BETA-AMYLASE

Beta-Amylase

Mario Montgomery

Independent Study

Beta-Amylase

Introduction

In this review, I will discuss the biochemical mechanism of beta amylase, factors that affects its expression, organ, tissue, and cell specific localization of the expression, plant species in which beta amylase function has been demonstrated, and finally recent updates about beta amylase gene function. Beta-amylase is an exoamylase that hydrolyses a 1,4 glycosidic linkages of polyglucan chains at the non-reducing end to produce maltose (4-O- a -D-Glucopyranosyl-ß-D-glucose) during starch degradation. Maltose is exported to the cytosol by a maltose translocator found in the chloroplast membrane to be further metabolized by cytosolic glucosyltransferases (Nakamura et al, 1991).

Studies showed that beta-amylase expression and/or activity was induced during temperature stress; and the levels of its product (maltose) also increased during temperature stress For example, when Arabidopsis was cold-stressed at 4oC for 12 h, beta-amylase (AJ25034; ct-Bmy or BMY8) transcript increased about 14-fold and induction was found to occur as early as 2 h of exposure to cold stress. The BMY7 expression increased about 5-fold when Arabidopsis plants grown at 20oC were exposed for 1 h to 40oC (Krapp et al,, 2129-2141). Studies with potato tubers also showed that an increase in beta-amylase activity was accompanied by maltose production during cold shock.

The role of beta-amylase induction during temperature shock is unknown (Scheidig, 2002, 581-591). The study showed that beta-amylase induction precedes the appearance of maltose, which has compatible solute-like protective abilities as demonstrated by in vitro assays. The BMY8 RNAi lines with reduced beta-amylase transcript levels had less maltose accumulation in response to cold shock, compared with WT (Krapp et al,, 2129-2141). Freezing stress assays showed that BMY8 RNAi lines had reduced chlorophyll fluorescence parameters. Therefore, stress-induced beta-amylase results in maltose accumulation, which can contribute to the protection of the photosynthetic electron transport chain, proteins, and membranes inside the chloroplast during temperature, shock (Mita, 1997).

Role of Beta-Amylases in Starch Degradation

In this part of the paper I intend to anlayse the role of beta-analyses in starch degradation. The primary function of beta-amylase in plants is in starch breakdown. Down-regulation of a chloroplast-localized beta-amylase by antisense mRNA resulted in a starch-excess phenotype in potato leaves compared to wild type plants. Plants with reduced chloroplastic beta-amylase activity could degrade only 8-30% of their total starch in the dark, while wild type could degrade 50%; indicating that hydrolytic rather than phosphorolytic cleavage is the predominant during starch degradation. Furthermore, it produces maltose by hydrolyzing long un-branched polyglucan chains that are breakdown products of native starch grains by other amylolytic enzymes like alpha-amylase based on in vitro studies conducted by Krapp (1995, 313-323).

It has been suggested that glucose and maltose are exported to the cytosol during hydrolytic cleavage. Several studies show that export of glucose and maltose occurs from isolated chloroplasts. This is further supported by the recent discovery of a maltose translocator (MEX) in the chloroplast membrane. Mutations in the maltose translocator, a single copy gene, resulted in a starch excess phenotype and elevated maltose content (Lu, 2004, 466-473).

Once maltose is exported ...