Lost in traffic? The K(+) channel of lily pollen, LilKT1, is detected at the endomembranes inside yeast cells, tobacco leaves, and lily pollen.

Minou J Safiarian,Gerhard Obermeyer,Adam Bertl,Heidi Pertl-Obermeyer,Rene Hude,Peter Lughofer

doi:10.3389/fpls.2015.00047

Minou J Safiarian, Gerhard Obermeyer + Show 4 more

Open Access

https://doi.org/10.3389/fpls.2015.00047

Copy DOI

Abstract

Fertilization in plants relies on fast growth of pollen tubes through the style tissue toward the ovules. This polarized growth depends on influx of ions and water to increase the tube’s volume. K+ inward rectifying channels were detected in many pollen species, with one identified in Arabidopsis. Here, an Arabidopsis AKT1-like channel (LilKT1) was identified from Lilium longiflorum pollen. Complementation of K+ uptake deficient yeast mutants was only successful when the entire LilKT1 C-terminus was replaced by the AKT1 C-terminus. No signals were observed in the plasma membrane (PM) of pollen tubes after expression of fluorescence-tagged LilKT1 nor were any LilKT1-derived peptides detectable in the pollen PM by mass spectrometry analysis. In contrast, fluorescent LilKT1 partly co-localized with the lily PM H+ ATPase LilHA2 in the PM of tobacco leaf cells, but exhibited a punctual fluorescence pattern and also sub-plasma membrane localization. Thus, incorporation of LilKT1 into the pollen PM seems tighter controlled than in other cells with still unknown trafficking signals in LilKT1’s C-terminus, resulting in channel densities below detection limits. This highly controlled incorporation might have physiological reasons: an uncontrolled number of K+ inward channels in the pollen PM will give an increased water influx due to the raising cytosolic K+ concentration, and finally, causing the tube to burst.

Highlights

A pollen grain landing on the stigma surface generates a rapidly elongating pollen tube that is on target for the ovules which are often located at a far distance
At the pollen grain and partially along the tube shank, an active plasma membrane (PM) H+ ATPase transports H+ into the extracellular medium hyperpolarizing the PM and generating an outward current carried by H+ (Weisenseel and Jaffe, 1976; Obermeyer et al, 1992; Pertl et al, 2001; Certal et al, 2008) while chloride currents are detectable at the tube tip as effluxes and at the tube shank as influxes (Zonia et al, 2002; Messerli et al, 2004)
No K+ channels could be detected, two PM H+ ATPases were identified in a parallel screen of the same cDNA library

Summary

Introduction

A pollen grain landing on the stigma surface generates a rapidly elongating pollen tube that is on target for the ovules which are often located at a far distance. Ion currents (Ca2+, H+, K+, Cl−) surrounding the growing pollen tube, have been identified as pacemakers of the growth rate and controllers of the direction of pollen tubes (Holdaway-Clarke and Hepler, 2003; Michard et al, 2009). Ca2+-dependent anion channels are probably involved in the generation of these Cl− currents (Tavares et al., 2011) Another major component of these currents are potassium ions (Weisenseel and Jaffe, 1976) which enter the pollen tube and leave at the tube tip (Michard et al, 2009). The uptake of K+ is important for tube growth by probably balancing the www.frontiersin.org

Methods

Results

Discussion

Conclusion