Border cells of Fragaria and Malus, as first demonstrated here for Rosaceae, were also found to contain flavan-3-ols. Transcript analyses with cDNA demonstrated root expression of known flavonoid genes expressed in the respective fruits and leaves. Primarily, this proves in situ biosynthesis of flavan-3-ols in these roots. Knowledge of the distinct cellular distribution patterns and their in situ biosynthesis in roots provides a basis for analysis of the functional roles of Rosaceae root flavan-3-ols.

Flavonoids are a large family of polyphenolic compounds with manifold functions in plants including pathogen defence. Present in a wide range of vegetables and fruits, flavonoids form an integral part of the human diet and confer multiple health benefits. Modifying flavonoid biosynthesis in fruit crops, such as apple, offers the opportunity to increase plant resistance against pathogens and the health benefit potential of the fruit. Both overexpression and RNAi-based suppression strategies were used to modify flavonoid biosynthesis in apple. Introducing the maize Lc transcription factor gene, responsible for controlling the expression of structural genes of the flavonoid biosynthetic pathway in maize, into Malus domestica Borkh. cv. 'Holsteiner Cox' resulted in increased mRNA levels for most of the structural genes of the flavonoid pathway. Lc transgenic plants accumulated higher levels of the anthocyanin idaein, the flavan-3-ols epicatechin, catechin, and some distinct dimeric proanthocyanidins. In a second approach, the consequences of RNAi silencing of the genes for anthocyanidin synthase (ANS) and UDPgalactose: flavonoid 3-O-galactosyltransferase (FGT) gene to induce a shift towards flavan-3-ols were examined. Preliminary results obtained from transgenic plants of the cv. 'Holsteiner Cox' and of the red-leaved type TNR 31-35 (a descendent from Malus sieversii var. sieversii f. niedzwetzkyana (Dieck)) revealed that suppression of ANS or FGT resulted in increased amounts of flavan-3-ols and flavonols. In the red leaved type TNR 31-35, anthocyanin accumulation decreased after ANS silencing.