## 1 Introduction  
Broccoli and other Brassica vegetables, with a global production of 71.7 million tons, have attracted increasing attention in the context of healthy diets in recent years [1]. Most cruciferous vegetables are green because of their association with photosynthesis. However, in certain other species, red pigments can persist throughout the life span of the leaves, or they may be induced and retained only after the plant has experienced stress [2, 3]. Broccoli (_Brassica oleracea_ var. _italica_) has a traditional, dark bluish-green head, some of which can develop a slight purple pigmentation at low temperatures. Green broccoli is rich in dietary fiber, vitamins, and minerals and contains valuable amounts of bioactive compounds such as sulforphane, glucosinolates, glycosylated flavonoids, and vitamin C [4, 5]. Purple broccoli was less well-known in marketplaces until [PERSON] and [PERSON] introduced the seeds of a cold-tolerant and better-tasting cultivar for commercialization in the early 1980s [6]. Purple broccoli is gaining popularity as a gourmet produce item and has similar morphology to regular broccoli, except that its florets have a deep purple tinge and the heads are typically smaller. It has been demonstrated that purple broccoli contains higher nutrition levels of antioxidant compounds, vitamin C, phenolic compounds, and thioglucosides but has lower productivity [7; 8; 9]. The purple color of broccoli is due to the accumulation of anthocyanins [10; 11]. Anthocyanins are water-soluble pigments. They not only provide plants with a range of colors, such as pink, red, purple, and blue, for attracting animal pollinators [12; 13] but also play important roles in protecting plants against biotic and abiotic stresses, such as UV light, pests, and diseases [14]. For humans, the antioxidant function of anthocyanins allows them to serve as strong free radical scavengers against many chronic health issues related to aging, mutagenesis, and cardiovascular diseases [11; 15]. Purple vegetables, including purple-leaved sweet potato, heartleaf houttuynia, purple-leaved perilla line 1, and bicolored-leaved perilla line 2, can protect lymphocyte DNA against oxidative damage due to their strong antioxidant activities [16]. A study showed that red kale extracts at a concentration of 2500 \(\mathrm{\SIUnitSymbolMicro g}\)/mL were effective in inhibiting cancer cell growth and increasing the death rate of cancer cells [17]. More than 20 naturally occurring anthocyanins have been identified, and only six are commonly found in higher plants: cyanidin, delphinidin, pelargonidin, peonidin, petunidin, and malvidin [18]. Studies have reported that the main component of anthocyanin in purple _Brassica_ vegetables is cyanidin, with small amounts of delphinidin and pelargonidin [11]. The biosynthesis and regulatory mechanisms of anthocyanins have been reported in a variety of model plants, such as _Arabidopsis_[19], maize [20], petunia [21], and grapes [22], as well as many other cruciferous plants, including oilseed rape [23], radish [24], kale [25], cabbage [26], and cauliflower [27]. The genes involved in anthocyanin accumulation are divided into two categories: regulatory genes and structural genes [28]. The regulatory genes mainly consist of three major classes of transcription factors (R2R3-MYB, bHLH, and WD40), which directly affect the transcription and expression of structural genes through the formation of the MYB-bHLH complex or MYB-bHLH-WD40 complex (MBW complex). In _Brassica oleracea_, the anthocyanin biosynthetic pathway is controlled by the R2R3 MYB transcription factor _BoMYB2_. Studies on transgenic plants have also shown that the _BoMYB2_ gene serves as a key regulator of anthocyanin accumulation in flower coloration through interaction with BobHLH and BoWD40 proteins and binding to the promoter of late anthocyanin biosynthesis genes [29; 30; 31; 25]. Structural genes encode enzymes required by the anthocyanin synthesis pathway.