Understanding the transcriptional regulatory mechanisms controlling fruit flesh pigmentation in Solanum muricatum

Abstract

Anthocyanins are pigmented molecules that contribute to the red-blue colour seen in many plants, which have nutritional benefits for consumers. Anthocyanins are commonly in fruit skin, as well as other plant tissues, while less often found in the flesh of fruits. Anthocyanins are present in many important crop species of the Solanaceae plant family, including potato and tomato. Some related ancestral species also accumulate anthocyanin to varying degrees, including the orphan crop Solanum muricatum (pepino). Pepino presents high phenotypic variation of pigment in the fruit skin, and fruits with purple, yellow and green pigmented skin exist. Exploration into the differences in genetic regulatory mechanisms responsible for the varied pigmented fruit skin found in different pepino cultivars has the potential to discover new ways to enhance pigmentation in fruit flesh. Previous research into potato and tomato, closely related to pepino, indicates possible regulatory mechanisms for anthocyanin biosynthesis. MYB proteins are a family of transcription factors involved in the transcriptional regulation of genes. In various Solanaceous species, MYBs have been identified as regulators of anthocyanin biosynthetic pathway genes, either by enhancing or inhibiting expression. Based on conserved homology between Solanaceae species, this study aimed to decipher the transcriptional regulatory mechanisms of activation and repression of anthocyanin accumulation in pepino. Bioinformatic analysis of the pepino transcriptome was able to identify a homolog of potato StAN1, SmuAN1, encoding for an R2R3-MYB enhancer MYB. Also isolated was as an R3-MYB repressor transcription factor, SmuATV. The functionality of SmuAN1 was enhanced upon the formation of an MBW-complex, a ternary complex additionally consisting of a bHLH protein with a WD-40 protein. A homolog of potato bHLH1 was also identified and functionally characterized in pepino, and was required for anthocyanin biosynthesis. Understanding the transcriptional regulatory mechanisms underlying anthocyanin biosynthesis could lead to the development of plant products with upregulated anthocyanin production in flesh tissue, substantially increasing the nutritional value, and economic potential of the orphan crop.