An unconventional proanthocyanidin pathway in maize

Maize has an unconventional proanthocyanidin (PA) biosynthetic pathway which produces rare stereoisomers of PA monomers and dimers, highlighting an opportunity for further improving the nutritional value of maize. Proanthocyanidins (PAs), flavonoid polymers involved in plant defense, are also beneficial to human health and ruminant nutrition. To date, there is little evidence for accumulation of PAs in maize (Zea mays), although maize makes anthocyanins and possesses the key enzyme of the PA pathway, anthocyanidin reductase (ANR). Here, we explore whether there is a functional PA biosynthesis pathway in maize using a combination of analytical chemistry and genetic approaches. The endogenous PA biosynthetic machinery in maize preferentially produces the unusual PA precursor (+)-epicatechin, as well as 4 & beta;-(S-cysteinyl)-catechin, as potential PA starter and extension units. Uncommon procyanidin dimers with (+)-epicatechin as starter unit are also found. Expression of soybean (Glycine max) anthocyanidin reductase 1 (ANR1) in maize seeds increases the levels of 4 & beta;-(S-cysteinyl)-epicatechin and procyanidin dimers mainly using (-)-epicatechin as starter units. Introducing a Sorghum bicolor transcription factor (SbTT2) specifically regulating PA biosynthesis into a maize inbred deficient in anthocyanin biosynthesis activates both anthocyanin and PA biosynthesis pathways, suggesting conservation of the PA regulatory machinery across species. Our data support the divergence of PA biosynthesis across plant species and offer perspectives for future agricultrural applications in maize.

Automated summary

Maize possesses a unique proanthocyanidin (PA) biosynthetic pathway that produces rare stereoisomers, offering opportunities to enhance its nutritional value. Previous evidence of PA accumulation in maize has been limited, despite the presence of the key enzyme anthocyanidin reductase (ANR) involved in PA synthesis. In this study, a combination of analytical chemistry and genetic approaches confirmed the presence of a functional PA biosynthesis pathway in maize. Introduction of soybean ANR1 into maize seeds increased the levels of specific PA components, and the activation of PA biosynthesis in a maize inbred deficient in anthocyanin synthesis was achieved by integrating a Sorghum bicolor transcription factor (SbTT2) that regulates PA production. These findings highlight the potential for agricultural applications and the divergence of PA biosynthesis across plant species.