Biotechnological production and emerging applications of betalains: A review

Betalains are food-grade hydrophilic pigments with antioxidant and biological activities, predominantly found in plants. Betanin is a red-violet betalain synthesized from tyrosine through L-DOPA formation, its subsequent aromatic ring-opening, spontaneous cyclization to betalamic acid, and then pH-dependent condensation with i) cyclo-DOPA-5-O-glucoside or ii) cyclo-DOPA followed by 5-O-glucosylation. This short pathway in plants for betanin biosynthesis has been heterologously expressed in other organisms (e.g. non-betalainic plants, yeasts, and fungi) using CYP76AD1, DOD1, and cDOPA5GT or B5GT, corresponding to the enzymatic steps mentioned above. For the red-violet color formation through heterologous expression of the pathway genes in non-betalainic plants, a simplified reporter gene called RUBY has been developed recently. Without any systems engineering, expression of RUBY in non-betalainic plants resulted in accumulation of up to 203 mg betalains/100 g fresh weight of peanut leaves. In yeasts, Saccharomyces cerevisiae and Yarrowia lipolytica, and fungus Fusarium venenatum, betanin production has been achieved through overexpression of the pathway genes, with productivity reaching up to 0.62 mg/L/h, 26 mg/L/h, and 26.4 mg/L/h from D-glucose as carbon source, respectively, after considerable systems engineering and gene copy number augmentation. This review critically analyzes recent biotechnological production of betalains to highlight the advancements and strategies for improvement in the technology. Also, emerging applications of betalain biosynthetic gene products or betalains as biosensors, fluorescent probes, meat analog colors, and others are discussed to strengthen the need for systems engineering and process optimization for large-scale industrial production of these pigments.