Beta vulgaris L. (Sugar Beet)
Sugar beet (Beta vulgaris L.) grows wild on the eastern coast of the Mediterranean Sea and in Central Asia. About 40% of the demand for sugar in the world is supplied by sugar beet. It is cultivated in the cold regions of the globe and in the temperate zone, where the climate is not suitable for cultivating sugarcane. The main producing countries are the former USSR, the USA, France, Poland, Germany, and Italy. About 85% of world production is from Europe.
Numerous studies have been carried out to elucidate the sucrose storage mechanism in sugar-beet roots. In sugarcane, it has been suggested that invertase plays an important role during sucrose accumulation. Sucrose translocated to the stalk enters the free space, where it is hydrolyzed by a cell wall acid invertase. The resulting hexoses are actively accumulated into the storage parenchyma and resynthesized to sucrose phosphate by sucrose phosphate synthetase. In sugar beet, however, sucrose synthetase plays a pivotal role during sucrose accumulation. Giaquinta (1979) has shown that the onset of storage is accompanied by the appearance of sucrose synthetase activity. Doney’s group has shown that sugar beet root cell size is negatively correlated with sucrose concentration. High-sucrose genotypes have small cells, and low-sucrose genotypes have large cells. Therefore, genetic selection methods that increase root yield decrease sucrose content, and those that increase sucrose content decrease yield. Stein and Willenbrink (1976) have shown a correlation between sucrose accumulation and energy charge during beet development.
In recent years, it has become clear that sugar beet contains interesting oc-glucosidases. The α-glucosidases (EC 3.2.1.20) have been studied extensively in animals, higher plants, yeasts, and molds. Most of these enzymes readily hydrolyze soluble starch, liberating glucose. The enzyme from Mucor javanicus () hydrolyzes soluble starch at the same rate as maltose, but the enzymes from Aspergillus () and some bacteria hydrolyze soluble starch very weakly or not at all. The enzymes from higher plants hydrolyze soluble starch more weakly than does the enzyme from M. javanicus. The in-vivo significance of glucose production from soluble starch in plant tissues by α-glucosidase is not yet clear, although some authors have suggested that the enzyme forms a part of the nonphosphorolytic pathway for the breakdown of starch, and is functioning in seed germination by hydrolyzing the oligosaccharides produced by α- and β-amylases.
The α-glucosidases from sugar-beet seeds hydrolyze soluble starch at a faster rate than maltose, as if they were glucoamylases. This strong soluble-starch-hydrolyzing activity suggests that, like glucoamylase, the enzymes may even hydrolyze starch to glucose without the preceding action of α- and β-amylases in the plant tissue. On the other hand, another type of α-glucosidase, which hydrolyzes soluble starch only very weakly, was also isolated from sugar-beet seeds at the same time. The two enzymes from sugar-beet seeds exhibit very different properties, particularly in their soluble-starch-hydrolyzing activity. They may be located in a different part of the plant tissue and play different roles there. Therefore, sugar-beet seeds offer an excellent system with which to examine the physiological function of α-glucosidase. Plant tissue cultures are often useful in studying the various physiological phenomena of higher plants. In this chapter, the purification and properties of the α-glucosidases produced by suspension-cultured sugar-beet cells are described.
It has been suggested that α-glucosidase forms a part of the nonphosphorolytic pathway for the breakdown of starch, and functions in seed germination by hydrolyzing the oligosaccharides produced by α- and β-amylases. However, α-glucosidases from higher plants readily hydrolyze soluble starch, liberating glucose. In higher plants, an α-glucosidase that hydrolyzes soluble starch weakly is found only in sugar-beet seeds. Sugar-beet seeds contain two α-glucosidases which exhibit very different properties, particularly in soluble-starch-hydrolyzing activity. Therefore, sugar beet offers an excellent system with which to examine the physiological function of α-glucosidase. Four α-glucosidases have been fractionated from sugar-beet cells. One of them (G-II), which is a main enzyme of four α-glucosidases, hydrolyzes soluble starch at a faster rate than maltose, is not detected during the phase of starch accumulation in cells, and undergoes a marked increase with the decrease in starch content. Therefore, a possible role for the enzyme is in the metabolism of starch in the cell. The molecular weight of the enzyme is calculated to be 53 000 and the enzyme does not contain quaternary structure. On the other hand, two α-glucosidases (N-I and N-II) are secreted into the culture medium when sugar-beet cells are grown in MS medium with shaking. α-Glucosidase activity of N-I is about three times as high as that of N-II. However, α-glucosidase activity of N-I, which inhibited the glycosylation by tunicamycin, is much lower than that of N-II. The secreted enzyme protein is proposed to possess a signal for secretion into the culture medium. In the two α-glucosidases from sugar-beet cells, the degree of glycosylation affects the destination of the enzyme proteins out of or within the cells.
Selections from the book: Medicinal and Aromatic Plants VII (1994).