The Subtleties of Subcellular Distribution

Crosstalk between Flavonoids and the Plant Circadian Clock

“…While the primary site of flavonoid metabolism in cells has long been held to be the cytoplasmic face of the endoplasmic reticulum, a number of studies in recent years point to the nucleus as another site of active flavonoid biosynthesis as well as potential moonlighting roles for the enzymes (Winkel, 2019; Zhang et al, 2020). This raises the possibility that flavonoid enzymes or end products may directly or indirectly influence gene expression.…”

“…Despite decades of research on the physiological effects of flavonoids in plants, understanding of the underlying modes of action remains remarkably murky (Winkel, 2019; Agati et al, 2020). Flavonoid bioactivity has invariably been ascribed to the strong antioxidant potential of these compounds, which can be present at quite high levels in certain tissues and in response to diverse biotic and abiotic stresses.…”

“…In fact, there has been substantial debate regarding the role of flavonoids as antioxidants in plants, including from the viewpoint that intracellular concentrations vary tremendously across compartments, with the majority of flavonoids localized within the vacuole, the cell wall, and in at least some cases the nucleus (Winkel, 2019; Agati et al, 2020). There has been little exploration of alternative explanations for flavonoid action in plants, such as modulation of the activities of enzymes or signaling molecules, despite long-standing evidence of their potent and specific protein-binding potential (Gebicka, 2020).…”

“…This enzyme complex is localized, in part, at the cytoplasmic face of the endoplasmic reticulum, with the resulting end products (flavonols, anthocyanins, and proanthocyanidins in Arabidopsis) transported primarily to the vacuole and cell wall (Agati et al, 2012; Zhao, 2015). Flavonoid enzymes have also been reported to localize to the nucleus in numerous species, where they may be responsible for the synthesis of flavonoids in situ (Winkel, 2019; Zhang et al, 2020). These proteins may also have alternative functions, as suggested by our recent report that CHS interacts with MOS9, a nuclear protein required for defense gene expression with no known function in flavonoid metabolism (Watkinson et al, 2018).…”

Sterol 3-beta-Glucosyltransferase TRANSPARENT TESTA15 Controls Seed Development and Flavanol Accumulation through its Role in Vacuole Biogenesis and Maintenance in Arabidopsis

“…The mechanisms mediating flavanol transport to the vacuole in wild-type seeds also are still a matter of debate (Dixon and Sarnala, 2020). The current working model for flavanol trafficking based on Arabidopsis and Medicago biochemistry and genetics postulates that after biosynthesis by a metabolon anchored at the external side of the ER, EC is glycosylated and transported to the vacuole where it would hypothetically be hydrolyzed by a glycosidase and polymerized to PCs before migrating to the cell wall according to an unknown mechanism (Winkel, 2019; Dixon and Sarnala, 2020) (Supplemental Figure S2). The vacuolar transport of glycosylated EC involves the tonoplastic Multidrug And Toxin Extrusion (MATE) transporter TT12 / Detoxification 41 (DTX41) / TDS3 (Debeaujon et al, 2001; Marinova et al, 2007; Zhao and Dixon, 2009; Appelhagen et al, 2014) that would be energized by the tonoplastic P 3A -ATPase TT13/Autoinhibited H + -ATPase isoform 10 (AHA10) / TDS5 (Baxter et al, 2005; Appelhagen et al, 2014; Appelhagen et al, 2015).…”