The Maize Tapetum Employs Diverse Mechanisms to Synthesize and Store Proteins and Flavonoids and Transfer Them to the Pollen Surface

Abstract

Abstract In anthers, the tapetum synthesizes and stores proteins and flavonoids, which will be transferred to the surface of adjacent microspores. The mechanism of synthesis, storage, and transfer of these pollen-coat materials in maize (Zea mays) differs completely from that reported in Arabidopsis (Arabidopsis thaliana), which stores major pollen-coat materials in tapetosomes and elaioplasts. On maize pollen, three proteins, glucanase, xylanase, and a novel protease, Zea mays pollen coat protease (ZmPCP), are predominant. During anther development, glucanase and xylanase transcripts appeared at a mid developmental stage, whereas protease transcript emerged at a late developmental stage. Protease and xylanase transcripts were present only in the anther tapetum of the plant, whereas glucanase transcript was distributed ubiquitously. ZmPCP belongs to the cysteine protease family but has no closely related paralogs. Its nascent polypeptide has a putative amino-terminal endoplasmic reticulum (ER)-targeting peptide and a propeptide. All three proteins were synthesized in the tapetum and were present on mature pollen after tapetum death. Electron microscopy of tapetum cells of mid to late developmental stages revealed small vacuoles distributed throughout the cytoplasm and numerous secretory vesicles concentrated near the locular side. Immunofluorescence microscopy and subcellular fractionation localized glucanase in ER-derived vesicles in the cytoplasm and the wall facing the locule, xylanase in the cytosol, protease in vacuoles, and flavonoids in subdomains of ER rather than in vacuoles. The nonoverlapping subcellular locations of the three proteins and flavonoids indicate distinct modes of their storage in tapetum cells and transfer to the pollen surface, which in turn reflect their respective functions in tapetum cells or the pollen surface.