An anther-specific cysteine-rich protein of tomato localized to the tapetum and microspores

The tapetum is a nutritive tissue of the stamen that is essential for normal microspore development. While numerous tapetal-specific genes have been identified, little information is available on the localization and function of the proteins produced by these genes. The tapetally produced protein 5B-CRP is cysteine-rich, has a secretory signal sequence and lacks an endoplasmic reticulum retention sequence. The 5B-CRP mRNA is expressed specifically within the tapetum and accumulates from premeiosis to tetrad release. Antibodies generated against an Escherichia coli fusion protein only recognized 5B-CRP in the reduced state. The 5B-CRP was detected as a 6 kDa 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 the tetrad through early free microspore stages. Based on similarity to proteins with known functions, 5B-CRP may inhibit proteasome activity within the stamen locule.