The proteins encoded by two tapetum-specific transcripts, Sa tap35 and Sa tap44, from Sinapis alba L. are localized in the exine cell wall layer of developing microspores

Abstract

By differential screening of a copy DNA (cDNA) library from flowering Sinapis alba L. apices against cDNAs from vegetative apices, two cDNA clones were isolated representing transcripts that are expressed transiently at an early stage of tapetum development. The Sa tap35 cDNA encodes a polypeptide with a predicted molecular weight of 12.7 kDa and an isoelectric point of 10.4. The sa tap44 cDNA codes for a putative 12.4-kDa polypeptide with an isoelectric point of 7.5. The deduced amino-acid sequences display 76% sequence identity and contain an N-terminal stretch of hydrophobic amino acids which has characteristics of secretory signal sequences. In-vitro transcription of the cDNAs and translation of the resulting RNAs in the presence of canine pancreatic microsomes demonstrates that the two proteins are translocated into the microsomes and that the putative preproteins are proteolytically processed to the mature forms. By immunoelectron microscopy the Sa TAP35 and Sa TAP44 proteins were detected at the developing peritapetal membrane between the tapetal cytoplasm and the adjacent middle layer of the anther wall. Furthermore, labelling was observed within the locule in association with globules resembling pro-Ubisch bodies which appeared at the tetrad stage. During the early vacuolate stage of microspore development the young exine was strongly labelled. The exine and the peritapetal membrane both are composed of sporopollenin, and the pro-Ubisch bodies are thought to contain sporopollenin precursors. Thus, Sa TAP35 and Sa TAP44 might be involved in sporopollenin formation and/or deposition.