Betalains are health-promoting plant pigments accumulated in non-anthocyanic plants. Owing to its limited distribution in nature, metabolic trade-offs of inhibiting them are poorly understood. The aim of this study was to investigate the effects of inhibition of betalains in Amaranthus tricolor seedlings to gain insights into the relationship of betalain biosynthesis with other biosynthetic pathways in betalain-accumulating plants. Phenylalanine (Phe; 12.5, 25, and 50 mmol L⁻¹) and 3-methyl-2-benzothiazolinone hydrazone (MBTH) (25 mmol L⁻¹) were treated to inhibit betalain biosynthesis in ten-day-old A. tricolor (red) seedlings. After two and six days of treatment, target (betalain-related metabolites) and non-target metabolites were analyzed. In two days, Phe content increased by 2.6, 8.5, and 17.4-fold in Phe (12.5, 25, and 50 mmol L⁻¹)-treated seedlings, respectively, compared to control, indicating the uptake of Phe by the seedlings. Phe treatment led to a 10.1–18% decrease in betacyanins, while MBTH caused an 18.3% decrease in two days. In both treatments, cyclo-DOPA formation, which is essential for betacyanin biosynthesis, seems to be inhibited, albeit through different mechanisms. Betalain biosynthetic precursors and intermediates, viz., tyrosine, L-DOPA, and dopamine decreased differentially. Ascorbic acid, α-tocopherol, and retinol contents increased in both treatments concomitant with the reduction in betalains, total phenols, and antioxidant enzymes. Therefore, Phe treatment is beneficial in enhancing antioxidant metabolites in betalain-accumulating plants. However, the mechanism of increasing ascorbic acid on inhibiting betalains needs further investigation in other betalain-producing plants also to understand if ascorbic acid is involved in regulating betalain biosynthesis.

甜菜碱是在非花青素植物中积累的促进健康的植物色素。由于其在自然界中的分布有限，人们对抑制它们的代谢权衡知之甚少。本研究的目的是研究抑制甜菜红素对三色苋幼苗的影响，以深入了解甜菜红素生物合成与甜菜红素积累植物中其他生物合成途径的关系。苯丙氨酸（Phe；12.5、25 和 50 mmol L ^-1）和 3-甲基-2-苯并噻唑啉酮腙（MBTH）（25 mmol L ^-1）经过处理可抑制 10 日龄三色A. tricolor中甜菜红素的生物合成。红色）幼苗。治疗两天和六天后，对目标代谢物（甜菜素相关代谢物）和非目标代谢物进行分析。两天内，与对照相比，Phe（12.5、25和50 mmol L ^{-1 ）处理的幼苗中的Phe含量分别增加了2.6、8.5和17.4倍，表明幼苗对Phe的吸收。}Phe 处理导致甜菜红素减少 10.1-18%，而 MBTH 则在两天内减少 18.3%。在这两种治疗中，对于甜菜花青素生物合成至关重要的环多巴的形成似乎受到抑制，尽管是通过不同的机制。甜菜碱生物合成前体和中间体，即酪氨酸、L-多巴和多巴胺有差异地减少。两种处理中抗坏血酸、α-生育酚和视黄醇含量均增加，同时甜菜红素、总酚和抗氧化酶减少。因此，Phe 处理有利于增强甜菜碱积累植物的抗氧化代谢。然而，增加抗坏血酸抑制甜菜红素的机制还需要在其他产生甜菜红素的植物中进行进一步研究，以了解抗坏血酸是否参与调节甜菜红素生物合成。