Investigating anthocyanin and sugar development in purple pericarp sweetcorn (Biochemical and Molecular Genetics)

Anirban, Apurba¹; Hong, Hung T.¹; O’Hare, Tim J.^1
1 Queensland Alliance for Agriculture & Food Innovation, The University of Queensland, Queensland 4072, Australia.

Anthocyanins are important plant secondary metabolites that give maize kernels purple/red pigmentation, but also have putative health benefits, including preventing cancers and hypertension. Anthocyanin in maize can be produced in either the kernel aleurone or pericarp. The pericarp is maternal tissue, meaning that all kernels on a cob are therefore similarly coloured, regardless of pollen source. Anthocyanin concentration in pericarp-pigmented kernels is most commonly higher than aleurone-pigmented kernels, as pericarp comprises approximately four layers of cells, as opposed to a single layer for aleurone tissue. Most anthocyanin-pigmented maize is starchy. By contrast, super-sweet sweetcorn, which lacks anthocyanin, has a high sugar content due to the sh2 (shrunken2) mutation, which prevents starch formation. Because of an extremely close genetic linkage (0.1 cM) between a non-functional anthocyanin biosynthesis gene, anthocyaninless-1(a1), and the supersweet mutation, sh2, the development of purple supersweet sweetcorn is challenging. To overcome this, we crossed a white supersweet sweetcorn (a1sh2.a1sh2) with a purple-pericarp Peruvian maize (A1Sh2.A1Sh2). We initially developed F3 heterozygous purple sweetcorn (A1sh2.a1sh2) in a large field experiment by breaking the tight genetic linkage, which was followed by two consecutive field experiments to successfully develop a homozygous purple-pericarp supersweet sweetcorn (A1sh2.A1sh2) accession. Biochemical analysis revealed that the mature purple-pericarp maize parent produced sevenfold more anthocyanin compared to aleurone-pigmented kernels. The developed supersweet lines also produced similar levels of anthocyanin. The principal pigments consisted of, in decreasing order, of cyanidin-, peonidin-, and pelardonidin-based anthocyanins. The sugar content in the developed lines at eating stage was similar to that of the sweetcorn parent, with the main sugar components identified as, in increasing order, of fructose, glucose and sucrose in both the white supersweet parent and the derived purple supersweet accession. We believe this is the first report of a purple-pericarp supersweet sweetcorn, based on the sh2 mutation.