Apple B-box factors regulate light-responsive anthocyanin biosynthesis genes

Abstract

Environmentally-responsive genes can affect fruit red colour via the activation of MYB transcription factors. The apple B-box (BBX) gene, BBX33/CONSTANS-like 11 (COL11) has been reported to influence apple red-skin colour in a light- and temperature-dependent manner. To further understand the role of apple BBX genes, other members of the BBX family were examined for effects on colour regulation. Expression of 23 BBX genes in apple skin was analysed during fruit development. We investigated the diurnal rhythm of expression of the BBX genes, the anthocyanin biosynthetic genes and a MYB activator, MYB10. Transactivation assays on the MYB10 promoter, showed that BBX proteins 1, 17, 15, 35, 51, and 54 were able to directly function as activators. Using truncated versions of the MYB10 promoter, a key region was identified for activation by BBX1. BBX1 enhanced the activation of MYB10 and MdbHLH3 on the promoter of the anthocyanin biosynthetic gene DFR. In transformed apple lines, over-expression of BBX1 reduced internal ethylene content and altered both cyanidin concentration and associated gene expression. We propose that, along with environmental signals, the control of MYB10 expression by BBXs in ‘Royal Gala’ fruit involves the integration of the expression of multiple BBXs to regulate fruit colour.

MYB-Mediated Regulation of Anthocyanin Biosynthesis

“…Leucine Zip TF elongated hypocotyl 5 (HY5) is part of the light signaling pathway downstream of PHY, CRY and UV-B photoreceptors, which regulating the regulatory network of PAP1 activates anthocyanin biosynthesis [ 24 ]. In Arabidopsis , hy5 regulated pap1 expression by directly combining G-box and ACE elements, while pap1 acted on the promoters of EBGs and LBGs to induce the downstream network activation of anthocyanins [ 24 , 160 ]. PAP1 expression was highly dependent on the presence of the HY5 TF, and this optical signaling pathway was affected by photosynthesis [ 24 ].…”

“…MYBx repressors inhibit the formation of the MBW complex by binding to bHLH proteins, preventing the activation of the target gene DFR [ 44 ]. When the mature flowers and fruits of Petunia are photoinduced, the light signaling pathway activates R2R3-MYBs and MBW (including bHLH1) to form an activation complex, inducing the expression of bHLH [ 103 , 106 , 160 ]. Consequent bHLH2 forms an MBW complex with the activator and WDR protein and acts on the promoter of the late acting anthocyanin structural gene DFR to regulate the downstream network of anthocyanin biosynthesis [ 160 ].…”

“…When the mature flowers and fruits of Petunia are photoinduced, the light signaling pathway activates R2R3-MYBs and MBW (including bHLH1) to form an activation complex, inducing the expression of bHLH [ 103 , 106 , 160 ]. Consequent bHLH2 forms an MBW complex with the activator and WDR protein and acts on the promoter of the late acting anthocyanin structural gene DFR to regulate the downstream network of anthocyanin biosynthesis [ 160 ]. PhMYBx repressors are also activated by MBW activating complexes, competing with activators for target gene sites for feedback inhibition [ 44 ].…”

B‐box proteins: Pivotal players in light‐mediated development in plants

“…Anthocyanin accumulation in fruit peels, which is affected predominantly by light signals, is extremely critical for the visual quality of fruit. In apple, MdBBX1, MdBBX17, MdBBX15, MdBBX35, MdBBX51, and MdBBX54 can directly associate with MdMYB10 promoter regions to activate its transcription, which in turn promotes light-dependent anthocyanin accumulation (Plunkett et al 2019). MdBBX37 is associated with MdMYB1 and MdMYB9 and represses their ability to bind downstream targets, thereby inhibiting anthocyanin biosynthesis (An et al 2019a).…”

“…Heterologous expression of AtBBX21 through a transgenic approach was found to increase chlorophyll and anthocyanin contents in potato plants (Crocco et al 2018). Furthermore, alternation of the transcription of several MdBBXs and PpBBXs led to significant changes in anthocyanin contents in the peels of apple or pear, respectively (An et al 2019a(An et al , 2019bBai et al 2019cBai et al , 2019dFang et al 2019;Plunkett et al 2019). In tomato, overexpression of SlBBX20 caused elevated levels of carotenoids (Xiong et al 2019).…”