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Published on September 20, 2007; 10.1104/pp.107.107326

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Received August 13, 2007
Accepted September 6, 2007

Early Steps in Proanthocyanidin Biosynthesis in the Model Legume Medicago truncatula

Yongzhen Pang , Gregory J. Peel , Elane Wright , Zengyu Wang , and Richard A. Dixon *

Plant Biology Division and Forage Improvement Division, Samuel Roberts Noble Foundation, 2510 Sam Noble Parkway, Ardmore, OK, 73401, USA

* Corresponding author; email: radixon{at}noble.org.

Oligomeric proanthocyanidins (PAs) composed primarily of epicatechin units accumulate in the seed coats of the model legume Medicago truncatula, reaching maximum levels at around 20 days after pollination. Genes encoding the single Medicago anthocyanidin synthase (ANS, EC 1.14.11.19) and leucoanthocyanidin reductase (LAR, EC 1.17.1.3) were cloned and the corresponding enzymes functionally identified. Recombinant MtANS converted leucocyanidin to cyanidin, and, more efficiently, dihydroquercetin to the flavonol quercetin. Levels of transcripts encoding dihydroflavonol reductase, ANS and anthocyanidin reductase (ANR), the enzyme responsible for conversion of anthocyanidin to (-)-epicatechin, paralleled the accumulation of PAs in developing seeds, whereas LAR transcripts appeared to be more transiently expressed. LAR, ANS and ANR proteins were localized to the cytosol in transfected tobacco leaves. Antisense down-regulation of ANS in M. truncatula resulted in reduced anthocyanin and PA levels, but had no impact on flavonol levels. Transgenic tobacco plants constitutively over-expressing MtLAR showed reduced anthocyanin content, but no catechin or increased levels of PAs were detected either in leaves or flowers. Our results confirm previously ascribed in vivo functions for ANS and ANR. However, the apparent lack of catechin in M. truncatula PAs, the poor correlation between LAR expression and PA accumulation, and the lack of production of catechin monomers or oligomers in transgenic plants over-expressing MtLAR, question the role of MtLAR in PA biosynthesis in Medicago.




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