biomolecules Review Recent Insights into Anthocyanin Pigmentation, Synthesis, Trafficking, and Regulatory Mechanisms in Rice (Oryza sativa L.) Caryopsis Enerand Mackon 1 , Guibeline Charlie Jeazet Dongho Epse Mackon 1 , Yafei Ma 1 , Muhammad Haneef Kashif 2 , Niyaz Ali 3 , Babar Usman 1 and Piqing Liu 1, *   Citation: Mackon, E.; Jeazet Dongho Epse Mackon, G.C.; Ma, Y.; Haneef Kashif, M.; Ali, N.; Usman, B.; Liu, P. Recent Insights into Anthocyanin Pigmentation, Synthesis, Trafficking, and Regulatory Mechanisms in Rice (Oryza sativa L.) Caryopsis. Biomolecules 2021, 11, 394. https:// doi.org/10.3390/biom11030394 Academic Editors: Roque Bru-Martinez and Carmen Socaciu Received: 20 December 2020 Accepted: 1 March 2021 Published: 7 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 State Key Laboratory of Conservation and Utilization of Subtropical Agro-Bioresources, College of Agriculture, Guangxi University, Nanning 530005, China; breedermackon@st.gxu.edu.cn (E.M.); msmackon@st.gxu.edu.cn (G.C.J.D.E.M.); mayafei@st.gxu.edu.cn (Y.M.); babarusman119@gmail.com (B.U.) 2 Key Laboratory of Plant Genetics and Breeding, College of Agriculture, Guangxi University, Nanning 530005, China; tipukashif@st.gxu.edu.cn 3 State Key Laboratory for Conservation and Utilization of Subtropical Agro-Bio-Resources, College of Life Science and Technology, Guangxi University, Nanning 530004, China; niyazali@st.gxu.edu.cn * Correspondence: liupq@gxu.edu.cn; Tel.: +86-133-9770-0471 Abstract: Anthocyanins are antioxidants used as natural colorants and are beneficial to human health. Anthocyanins contribute to reactive oxygen species detoxification and sustain plant growth and development under different environmental stresses. They are phenolic compounds that are broadly distributed in nature and are responsible for a wide range of attractive coloration in many plant organs. Anthocyanins are found in various parts of plants such as flowers, leaves, stems, shoots, and grains. Considering their nutritional and health attributes, anthocyanin-enriched rice or pigmented rice cultivars are a possible alternative to reduce malnutrition around the globe. Anthocyanin biosynthesis and storage in rice are complex processes in which several structural and regulatory genes are involved. In recent years, significant progress has been achieved in the molecular and genetic mechanism of anthocyanins, and their synthesis is of great interest to researchers and the scientific community. However, limited studies have reported anthocyanin synthesis, transportation, and environmental conditions that can hinder anthocyanin production in rice. Rice is a staple food around the globe, and further research on anthocyanin in rice warrants more attention. In this review, metabolic and pre-biotic activities, the underlying transportation, and storage mechanisms of anthocyanins in rice are discussed in detail. This review provides potential information for the food industry and clues for rice breeding and genetic engineering of rice. Keywords: antioxidant; anthocyanin; cyanidin-3-glucoside; black rice; transcription factor; antho- cyanin vacuolar intrusion; transporters; MBW complex 1. Introduction Anthocyanins are water-soluble plant pigments, responsible for a wide range of at- tractive colors in leaves, fruits, grains, and flowers [1], with most colors being red, blue, purple, and dark purple [2]. They belong to the group of phenolic compounds derived from flavonoids which play an important biological role in plants [3]. The pigments are glycosylated (aglycone), methylated or acylated forms of anthocyanidin [4,5]. Thus far, four putative functions of anthocyanins have been reported: (1) reactive oxygen species (ROS) mediators, (2) strong antioxidants, (3) chelating agents for metalloids and metals, and (4) sunscreen and promoter of leaf turgor, mainly during nutrient shortage [6]. Some scien- tific studies such as animal models and human clinical trials revealed that anthocyanins have potential antioxidant and antimicrobial properties, improve visual and neurological health, and protect against various non-communicable diseases [5]. Consequently, they are considered health-promoting compounds [7] and may play a fundamental role in Biomolecules 2021, 11, 394. https://doi.org/10.3390/biom11030394 https://www.mdpi.com/journal/biomolecules