Anthocyanins are a class of water-soluble flavonoids present in fruits, flowers, and vegetative organs of plants which have antioxidant activities in protecting plants under abiotic stress. Anthocyanidins are synthesized on the cytoplasmic surface of the endoplasmic reticulum and further undergo various modifications such as methylation, glycosylation, hydroxylation, and acylation in the endoplasmic reticulum. Afterward, anthocyanins (the forms of anthocyanidin glycosides and acylated anthocyanins) enter the vacuoles to store and accumulate with the assistance of transporters and transport vesicles
[6]. Anthocyanins not only determine the blue, red, and purple pigments of plants for attracting pollinators but also play roles in various biotic and abiotic stresses. In addition to physiological roles for plants, anthocyanins also have potential benefits for human health, such as decreasing the risk of heart disease, diabetes, cardiovascular disease, and metabolic diseases
[7][8][9][10]. The anthocyanin biosynthetic pathway has been elucidated and most of the regulatory genes involved in anthocyanin biosynthesis have been identified
[11][12][13]. The enzymes involved in anthocyanin biosynthesis include CHS (chalcone synthase), CHI (chalcone isomerase), F3H (flavanone 3-hydroxylase), F3′H (flavonoid 3′-hydroxylase), F3′5′H (flavonoid 3′,5′-hydroxylase) which correspond to early biosynthetic genes (EBGs), and DFR (dihydroflavonol 4-reductase), ANS (anthocyanin synthase), OMT (O-methyltransferase) and UFGT (UDP flavonoid glucosyltransferase) which correspond to late biosynthetic genes (LBGs)
[14]. Moreover, the anthocyanin biosynthesis is regulated by the MYB–bHLH–WD40 (MBW) protein complex, which is composed of MYB, bHLH transcription factors, and a WD40 protein
[15][16]. In addition to the MBW complex, PybHLH3-PyMYB114-PyERF3
[17] transcription complex in pear and WRKY
[18][19], NAC
[20], MADS
[21], HY5
[22], BBX
[23], bZIP
[24], SPL
[25] regulatory factors in various fruit crops are also involved in the regulation of anthocyanin biosynthesis (
Figure 1). Recently, a great number of studies have revealed that anthocyanins increasingly accumulate when plants are under environmental stress. In
Arabidopsis, the low nitrogen (N)-induced anthocyanin accumulation plays a substantial role in plant tolerance to low N stress
[26]. In addition, the flavonoid biosynthesis and accumulation in
Arabidopsis improves salt resistance under salt stress
[27]. In
Cymbidium hybrid flowers, anthocyanin pigmentation has been demonstrated to be organ-specific and temperature-dependent synthesized
[28]. Moreover, anthocyanin accumulation is related to salt stress response in MdZAT5-overexpressing apple Calli and
Arabidopsis [29]. Furthermore, the anthocyanin accumulation in
AN1-overexpressing tobacco plants has a higher drought tolerance compared to the wild-type plants
[30]. Also, the overexpression of
UGTs enhanced plant tolerance to low temperatures, drought, and salt stresses by modulating the anthocyanin accumulation
[31]. Under abiotic stresses, a reduction in electron transport in the Calvin cycle and a higher electron leakage during photosynthesis in the Mehler reaction of cells lead to the plants producing extensive reactive oxygen species (ROS) including O
2−, H
2O
2, OH
-, and
1O
2 [32], which cause oxidative damage to plants and can be used as signaling molecules to activate stress-tolerance mechanisms. Anthocyanin can directly scavenge active oxygen species, such as singlet oxygen, superoxide, hydrogen peroxide, hydroxyl, and peroxyl radicals
[33]. Consequently, the anthocyanins accumulate in plants after ROS signals induce the transcription of anthocyanin biosynthesis pathway genes to scavenge excess ROS and avoid oxidative damage
[34][35]. Therefore, ROS as signal factors in the response to plant abiotic stresses activate the transcription of anthocyanin biosynthetic genes to produce anthocyanins for stress tolerance.