Analysis of cysteine- and glycine -rich proteins encoded by genes expressed during anther development in tomato (Lycopersicon esculentum Mill.)

The tapetum is a nutritive tissue of the stamen that is essential for normal microspore development. While tapetal-specific genes have been identified, little information is available on the localization and function of the proteins produced by these genes. Presented here is the characterization and localization of two proteins that are synthesized in the tapetum early in microsporogenesis.;The cysteine-rich, tapetally-expressed protein 5B-CRP has a secretory signal sequence and lacks an endoplasmic reticulum retention sequence. The 5B-CRP mRNA accumulates specifically within the tapetum, and accumulates from premeiosis to tetrad release. Antibodies generated against an E. coli fusion protein only recognized 5B-CRP in a reduced state. The 5B-CRP was detected as a 6kDa protein in extracts of stamens from microspore meiosis through anthesis, and was also observed in extracts from dehisced pollen. In situ, 5B-CRP was localized in stamens to the tapetum and the developing microspores, from tetrad through early free microspore. Based on similarity to proteins with known functions, 5B-CRP may inhibit proteasome activity within the stamen locule, or transfer lipophilic compounds from the tapetum to the developing microspores.;The 92-GRP is a small molecular weight, glycine-rich protein expressed specifically in the stamen. Except for the high glycine content the protein has little similarity with other reported proteins. Four major forms of 92-GRP were detected in protein extracts from stamens. The three largest molecular weight forms were present in early microspore development, while the smallest molecular weight form of 92-GRP was present in stamens from early microsporogenesis through anthesis, and was the only form present in dehisced pollen. The 92-GRP was detected in callose surrounding the microspore mother cells and tetrads, as well as in microspore and pollen exine. Additionally, 92-GRP was present in orbicules (sporopollenin-rich bodies that line the locule wall). In plants with reduced levels of 92-GRP, pollen had altered exine formation and pollen viability was reduced. The 92-GRP is proposed to be involved in exine formation, possibly functioning as an intermediary between the hydrophilic and hydrophobic components of the exine wall.