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Query: UMLS:C0277787 (stigma)
 13,352 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

In plants, the pollen coat covers the exine wall of the pollen and is the outermost layer that makes the initial contact with the stigma surface during sexual reproduction. Little is known about the constituents of the pollen coat, especially in wind-pollinated species. The pollen coat was extracted with diethyl ether from the pollen of maize (Zea mays L.), and a predominant protein of 35 kDa was identified. On the basis of the N-terminal sequence of this protein, a cDNA clone of the Xyl gene was obtained by reverse transcriptase-polymerase chain reaction. The deduced amino acid sequence of the 35-kDa protein shared similarities with the sequences of many microbial xylanases and a barley aleurone-layer xylanase. The 35-kDa protein in the pollen-coat extract was purified to homogeneity by fast protein liquid chromatography and determined to be an acidic endoxylanase that was most active on oat spelt xylan. Northern and in situ hybridization showed that Xyl was specifically expressed in the tapetum of the anther after the tetrad microspores had become individual microspores. Southern hybridization and gene-copy reconstruction studies showed only one copy of the Xyl gene per haploid genome. Analyses of the genomic DNA sequence of Xyl and RNase protection studies with the transcript revealed many regulatory motifs at the promoter region and an intron at the 5' leader region of the transcript. The Xyl transcript had a 562-nucleotide (nt) 5' leader, a 54-nt sequence encoding a putative signal peptide, a 933-nt coding sequence, and a 420-nt 3'-untranslated sequence. The unusually long 5' leader had an open reading frame encoding a putative 175-residue protein whose sequence was most similar to that of a microbial arabinosidase. The maize xylanase is the first enzyme documented to be present in the pollen coat. Its possible role in the hydrolysis of the maize type II primary cell wall (having xylose, glucose, and arabinose as the major moieties) of the tapetum cells and the stigma surface is discussed.
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PMID:The predominant protein on the surface of maize pollen is an endoxylanase synthesized by a tapetum mRNA with a long 5' leader. 1042 75

Pollen coat contains ingredients that interact with the stigma surface during sexual reproduction. In maize (Zea mays L.) pollen coat, the predominant protein is a 35-kDa endoxylanase, whose mRNA is located in the tapetum cells enclosing the maturing pollen in the anthers. This 2.0-kb mRNA was found to have an open reading frame of 1,635 nucleotides encoding a 60-kDa pre-xylanase. In developing anthers, the pre-xylanase protein appeared prior to the 35-kDa xylanase protein and enzyme activity and then peaked and declined, whereas the 35-kDa xylanase protein and activity continued to increase until anther maturation. An acid protease in the anther extract converted the inactive pre-xylanase to the active 35-kDa xylanase in vitro. The protease activity was inhibited by inhibitors of serine proteases but unaffected by inhibitors of cysteine, aspartic, or metallic proteases. Sequence analysis revealed that the 60-kDa pre-xylanase was converted to the 35-kDa xylanase with the removal of 198 and 48 residues from the N and C termini, respectively. During in vitro and in vivo conversions, no intermediates of 60-35 kDa were observed, and the 35-kDa xylanase was highly stable. The pre-xylanase was localized in the tapetum-containing anther wall, whereas the 35-kDa xylanase was found in the pollen coat. The significance of having a large non-active pre-xylanase and the mode of transfer of the xylanase to the pollen coat are discussed. A gene encoding the barley (Hordeum vulgare L.) tapetum xylanase was cloned; this gene and the gene encoding the seed aleurone-layer xylanase had strict tissue-specific expressions.
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PMID:Maize tapetum xylanase is synthesized as a precursor, processed and activated by a serine protease, and deposited on the pollen. 1236 81