The stromacentre inAvena plastids: an aggregation ofβ-glucosidase responsible for the activation of oat-leaf saponins

Abstract

The stromacentre, a particular structure in the plastids of mostAvena species, was isolated from etioplasts ofAvena sativa and then characterized to determine its biological function. When comparing differentAvena species with or without stromacentre, it was shown that the stromacentre, a 63-kDa protein, and saponins (characteristic compounds ofAvena sativa) either occur together or not at all. This linkage was confirmed by demonstrating a transformation of saponins by the isolated stromacentre protein: avenacosides were hydrolyzed to 26-desgluco-avenacosides. Therefore, the stromacentre protein had to be regarded as aβ-glucosidase. Enzyme assays usingp-nitrophenyl-β-d-glucopyranoside as substrate showed that thisβ-glucosidase has a pH optimum at pH 6.0. The calculatedK _m value for this substrate was 2.2·10^-3 M. Antibodies against the stromacentre protein inhibitedβ-glucosidase activity. The determination of the molecular weight of theβ-glucosidase by sodium dodecyl sulfate-gel electrophoresis showed that it consists of subunits of 63 kDa. After gel electrophoresis under non-denaturing conditions, enzymatically active molecules were shown to consist of at least two of these subunits. Molecules aggregated up to about 10⁶ Da also had enzyme activity. Enzyme assays using avenacosides as substrate showed a pH optimum at pH 6.0. The calculatedK _m value for this substrate was 1.2·10^-5 M. The high affinity to the avenacosides and the high specificity for the C-26 bound glucose indicate that avenacosides are the natural substrates for thisβ-glucosidase. Assuming that the avenacosides in oat leaves play a role as preformed chemical inhibitory substances against phytopathogenic microorganisms, a model is presented showing the stromacentre with a central role in activating the fungitoxicity of avenacosides.