External Ca Is Essential for Chloroplast Movement Induced by Mechanical Stimulation But Not by Light Stimulation

In the fern Adiantum capillus-veneris, chloroplast movement is induced by mechanical stimulation as well as by light stimulation. Directional movement of both types depends on an actin-based motile system. To investigate the physiological relationship between mechanical and light signaling in the regulation of chloroplast movement, we examined the mechanoresponse of chloroplasts whose motility had been already restricted after photo-relocation. Chloroplast mechano-avoidance movement was induced under all of the photo-relocation conditions tested, indicating that mechano-specific signals generated by mechanical stimulation dominate over the light signals and reactivate the motility of chloroplasts. When the effects of external Ca on the induction of mechanoand light responses were examined, strikingly different requirements of external Ca were found for each. In medium without Ca , the mechano-response was suppressed but no effects were observed on photo-response. Mechano-relocation movement of chloroplasts was inhibited by 100 m lanthanum (La ), a plasma membrane calcium channel blocker, and by 10 m gadolinium (Gd ), a stretch-activated channel blocker. However, the same concentrations of these drugs did not affect the photo-relocation movement at all. These results suggest that the influx of external Ca is crucial for the early signaling step of chloroplast mechano-relocation but not for that of photo-relocation. This is the first report showing the separation of signaling pathways in mechanoand photo-relocation of chloroplasts.