Sugar beet is the major source of commercially produced sugar and bioenergy in the form of ethanol. The green leaves synthesize sugars by photosynthesis and these sugars are stored in the tap root underground. Sugars are mostly stored inside the vacuoles of cells of the taproot which act like storage bags. However the specific mechanism and transporter by which the sugars are transported and stored into the vacuoles of tap root has eluded plant biologists for decades now.
In plants (vascular), sugars and water are circulated throughout the plant body by phloem and xylem respectively. Xylem and phloem are tubular structures made up of specialized cells that run all over, connecting different organs of the plant. Sucrose reaches the vacuoles for storage via this vascular network which is used for the growth, metabolism, survival of osmotic or drought stress and various other functions.
The vacuoles are surrounded by a membrane named vacuolar membrane or tonoplast made up of lipoproteins. Certain proteins on the membrane act as carriers or transporters that transport sugars into the vacuoles with or without using the cellular energy molecules, ATPs. Vacuolar glucose transporter (VGT), Tonoplast monosaccharide transporter (TMT), Early response to dehydration like 6 protein (ERDL6) and SUC4 in Arabidopsis are a few examples of transporters that act as primary or secondary carriers that export sugars in the vacuoles of leaves in plants. Not much was known about the transporters that transport sugars across the vacuolar membrane of sugar storing organs.
A research article published in Nature Plants by 3 lead authors Benjamin Jung, Frank Ludewig and Alexander Schulz from University Würzburg and University Kaiserslautern in Germany identified a transporter protein named BvTST2.1 as the key player in the uptake of sucrose inside the vacuoles of sugar beet taproot. BvTST2.1 is identified as the prominent and abundant protein present on the tonoplast of taproot vacuoles. It imports sucrose into the highly concentrated vacuole against the osmotic gradient by using proton motive force. Moreover this BvTST2.1 is highly specific to sucrose import and does not transfer glucose that makes it unique for a sucrose storing plant like sugar beet.
The identification of BvTST2.1 as the key transporter protein, could help scientists to increase sugar yields from sugar beet and other sucrose accumulating pants by plant breeding programs. This is especially significant now, since agriculturists are trying to meet the demands of the growing population.
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