Anthocyanin in the Vacuole of Red Onion Epidermal Cells Quenches Other Fluorescent Molecules

Abstract

Peels from the inner epidermis of onion bulbs are a model system in plant cell biology. While the inner epidermis of red onions is characteristically white, small patches of cells sometimes redden, containing vacuolar anthocyanin. This study investigated the spectroscopic properties of these anthocyanic cells. When fluorescent dyes were loaded into the vacuole of onion epidermal cells, the anthocyanic cells showed decreased dye fluorescence. This decrease was observed for fluorescein and carboxyfluorescein that are pumped into the vacuole by anion transporters, for acridine orange which acid loads into the vacuole, and for the fluorescent sugar analogue esculin loaded into the vacuole by sucrose transporters. Similar decreases in carboxyfluorescein fluorescence were observed when dye was loaded into the vacuoles of several other plant species, but decreases were not observed for dyes resident in the tonoplast membrane. As cellular physiology was unaffected in the anthocyanic cells, with cytoplasmic streaming, vacuolar and cytoplasmic pH not being altered, the decreased dye fluorescence from the anthocyanic cells can be attributed to fluorescence quenching. Furthermore, because quenching decreased with increasing temperature. It was concluded, therefore, that vacuolar anthocyanin can statically quench other fluorescent molecules in vivo, an effect previously demonstrated for anthocyanin in vitro.2 of 15 pH-dependant, which means that anthocyanin colour, and the colour of the plant, will depend upon vacuolar pH. Anthocyanin colour may also vary because anthocyanins can interact with themselves, stacking to form super-molecular structures, and can interact with other flavonoid pigments, stabilising the coloured forms at acidic pHs, and also interact with metal ions [1][2][3][4].Like many flavonoids, anthocyanins can be weakly autofluorescent with in vitro excitation and emission peaks in the UV [4,5]. However, in vivo excitation of anthocyanins in both Arabidopsis thaliana and onion (Allium cepa) results in red fluorescence, and has allowed for direct visualisations of vacuole structure and dynamics [6][7][8].The flavonoid biosynthetic pathway, a complex series of reactions in which coumaroyl-CoA is sequentially modified to form various end-products, results in the formation of anthocyanins [9]. The enzymes controlling this pathway vary slightly between species resulting in the variability in the anthocyanidin core and attached sugars. Following their synthesis in the cytoplasm, the vacuolar transport of anthocyanins occurs through several pathways. These include ER-derived vesicles [6,10] and a tonoplast-bound glutathione S-transferase-like transporter that sequesters various glutathione conjugates and anthocyanins into the vacuole [11,12]. The mechanism(s) underlying this second process are unclear as anthocyanin-glutathione conjugates remain undiscovered [12]. The multi-drug resistance-like protein is another membrane-bound transporter known to transport anthocyanin into maize vacuoles [1].It is thought that anth...

Metabolome-Based Discrimination Analysis of Shallot Landraces and Bulb Onion Cultivars Associated with Differences in the Amino Acid and Flavonoid Profiles

“…In the present study, several flavonols and anthocyanins were accumulated in shallot landraces, as compared with bulb onion cultivars, and the increase in flavonoid contents in shallots was highly coordinated with the high expression of several flavonol and anthocyanin biosynthesis and regulatory genes in the DHA compared with the DHC ( Figure 3 B and Figure S2; Tables S2 and S4 ). Flavonoids are potent antioxidants that play prominent roles in protecting plant cells against ROS, generated from photosynthesis and the respiration process under normal and stressed conditions, that cause cellular damage to membranes, proteins, and nucleic acids, as they contain unpaired electrons [ 31 ]. In addition, flavonoids located mainly in the outer skin of the bulb tissue act as a chemical barrier against pathogens [ 10 ].…”

Pemanfaatan Ekstrak Antosianin dari Limbah Kulit Bawang Merah (Allium cepa) sebagai Zat Pemeka (Sensitizer) pada Dye Sensitized Solar Cell (DSSC)

“…Salah satu limbah biomassa yang berpotensi sebagai sumber zat warna alami untuk digunakan sebagai zat pemeka pada DSSC adalah kulit bawang merah (Allium cepa). Hasil analisa kandungan senyawa kimia kulit bawang merah memperlihatkan bahwa kulit bawang merah tersebut banyak mengandung antosianin (Collings, 2019;Hemmatzadeh and Jamali, 2015;Oancea and Drághici, 2013). Antosianin merupakan salah satu pigmen berwarna gugus fenolik yang disusun oleh antosianidin sebagai struktur dasar yang terikat pada gula.…”

Anthocyanins From Clitoria ternatea Flower: Biosynthesis, Extraction, Stability, Antioxidant Activity, and Applications

“…Anthocyanins are synthesised in the cytoplasm of the cell and then transported to the vacuole. The vacuolar transportation of anthocyanins takes place in several pathways that include endoplasmic reticulum-derived vesicles and a tonoplast-bound glutathione S-transferase-like transporter (Collings, 2019). However, acylation of anthocyanins, catalysed by acyltransferases (ATs) and the final modification of anthocyanins occurs after being transported to the vacuole (Lu et al, 2021).…”