Functional Plant Biology Functional Plant Biology Society
Plant function and evolutionary biology
RESEARCH ARTICLE

Actin filaments modulate hypoosmotic-responsive K+ efflux channels in specialised cells of developing bean seed coats

Wen-Hao Zhang A D , John W. Patrick B and Stephen D. Tyerman C
+ Author Affiliations
- Author Affiliations

A Key Laboratory of Vegetation and Environmental Change, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, PR China.

B School of Biological and Chemical Sciences, The University of Newcastle, Newcastle, NSW 2308, Australia.

C School of Agriculture, Food and Wine, The University of Adelaide, PMB#1 Glen Osmond, SA 5064, Australia.

D Corresponding author. Email: whzhang@ibcas.ac.cn

Functional Plant Biology 34(10) 874-884 https://doi.org/10.1071/FP07138
Submitted: 31 May 2007  Accepted: 24 July 2007   Published: 13 September 2007

Abstract

In developing bean (Phaseolus vulgaris L.) seeds, nutrients move in the symplasm from sieve elements to ground-parenchyma cells where they are transported across the plasma membrane into the seed apoplasm. Release of nutrients to the seed apoplasm is related to the osmotic conditions of the apoplasm. A hypoosmotic solution, resulting from enhanced uptake of nutrients by cotyledons, stimulates nutrient release from seed coat to the apoplasm. We investigated hypoosmotic nutrient release by examining the ionic membrane currents that respond to hypoosmotic treatment in protoplasts derived from three important cell types that occur at the seed coat–cotyledonary boundary. A non-selective but predominantly K+ efflux current that displayed a distinct time-dependent inactivation was elicited by membrane depolarisation under hypoosmotic conditions only in ground-parenchyma protoplasts. Hypoosmotic treatment had little effect on whole-cell ionic currents in protoplasts derived from coat chlorenchyma cells and cotyledon dermal cells. The inactivating K+ efflux current was elicited under isosmotic conditions by treatment with cytochalasin D, which disrupts actin filaments. Hypoosmotic treatment and cytochalasin D failed to induce the K+ current in ground-parenchyma protoplasts in the presence of the actin stabiliser, phalloidin. The net efflux of K+ from intact seed coats was enhanced by hypoosmotic treatment and cytochalasin D, and the stimulation of K+ efflux induced by the hypoosmotic treatment and cytochalasin D was abolished by phalloidin. A bursting Cl channel previously described showed a similar pattern of responses. These results suggest that hypoosmotic-dependent KCl efflux from seed coats is mediated by the inactivating K+ outward current and bursting Cl channel, and that actin filaments act as components of the transduction process that is a function of cell volume.

Additional keywords: cytoskeleton, hypoosmotic treatment, K+ outward current, patch clamp, Phaseolus vulgaris, seed coat.


Acknowledgements

This study was supported by Australian Research Council grants to S.D. Tyerman and J.W. Patrick, Natural Science Foundation of China (No. 30521002 and No. 30570136) and Chinese Academy of Sciences’ One Hundred Talent Project. We thank Wendy Sullivan for her excellent technical assistance.


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