The role of calcium in blue-light-dependent chloroplast movement in Lemna trisulca L.

Chloroplast movements are a normal physiological response to changes in lig ht intensity and provide a good model system to analyse the signal transduc tion 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. C hloroplasts 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 ([Ca2 +](cyt)) forms part of the signalling system in response to SBL as movement s were associated with small increases in [Ca2+](cyt) and were blocked by a ntagonists of calcium homeostasis, including EGTA, nifedipine, verapamil, c affeine, thapsigargin, TFP (trifluoperazine), W7 and compound 48/80. Treatm ents predicted to affect internal Ca2+ stores gave the most rapid and prono unced effects. In addition, artificially increasing [Ca2+](cyt) in darkness using the Ca2+ ionophore A23187 and high external Ca2+ (or Sr2+), triggere d partial movement of chloroplasts to profile position analogous to a SBL r esponse. These data are all consistent with [Ca2+](cyt) acting as a signal in SBL responses; however, the situation is more complex given that both WB L and SBL responses were inhibited to a similar extent by all the Ca2+-sign alling antagonists used. As the direction of chloroplast movement in WBL is exactly opposite to that in SBL, we conclude that, whilst proper regulatio n of [Ca2+](cyt) homeostasis is critical for both SBL and WBL responses, ad ditional factors may be required to specify the direction of chloroplast mo vement.