The role of calcium in blue-light-dependent chloroplast movement in Lemna trisulca L.
Tlałka M., Fricker M.
Chloroplast movements are a normal physiological response to changes in light intensity and provide a good model system to analyse the signal transduction pathways following light perception. Blue-light-dependent chloroplast movements were observed in Lemna trisulca using confocal optical sectioning and 3-D reconstruction or photometric measurements of leaf transmission. Chloroplasts moved away from strong blue light (SBL) towards the anticlinal walls (profile position), and towards the periclinal walls (face position) under weak blue light (WBL) over about 20-40 min. Cytoplasmic calcium ([Ca 2+ ](cyt)) forms part of the signalling system in response to SBL as movements were associated with small increases in [Ca 2+ ](cyt) and were blocked by antagonists of calcium homeostasis, including EGTA, nifedipine, verapamil, caffeine, thapsigargin, TFP (trifluoperazine), W7 and compound 48/80. Treatments predicted to affect internal Ca 2+ stores gave the most rapid and pronounced effects. In addition, artificially increasing [Ca 2+ ](cyt) in darkness using the Ca 2+ ionophore A23187 and high external Ca 2+ (or Sr 2+ ), triggered partial movement of chloroplasts to profile position analogous to a SBL response. These data are all consistent with [Ca 2+ ](cyt) acting as a signal in SBL responses; however, the situation is more complex given that both WBL and SBL responses were inhibited to a similar extent by all the Ca 2+ -signalling antagonists used. As the direction of chloroplast movement in WBL is exactly opposite to that in SBL, we conclude that, whilst proper regulation of [Ca 2+ ](cyt) homeostasis is critical for both SBL and WBL responses, additional factors may be required to specify the direction of chloroplast movement.