The transcriptional regulation of plant pigments
The transcriptional regulation of anthocyanins in fruit has been well documented: at its core is a TF complex, termed the MYB-bHLH-WD40 (MBW) complex (Allan, Hellens & Laing 2008). The MYB component is derived from the subgroup (SG) 6 class of MYBs (as defined by Stracke et al. (2001) as opposed to the SG5 MYBs, which control proanthocyanidins) and has been characterised in many fruit species (Albert et al. 2014; Jaakola 2013). The MYBs are expressed in ripening fruit and activate either the whole biosynthetic pathway or a subset of genes in the pathway. Examples of activators include FaMYB10 and FvMYB10 in strawberry (Lin-Wang et al. 2014; Medina-Puche et al.2014), MYB10 in nectarine (Prunus persica ) (Ravaglia et al. 2013), VmMYBA in bilberry (Vaccinium myrtillus ) (Karppinenet al. 2021), PpMYB10.1 in peach (Prunus persica ) (Rahim, Busatto & Trainotti 2014), VcMYBA1 in blueberry (VacciniumSection Cyanococcus ) (Plunkett et al. 2018) and MYB114 in pear (Pyrus bretschneideri ) (Yao et al. 2017). MYBs can also act as repressors of anthocyanin, such as the SG4 MYBs, FaMYB1 from strawberry (Aharoni et al. 2001) and MdMYB16 from apple (Malus domestica ) (Xu et al. 2017), as recently reviewed by LaFountain & Yuan (2021). There is increasing evidence for the interactions of different types of MYBs on anthocyanin regulation, such as the co-ordinated actions of MYBA1 and MYBPA1.1 in blueberry (Laffertyet al. 2021). Transcriptional regulation of anthocyanin is not restricted to the MBW complex: other TFs, such as MADs VmTRD4 in bilberry (Jaakola et al. 2010) and PpNAC1 in peach (Zhou et al. 2015) have been shown to have regulatory control. This TF-mediated control of pigments occurs in response to both developmental cues, but also environmental stresses.
Carotenoid biosynthesis is stimulated by the presence of ROS (Panet al. 2009). Stress perceived in the leaves can trigger responses in the fruit, for example, the incidence of high light in leaves of citrus leads to increases in carotenoids in the fruit (Poiroux‐Gonord et al. 2013). Many TFs have been associated with carotenoid biosynthesis as part of the fruit ripening cascade, but the evidence for there being master transcriptional regulators of carotenoid pigments is less clear cut than for anthocyanins (Stanley & Yuan 2019). A number of TFs have recently been shown to control at least parts of the pathway. In citrus, the AGAMOUS-LIKE MADS TF, CsMADS6, was shown to activate the promoter of LCYb1 and drive the expression of carotenoid pathway genes PSY , PDS and CCD1, while repressing transcription of LCYE and so increasing the flux towards β-carotene (Lu et al. 2018). Another TF in citrus, CrMYB68, negatively regulates the expression of beta-carotene hydroxylase (BCH), controlling the α- and β-branches of carotenoid biosynthesis (Zhuet al. 2017). A MYB from kiwifruit (Actinidia chinensis var. deliciosa ), MYB7, was reported to activate the promoter ofLCYb and in heterologous systems elevated expression ofPSY , PDS and ZDS to drive carotenoid accumulation (Ampomah-Dwamena et al. 2019). In papaya, two other TF classes have been implicated in regulating carotenoids: CpbHLH1 and CpbHLH2 repress and activate respectively the promoters of CYCB and LCYB, while the NAC TFs, CpNAC1 and CpNAC2, bind the promoters of PDS (NAC1) and PDS, ZDS, LYCE and BCH (NAC2) (Fu et al. 2016; 2017). The greatest body of work on the transcriptional control of carotenoids has been carried out in tomato where the major ripening MADS TFs, Tomato AGAMOUS-LIKE (TAGL1, homologous to CsMADS6), Ripening Inhibitor (RIN), FRUITFULL1 (FUL1) and FUL2 exert largely positive effects on the carotenoid pathway, as reviewed in Stanley & Yuan (2019).