STRUCTURAL BASIS FOR THE RED-LIGHT INDUCED REPOLARIZATION OF TIP GROWTH IN CAULONEMA CELLS OF CERATODON PURPUREUS

Two dynamic changes are associated with the phytochrome-regulated phot otropic response in tip cells of the moss Ceratodon purpureus: a tip-l ocated gradient shift of chlortetracycline (CTC)-stained calcium and a structural reorganization of apical microfilaments (MFs). We examined the interdependence of these processes. Cells were treated with the a ntimicrotubule drug oryzalin, the antimicrofilament drug cytochalasin- D, and the calcium channel blocker nifedipine, respectively. The effec ts on phototropic growth, on the structural alignment of the cytoskele ton (microtubules, MTs; microfilaments) and on the distribution of CTC -stained calcium were studied under each of these conditions. In gravi tropically growing tip cells the apical MFs form a cortical collar-lik e structure, consisting of actin bundles with a parallel axial alignme nt. These MFs point towards the presumptive growing point, a weakly st ained region in the tip of the cell from which bundles are absent. MTs are present in the cortex and in the endoplasm of the tip, predominan tly oriented longitudinally. The MTs converge within the central apex. The cells show a steep tip-to-base CTC-calcium gradient with its high est signal in the central apex. Destruction of MTs by 1 mu M oryzalin induces several translocational effects: (i) the growing zone and phot otropic outgrowth shift from the apex to subapical parts of the cell; (ii) the structural integrity of the apical MFs and the tip-to-base al ignment of the CTC-calcium gradient are disturbed; and (iii) the red l ight induced gradient shift and the reorientation of MFs proceed in an expanded area spanning from the tip to subapical parts of the cell. C ytochalasin-D (10 mu g/ml) destroys the MFs. Under these conditions ti p growth stops and the phototropic outgrowth is suppressed. The apical MT-structure and the CTC-calcium gradient are not influenced by the a gent. Unilateral red light still induces the light-directed translocat ion of the gradient. Tip cells ''memorize'' a unilateral irradiation a pplied during growth inhibition with cytochalasin-D. After recovery in darkness the cells start to grow in the former light direction. The r estoration of the MFs precedes the outgrowth. The structural alignment of the rebuilt actin bundles indicates the future growth direction. T he calcium channel blocker nifedipine (10 mu M) also inhibits tip grow th and concurrently phototropic outgrowth. Nifedipine destroys the CTC -calcium gradient and apical MFs; MTs are not influenced by the channe l blocker.