Introduction

Callase is a complex of β-1,3-glucanase activities which play a crucial role in microsporogenesis. During microsporogenesis, archaesporial cells in the anther give rise to microsporocytes and tapetal cells (Fig. 1). In almost all higher plants a thick wall of callose, β-1,3-glucan polymer, is deposited between the cell membrane and the primary cell wall of the microsporocyte. As meiosis progresses, callose is also deposited along the cellular plates formed during cytokinesis, until each individual microspore is completely encased in callose. The tapetum forms a single layer of cells surrounding the anther locule. The tapetum is an extremely metabolically active tissue and is thought to play a nutritive role in microspore development (Chapman, 1987). Another critical function of the tapetum is the synthesis of callase which is required for the dissolution of the callose walls of the tetrad (Frankel, Izhar & Nitsan, 1969). After the primary and callosic walls of the tetrad have been degraded, the individual microspores are freed into the locule and continue their development into mature pollen grains. The developmental importance of callase activity is illustrated by the occurrence of mutants in petunia and sorghum where callase activity is premature (Frankel et al., 1969; Warmke & Overman, 1972) or delayed (Izhar & Frankel, 1971). These plants are male sterile because the inappropriate expression of callase results in microspore abortion. More recently, work by Worrall et al. (1992) demonstrated that premature secretion of an engineered β-1,3-glucanase from the tapetum of tobacco resulted in partial or total male sterility.