Sn, a maize bHLH gene, transactivates anthocyanin and condensed tannin pathways in Lotus corniculatus

Anthocyanins and condensed tannins are major flavonoid end products in higher plants. While the transactivation of anthocyanins by myc transcription factors is well documented very little is known about the transregulation of the pathway to condensed tannins. The present study analyses the effect of over-expressing an Sn transgene in Lotus corniculatus, a model legume which biosynthesises both anthocyanin and tannin end products. Contrary to expectation, effects on anthocyanin accumulation were subtle and restricted to the leaf midrib, leaf base and petiole tissues. However the accumulation of condensed tannin polymers was dramatically enhanced in the leaf blade and this increase was accompanied by a 50-fold increase in the number of tannin-containing cells in this tissue. A detailed analysis of selected lines indicated that this transactivational phenotype correlated with high steady state transcript levels of the introduced transgene and the introduction of a single copy of the CaMV35S-Sn construct into these clonal genotypes. While the levels of condensed tannins in leaves were increased by up to 1% of the dry weight, other major secondary end products (flavonols, lignins and inducible phytoalexins) were unaltered in transactivated lines. These results give us an initial insight into the developmental and higher-order regulation of polyphenolic metabolism in Lotus and other higher plant species.