MdGSTF6, activated by MdMYB1, plays an essential role in anthocyanin accumulation in apple

Anthocyanins are biosynthesized on the cytosolic surface of the endoplasmic reticulum and then transported into the vacuole for storage. Glutathione S-transferases (GSTs) are considered to be responsible for the transport of anthocyanins into the vacuole. However, the regulatory mechanisms of GSTs in plants are still unclear. Here, we performed a genome-wide analysis and identified 69 GST genes in apple. The expression of MdGSTF6 was positively correlated with the anthocyanin content (r = 0.949) during ‘Yanfu 8’ fruit development. The overexpression of MdGSTF6 in the Arabidopsis thaliana tt19 mutant resulted in seedlings of 35S::MdGSTF6-GFP/tt19 that could accumulate anthocyanin and rescue its phenotype, suggesting that MdGSTF6 was an anthocyanin transporter. The silencing of MdGSTF6 affected anthocyanin accumulation in apple fruit. Moreover, the knockdown of MdGSTF6 by RNA interference in cultured ‘Gala’ seedlings inhibited anthocyanin accumulation. The interaction experiments showed that MdMYB1 could bind directly to the MdGSTF6 promoter to transcriptionally activate its expression. Collectively, our results demonstrate that MdGSTF6 encodes an important GST transporter of anthocyanins in apple fruit and provide evidence for the associated regulatory mechanisms. Therefore, MdMYB1 can not only regulate anthocyanin synthesis, but also control the transport of anthocyanin in apples. This information may be useful for further clarifying the regulation of anthocyanin transport in apple.

Intracellular transport: Identifying how apple pigment is sent for storage The mystery of how pigmenting anthocyanins are transported to intracellular storage is starting to be revealed. Alongside giving fruit their color, anthocyanins confer benefits to plant and human health. Glutathione S-transferases (GSTs) are responsible for transporting anthocyanins to storage vacuoles, yet research is lacking on how GSTs are regulated. Xuesen Chen, from China’s Shandong Agricultural University, and his team analyzed the activity of 23 GST genes in apple, finding one, MdGSTF6, was most highly active in the fruit-coloration stage of apple development. MdGSTF6 expression correlated with fruit anthocyanin levels and also restored anthocyanin levels in transport-suppressed plants, confirming its function. Chen’s team then discovered that MdGSTF6 expression is activated by the protein MdMYB1. The MdMYB1 gene is the major regulatory gene in anthocyanin synthesis, and is now revealed to also influence their transport.