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

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.

Abundant Type III Lipid Transfer Proteins in Arabidopsis Tapetum Are Secreted to the Locule and Become a Constituent of the Pollen Exine

“…2A). The presence of rough ER and Golgi bodies in the tapetum cells has been observed by others (Murgia et al, 1991;Platt et al, 1998;Li et al, 2012). In the Arabidopsis tapetum, the secretory organelles were distributed throughout the cell rather than concentrated in the cell portion near the locule, as in maize (Li et al, 2012).…”

“…Type III LTPs in the Tapetum Were Secreted to the Anther Locule via the ER-trans-Golgi Network Although the tapetum in most higher plants is described as secretory, the mechanism of subcellular tapetum secretion has not been explored in much detail until recently with maize (Li et al, 2012). The mode of secretion of LTP from the tapetum has not been explored.…”

Flavonoid transport mechanisms: how to go, and with whom

“…The flavonols synthesized in tapetum cells are translocated to the ER-derived tapetosome vesicles, which traffic to the PM for secretion [31]. These processes require full functions of TT19 and TT12 [31,32] (Box 1). Therefore, GST is required for transport of all three types of flavonoid.…”

The Cysteine Protease CEP1, a Key Executor Involved in Tapetal Programmed Cell Death, Regulates Pollen Development in Arabidopsis

“…The steryl esters, alkanes, and oleosins secreted from the elaioplasts and tapetosomes of the tapetum are needed for mature pollen wall formation (Hsieh and Huang, 2007). Steryl esters and alkanes are lipids that waterproof the pollen; oleosins on the pollen surface assist with water uptake from the stigma to the pollen for germination (Hsieh and Huang, 2004;Li et al, 2012). Therefore, normal lipid and fatty acid metabolism is crucial for fertile pollen formation.…”