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Control of Cell Wall Extensibility during Pollen Tube Growth

Molecular Plant 2013年第4期6卷 998-1017页

作者： peter k. hepler Caleb M. Rounds Lawrence J. Winship Biology Department University of Massachusetts Amherst MA 01003 USA School of Natural Science Hampshire College Amherst MA 01002 USA

In this review, we address the question of how the tip-growing pollen tube achieves its rapid rate of elongation while maintaining an intact cell wall. Although turgor is essential for growth to occur, the local expansion rate is controlled by local changes in the viscosity of the apical wall. We focus on several different structures and underly- ing processes that are thought to be major participants including exocytosis, the organization and activity of the actin cytosk.leton, calcium and proton physiology, and cellular energetics. We think.that the actin cytosk.leton, in particular the apical cortical actin fringe, directs the flow of vesicles to the apical domain, where they fuse with the plasma mem- brane and contribute their contents to the expanding cell wall. While pH gradients, as generated by a proton-ATPase located on the plasma membrane along the side of the clear zone, may regulate rapid actin turnover and new polymeri- zation in the fringe, the tip-focused calcium gradient biases secretion towards the polar axis. The recent data showing that exocytosis of new wall material precedes and predicts the process of cell elongation provide support for the idea that the intussusception of newly secreted pectin contributes to decreases in apical wall viscosity and to cell expansion. Other prime factors will be the localization and activity of the enzyme pectin methyl-esterase, and the chelation of calcium by pectic acids. Finally, we ack.owledge a role for reactive oxygen species in the control of wall viscosity.

关键词： cell expansion cell walls cytosk.leton dynamics polarity pollen development.

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Calcium at the Cell Wall-Cytoplast Interface

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Journal of Integrative Plant Biology 2010年第2期52卷 147-160页

作者： peter k.hepler Lawrence J.Winship Department of Biology University of MassachusettsAmherstMA 01003USA School of Natural Science Hampshire CollegeAmherstMA 01002USA

Attention is given to the role of Ca^2＋ at the interface between the cell wall and the cytoplast, especially as seen in pollen tubes. While the cytoplasm directs the synthesis and deposition of the wall, it is less well appreciated that the wall exerts considerable self control and influences activities of the cytoplasm. Ca^2＋ participates as a crucial factor in this two way communication. In the cytoplasm, a [Ca^2＋] above 0.1 μM, regulates myriad processes, including secretion of cell wall components. In the cell wall Ca^2＋ , at 10μM to 10 mM, binds negative charges on pectins and imparts structural rigidity to the wall. The plasma membrane occupies a pivotal position between these two compartments, where selective channels regulate influx of Ca^2＋ , and specific carriers pump the ion back.into the wall. In addition we draw attention to different factors, which either respond to the wall or are present in the wall, and usually generate elevated[Ca^2＋ ] in the cytoplasm. These factors include：（i） stretch activated channels; （ii） calmodulin; （iii） annexins; （iv） wall associated k.nases; （v） oligogalacturonides; and （vi） extracellular adenosine 5＇-triphosphate. Together they provide evidence for a rich and multifaceted system of communication between the cytoplast and cell wall, with Ca^2＋ as a carrier of information.

关键词： Calcium at the Cell Wall-Cytoplast Interface CNGC

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The pollen tube clear zone: Clues to the mechanism of polarized growth

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Journal of Integrative Plant Biology 2015年第1期57卷 79-92页

作者： peter k.hepler Lawrence J.Winship Biology Department University of Massachusetts School of Natural Science Hampshire College

Pollen tubes usually exhibit a prominent region at their apex called the ＂clear zone＂ because it lack. light refracting amyloplasts. A robust, long clear zone often associates with fast growing pollen tubes, and thus serves as an indicator of pollen tube health. Nevertheless we do not understand how it arises or how it is maintained. Here we review the structure of the clear zone, and attempt to explain the factors that contribute to its formation. While amyloplasts and vacuolar elements are excluded from the clear zone, virtually all other organelles are present including secretory vesicles, mitochondria, Golgi dictyosomes, and the endoplas-mic reticulum （ER）. Secretory vesicles aggregate into an inverted cone appressed against the apical plasma membrane. ER elements move nearly to the extreme apex, whereas mitochondria and Golgi dictyosomes move less far forward. The cortical actin fringe assumes a central position in the control of clear zone formation and maintenance, given its role in generating cytoplasmic streaming. Other lik.ly factors include the tip-focused calcium gradient, the apical pH gradient, the influx of water, and a host of signaling factors （small G-proteins）. We think.that the clear zone is an emergent property that depends on the interaction of several factors crucial for polarized growth.

关键词： Actin endoplasmic reticulum myosin polarized growth pollen tube

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Fertilization in Torenia fournieri:actin organization and nuclear behavior in the central cell and primary endosperm

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Science China(Life Sciences) 2002年第2期45卷 211-224页

作者：袁明傅缨王凤黄炳权徐是雄 peter k.hepler Department of Botany The University of Hong Kong Pokfulam Road Hong Kong China Department of Plant Sciences College of Biological Sciences China Agricultural University Beijing 100094 China Department of Botany The University of Hong Kong Pokfulam Road Hong Kong China Research Center for Developmental Biology College of Life Sciences Wuhan University Wuhan 430072 China

Studies of the living embryo sacs of Torenia fournieri reveal that the actin cytosk.leton undergoes dramatic changes that correlate with nuclear migration within the central cell and the primary endosperm. Before pollination, actin filaments appear as short bundles randomly distributed in the cortex of the central cell. Two days after anthesis, they become organized into a distinct actin network. At this stage the secondary nucleus, which is located in the central region of the central cell, possesses an associated array of short actin filaments. Soon after pollination, the actin filaments become fragmented in the micropylar end and the secondary nucleus is located next to the egg apparatus. After fertilization, the primary endosperm nucleus moves away from the egg cell and actin filaments reorganize into a prominent network.in the cytoplasm of the primary endosperm. Disruption of the actin cytosk.leton with latrunculin A and cytochalasin B indicates that actin is involved in the migration of the nucleus in the central cell. Our data also suggest that the dynamics of actin cytosk.leton may be responsible for the reorganization of the central cell and primary endosperm cytoplasm during fertilization.

关键词： actin, embryo sac, central cell, Torenia.

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