Transcriptomic profiling reveals pigment regulation during peanut testa development

Although peanut (Arachis hypogaea L.) is one of the most important edible oil crops globally, pigments present in the testa influence both the processing efficiency and the quality of the oil. In peanut, polymeric phenolic compounds are present in the episperm rather than in the endothelium and their levels increase during ripening; therefore, to better understand testa development, and especially the accumulation of pigments, RNA-Seq was applied to elucidate the mechanisms underlying the regulation of peanut testae at three different developmental stages (i.e., at 20 days after flowering - 20DAF - and at 40DAF and 60DAF). A total of 5452 differentially expressed unigenes (DEGs) were obtained encompassing these three stages; comparative results showed that phenylpropanoid biosynthesis, phenylalanine metabolism, flavonoid biosynthesis, and plant hormone signal transduction comprised the principal KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways expressed during peanut testa development. Further studies revealed that the expression patterns of the flavonoid biosynthesis pathway genes PAL, C4H, CHS, and CHI (early biosynthetic genes - EBGs) were consistent with the accumulation of testa pigments. Thus, the results of this study demonstrate that EBGs, as well as the homologs of AtMYB111 (i.e., c35101_g4 and c37398_g2), are likely the principal regulators of testa pigment accumulation; the gene database assembled here is therefore a sequencing resource for future research and provides a foundation for understanding the regulation of pink testa pigmentation in peanuts.