## 1 Introduction  
Terrestrial plants survive diverse stresses, including droughts, low temperatures, salinity, UV-B, etc., and they face more challenges due to frequent abnormal climate changes [1, 2, 3, 4, 5, 6]. To defend and endure the stressful environments, they can produce plenty of protective phytochemicals, such as polyphenolic compounds, saponins, monoterpens, and phytates [7, 8, 9, 10, 11]. This is a universal phenomenon that plants produce complex secondary metabolite mixtures in response to various stresses, exhibiting the phytochemical diversity [12]. As the most famous and abundant protective phytochemicals, flavonoids perform multiple physiological functions in plant growth and in response to various stresses, including regulating plant hormone signals, antioxidation, scavenging free radicals, and participating in vital life processes such as photosynthesis, morphogenesis, and chemical signaling [13; 14; 15]. In addition, flavonoids are the largest class of polyphenols discovered to date, and are estimated to encompass over 8000 metabolites with a common diphenylpropane A skeleton structure (C6-C3-C6) [16]. This structure consists of two aromatic rings connected by a three-carbon chain and is classified into seven subclasses based on the degree of oxidation of the central ring: flavanols, anthocyanins, flavanones, flavonols, isoflaves, flavones, and chalcones [17]. The biosynthesis of flavonoids in plants occurs at the intersection of the acetic acid and shikimic acid pathways (polyketic pathway), which initiates from the general phenylpropane pathway (GPP). Subsequently, a series of consecutive enzymes, including phenylalanine ammonia lyase (PAL), cinnamate-4-hydroxylase (C4H), 4-cumarate-CoA ligase (4 CL), and chalcone synthase (CHS), synthesize various stable flavonoids [17]. Previous research has indicated that the levels of hormones within plants can be influenced by various stressors, thereby leading to the expression of hormone-related regulatory genes, and regulating the important genes of the biosynthetic pathway of the flavonoids [15; 18; 19]. The MYB-bHLH-WDR (MBW) tertiary complex was widely identified, which is composed of three transcription factor families (MYB, bHLH, and WD40) that regulate flavonoid synthesis-related genes [20; 21; 22; 23]. Research on the flowers of _Pyrus hopiensis_ demonstrated that _bHLH_ and _MYB_ regulate the expression of genes associated with flavonoid synthesis, thereby influencing flavonoid biosynthesis and responding to low-temperature stress [24]. The study of _Tetrastigma hemselayanum_ seedlings revealed that the expression of _PAL_, _CL_, _CHS_, _ANR_, _FLS_, and _LAR_ was up-regulated under cold stress, resulting in a significant increase in key flavonoids such as catechins, epicatechins, rutin, and quercetin [25]. In _Tagopyrum tatarium_ (_F. tatarium_) hairy roots, the biosynthesis of rutin was directly repressed by JA-responsive TFs, such as _FHMYB13_, _FHMYB14_, _FHMYB15_, and _FtMYB16_[26], and rutin synthesis in the sprouts could be up-regulated to a higher-level under cold stress [27]. In tobacco plants, overexpression of _CHS_ could induce flavonoid accumulation in tobacco leaves (rutin, quercetin, kaempfer-3-rutinoside, and kaempfer-glucopyranoside), and improve drought resistance [28]. Research on _Camelia sinensis_ has found that UV-B could induce the activation of _MYB12_ which, when combined with the promoters of flavonoid biosynthesis genes (_CsFLS_, _CsLAR_, and _CsDFR_), leads to changes in flavonoid content in the tea leaves [29]. In a word, the biosynthesis of flavonoids was largely triggered across different economic plant species under various stresses, with tissue- and species-specific regulations. As desert plants survive prolonged exposure to multiple stressful environments, they could evolve specific regulation mechanisms in flavonoid biosynthesis.