Abstract
BackgroundThe actin cytoskeleton is involved in the responses of plants to environmental signals. Actin bundles play the role of tracks in chloroplast movements activated by light. Chloroplasts redistribute in response to blue light in the mesophyll cells of Nicotiana tabacum. The aim of this work was to study the relationship between chloroplast responses and the organization of actin cytoskeleton in living tobacco cells. Chloroplast movements were measured photometrically as changes in light transmission through the leaves. The actin cytoskeleton, labeled with plastin-GFP, was visualised by confocal microscopy.ResultsThe actin cytoskeleton was affected by strong blue and red light. No blue light specific actin reorganization was detected. EGTA and trifluoperazine strongly inhibited chloroplast responses and disrupted the integrity of the cytoskeleton. This disruption was reversible by Ca2+ or Mg2+. Additionally, the effect of trifluoperazine was reversible by light. Wortmannin, an inhibitor of phosphoinositide kinases, potently inhibited chloroplast responses but did not influence the actin cytoskeleton at the same concentration. Also this inhibition was reversed by Ca2+ and Mg2+. Magnesium ions were equally or more effective than Ca2+ in restoring chloroplast motility after treatment with EGTA, trifluoperazine or wortmannin.ConclusionThe architecture of the actin cytoskeleton in the mesophyll of tobacco is significantly modulated by strong light. This modulation does not affect the direction of chloroplast redistribution in the cell. Calcium ions have multiple functions in the mechanism of the movements. Our results suggest also that Mg2+ is a regulatory molecule cooperating with Ca2+ in the signaling pathway of blue light-induced tobacco chloroplast movements.
Highlights
The actin cytoskeleton is involved in the responses of plants to environmental signals
Characteristics of the transgenic tobacco line The expression of plastin-green fluorescent protein (GFP) did not affect the responses of chloroplasts in Nicotiana tabacum
Our results point to the possibility that Mg2+ is a regulatory molecule cooperating with Ca2+ in the signaling pathway of BL-induced tobacco chloroplast movements
Summary
The actin cytoskeleton is involved in the responses of plants to environmental signals. Actin bundles play the role of tracks in chloroplast movements activated by light. Actin cytoskeleton (AC) provides tracks for myosin-mediated movements of organelles in plant cells [1]. On the basis of AC, chloroplasts change their intracellular arrangement in response to light. These movements are controlled only by blue light in higher plants [3]. Weak blue light (wBL) induces an accumulation response in which chloroplasts gather along the cell walls perpendicular to the light direction. The light signal is perceived by phototropins (phot and phot2), blue-light photoreceptors localised at the plasma membrane [4,5] Both phototropins mediate chloroplast accumulation, whereas phot mediates the avoidance response [6,7]. Chloroplasts of several algae, mosses, ferns and aquatic angiosperms respond to red light [8,9]
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