Characterization of Growth-related Osmiophilic Particles in Corn Coleoptiles and Deepwater Rice Internodes

Abstract

Osmiophilic particles are secreted from the epidermal cells into the outer epidermal cell wall of deepwater rice internodes and corn coleoptiles that have been induced to elongate rapidly by treatment with gibberellin or auxin, respectively. The diameter of the osmiophilic particles is 80 to 100 nm in deepwater rice and up to 300 nm in corn coleoptiles. Because these particles are secreted to the outer epidermal wall only, they may contain either cell wall components that are specific for the outer epidermal wall or components of the cuticle that coats the outer epidermal wall. Monensin inhibited the appearance of these particles, indicating that they are derived from the Golgi apparatus. An attempt was made to identify the contents of the osmiophilic particles by enzyme-gold or antibody-gold labelling. Cutinase-gold accumulated mostly in the region of the cell wall just below the cuticle. The osmiophilic particles became densely labelled after incubation with proteinase K-gold, indicating that they are, at least in part, proteinaceous. In excised tissue and ultrathin sections treated with proteinase K, the number of osmiophilic particles was reduced, while glucanase and cutinase treatments were without effect. Antibodies against lipid transfer protein, an extensin-like protein, an arabinogalactan protein, a peroxidase, and expansin bound to the cell wall or plasma membrane but not to the osmiophilic particles.