Polarized and persistent Ca²⁺ plumes define loci for formation of wall ingrowth papillae in transfer cells.

Hui-Ming Zhang,Derek R Laver,David W Mccurdy,Dirk F Van Helden,John W Patrick,Christina E Offler,Mohammad S Imtiaz

doi:10.1093/jxb/eru460

Abstract

Transfer cell morphology is characterized by a polarized ingrowth wall comprising a uniform wall upon which wall ingrowth papillae develop at right angles into the cytoplasm. The hypothesis that positional information directing construction of wall ingrowth papillae is mediated by Ca(2+) signals generated by spatiotemporal alterations in cytosolic Ca(2+) ([Ca(2+)]cyt) of cells trans-differentiating to a transfer cell morphology was tested. This hypothesis was examined using Vicia faba cotyledons. On transferring cotyledons to culture, their adaxial epidermal cells synchronously trans-differentiate to epidermal transfer cells. A polarized and persistent Ca(2+) signal, generated during epidermal cell trans-differentiation, was found to co-localize with the site of ingrowth wall formation. Dampening Ca(2+) signal intensity, by withdrawing extracellular Ca(2+) or blocking Ca(2+) channel activity, inhibited formation of wall ingrowth papillae. Maintenance of Ca(2+) signal polarity and persistence depended upon a rapid turnover (minutes) of cytosolic Ca(2+) by co-operative functioning of plasma membrane Ca(2+)-permeable channels and Ca(2+)-ATPases. Viewed paradermally, and proximal to the cytosol-plasma membrane interface, the Ca(2+) signal was organized into discrete patches that aligned spatially with clusters of Ca(2+)-permeable channels. Mathematical modelling demonstrated that these patches of cytosolic Ca(2+) were consistent with inward-directed plumes of elevated [Ca(2+)]cyt. Plume formation depended upon an alternating distribution of Ca(2+)-permeable channels and Ca(2+)-ATPase clusters. On further inward diffusion, the Ca(2+) plumes coalesced into a uniform Ca(2+) signal. Blocking or dispersing the Ca(2+) plumes inhibited deposition of wall ingrowth papillae, while uniform wall formation remained unaltered. A working model envisages that cytosolic Ca(2+) plumes define the loci at which wall ingrowth papillae are deposited.

Highlights

Cytosolic Ca2+ is a conserved signal directing polarized cell development in algae (Wheeler and Brownlee, 2008), animals (Tojima, 2012), fungi (Brand and Gow, 2009), and plants (Kudla et al, 2010)
The hypothesis that positional information directing construction of wall ingrowth papillae is mediated by Ca2+ signals generated by spatiotemporal alterations in cytosolic Ca2+ ([Ca2+]cyt) of cells trans-differentiating to a transfer cell morphology was tested
Induced remodelling of the actin cytoskeleton combined with alterations to the secretory apparatus (Cárdenas, 2009; Abbreviations: 2-APB, 2-aminoethoxydiphenyl borate; AM, acetoxymethyl; BAPTA, 1, 2-bis(o-aminophenoxy)ethane-N,N,N’,N’-tetraacetic acid; [Ca2+]cyt, cytosolic Ca2+ concentration; CCCP, carbonyl cyanide m-chlorophenyl hydrazine; CLSM, confocal laser scanning microscope; ER, endoplasmic reticulum; fl-DHP, DM-BODIPY(–)dihydropyridine; HPTS, 8-acetoxypyrene-l,3,6-trisulphonic acid; H2O2, hydrogen peroxide; Murashige and Skoog (MS), Murishige and Skoog; PBS, phosphate-buffered saline; PM, plasma membrane; RH-414, N-(3-triethylammoniumpropyl)-4-(4-(4-diethylamino)phenyl)butadienyl)pyridinium dibromide; SEM, standard error of the mean

Summary

Introduction

Cytosolic Ca2+ is a conserved signal directing polarized cell development in algae (Wheeler and Brownlee, 2008), animals (Tojima, 2012), fungi (Brand and Gow, 2009), and plants (Kudla et al, 2010). The most studied experimental cell models in which cytosolic Ca2+ functions as a polarity signal are elongating pollen tubes (Hepler et al, 2012) and root hairs (Cárdenas, 2009). In these cells, a tip-high gradient of [Ca2+]cyt directs polarized delivery of vesicles containing cargoes of cell wall building material for continued tip growth. The distinctive patterns arise from the co-operative activities of Ca2+-permeable channels releasing Ca2+ into the cell cytosol from extracellular and intracellular sources and Ca2+ retrieval back into these compartments by Ca2+-ATPases and Ca2+/proton antiporters (McAinsh and Pittman, 2009; Hepler et al, 2012)

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