Transcriptomic profiling of purple broccoli reveals light-induced anthocyanin biosynthetic signaling and structural genes

Abstract

Purple Broccoli (<i>Brassica oleracea</i> L. var <i>italica</i>) attracts growing attention as a functional food. Its purple coloration is due to high anthocyanin amounts. Light represents a critical parameter affecting anthocyanins biosynthesis. In this study, 'Purple Broccoli', a light-responding pigmentation cultivar, was assessed for exploring the mechanism underlying light-induced anthocyanin biosynthesis by RNA-Seq. Cyanidin, delphinidin and malvidin derivatives were detected in broccoli head samples. Shading assays and RNA-seq analysis identified the flower head as more critical organ compared with leaves. Anthocyanin levels were assessed at 0, 7 and 11 days, respectively, with further valuation by RNA-seq under head-shading and light conditions. RNA sequences were de novo assembled into 50,329 unigenes, of which 38,701 were annotated against four public protein databases. Cluster analysis demonstrated that anthocyanin/phenylpropanoid biosynthesis, photosynthesis, and flavonoid biosynthesis in cluster 8 were the main metabolic pathways regulated by light and had showed associations with flower head growth. A total of 2,400 unigenes showed differential expression between the light and head-shading groups in cluster 8, including 650 co-expressed, 373 specifically expressed under shading conditions and 1,377 specifically expressed under normal light. Digital gene expression (DGE) analysis demonstrated that light perception and the signal transducers <i>CRY3</i> and <i>HY5</i> may control anthocyanin accumulation. Following shading, 15 structural genes involved in anthocyanin biosynthesis were downregulated, including <i>PAL</i>, <i>C4H</i>, 4CL, <i>CHS</i>, <i>CHI</i>, <i>F3H</i> and <i>DFR</i>. Moreover, six BoMYB genes (<i>BoMYB6-1</i>, <i>BoMYB6-2</i>, <i>BoMYB6-3</i>, <i>BoMYB6-4</i>, <i>BoMYBL2-1</i> and <i>BoMYBL2-2</i>) and three BobHLH genes (<i>BoTT8_5-1</i>, <i>BoTT8_5-2</i> and <i>BoEGL5-3</i>) were critical transcription factors controlling anthocyanin accumulation under light conditions. Based on these data, a light-associated anthocyanin biosynthesis pathway in Broccoli was proposed. This information could help improve broccoli properties, providing novel insights into the molecular mechanisms underpinning light-associated anthocyanin production in purple vegetables.