Relocalization of the calcium gradient and a dihydropyridine receptor is involved in upward bending by bulging of Chara protonemata, but not in downward bending by bowing of Chara rhizoids

The localization of cytoplasmic free calcium and a dihydropyridine (DHP) re ceptor, a putative calcium channel, was recorded during the opposite gravir esponses of tip-growing Chara rhizoids and Chara protonemata by using the c alcium indicator Calcium Crimson and a fluorescently labeled dihydropyridin e (FL-DHP). In upward (negatively gravitropically) growing protonemata and downward (positively gravitropically) growing rhizoids, a steep Ca gradient and DHP receptors were found to be symmetrically localized in the tip. How ever, the localization of the Ca2+ gradient and DHP receptors differed cons iderably during the gravitropic responses upon horizontal positioning of th e two cell types. During the graviresponse of rhizoids, a continuous bowing downward by differential flank growth, the Ca2+ gradient and DHP receptors remained symmetrically localized in the tip at the centre of growth. Howev er, after tilting protonemata into a horizontal position, there was a drast ic displacement of the Ca2+ gradient and FL-DHP to the upper flank of the a pical dome. This displacement occurred after the apical intrusion and sedim entation of the statoliths but clearly before the change in the growth dire ction became evident. In protonemata, the reorientation of the growth direc tion started with the appearence of a bulge on that site of the upper flank which was predicted by the asymmetrically displaced Ca2+ gradient. With th e upward shift of the cell tip, which is suggested to result from a statoli th-induced displacement of the growth centre, the Ca2+ gradient and DHP rec eptors became symmetrically relocalized in the apical dome. No major asymme trical rearrangement was observed during the following phase of gravitropic curvature which is characterized by slower rates of bending. Labeling with FL-DHP was completely inhibited by a non-fluorescently labeled dihydropyri dine. From these results it is suggested that FL-DHP labels calcium channel s in rhizoids and protonemata. In rhizoids, positive gravitropic curvature is caused by differential growth limited to the opposite subapical flanks o f the apical dome, a process which does not involve displacement of the gro wth centre, the calcium gradient or calcium channels. In protonemata, howev er, it is proposed that a statolith-induced asymmetrical relocalization of calcium channels and the Ca2+ gradient precedes, and might mediate, the rea rrangement of the centre of growth, most likely by the displacement of the Spitzenkorper, to the upper flank, which results in the negative gravitropi c reorientation of the growth direction.