Premature dissolution of the microsporocyte callose wall causes male sterility in transgenic tobacco.

Abstract

Male sterility in a petunia cytoplasmic male sterile line has been attributed to the early appearance of active callase, a P-1,3-glucanase, in the anther locule. This leads to premature dissolution of the callose walls surrounding the microsporogenous cells. We have mimicked this aspect of the petunia line in transgenic tobacco by engineering the secretion of a modified pathogenesis-related vacuolar P-1,3-glucanase from the tapetum prior to the appearance of callase activity in the locule. Plants expressing the modified glucanase from tapetum-specific promoters exhibited reduced male fertility, ranging from complete to partia1 male sterility. Callose appearance and distribution are normal in the male sterile transgenic plants up to prophase I, whereupon callose is prematurely degraded. Meiosis and cell division occur normally. The resultant microspores have an abnormally thin cell wall that lacks sculpturing. The tapetum shows hypertrophy. Male sterility is probably caused by bursting of the aberrant microspores at a time corresponding to microspore release. These results demonstrate that premature callose degradation is sufficient to cause male sterility and suggest that callose is essential for the formation of a normal microspore cell wall.

Cell Adhesion, Separation and Guidance in Compatible Plant Reproduction

“…In many other plants and in normal pollen development, soon after meiosis, cell separation generally occurs to release the free microspores from the tetrads (Figure 4.1E and F). During this process, a decrease of pH (Izhar and Frankel, 1971;Worrall et al, 1992) and a peak of β-(1,3)-glucanase activity were observed in the anther locular fluid (Stieglitz, 1977) suggesting that this enzyme may be involved in the degradation of the tetrad wall. Degradation of the callose wall requires a complex of endo-and exo-enzymes, called callase.…”

“…A tight developmental regulation is de facto required to synchronize the production of wall-degrading enzymes by the tapetal cells with the maturation of the PMCs. Indeed, the premature dissolution of the callose wall or premature programmed cell death of the tapetum, which normally occurs during the late pollen formation, lead generally to a reduction in male fertility (Worrall et al, 1992;Tsuchiya et al, 1995;Ku et al, 2003). Several possible callase members have been described.…”

“…In contrast, callose is one of the main wall polysaccharides of the PMCs, microspores and pollen tubes but its role in pollen development is not clear. It was suggested that callose could isolate the microspores and/or serve as a template for the proper formation of exine -the outer wall layer of the mature pollen grains (Worrall et al, 1992;Scott et al, 2004). In pollen tubes, the callose wall was recently implicated in protection against tension and compression stresses (Parre and Geitmann, 2005).…”

“…In pollen tubes, the callose wall was recently implicated in protection against tension and compression stresses (Parre and Geitmann, 2005). Several studies with transgenic tobacco plants (Worrall et al, 1992;Tsuchiya et al, 1995) and Arabidopsis mutants (Dong et al, 2005b) have shown that an accurate callose wall deposition is a prerequisite for the normal development of the future pollen grains. In Arabidopsis, mutations in the CalS5 gene, encoding a callose synthase in PMCs, tetrads and microspores exhibited a lack of callose in the microspore wall leading to the abnormal formation of the exine wall and degeneration of the microspores (Dong et al, 2005b).…”

“…This study supports the hypothesis that callose deposition, at the plasma membrane, may serve as a template for the proper synthesis of exine and this exine wall has an important function in pollen development. Nevertheless, the PMC callosic wall is apparently not essential for the proper nuclear meiotic processes (Worrall et al, 1992;Scott et al, 2004). …”