A phytochrome-regulated phototropic response of the mossCeratodon purpureuswas investigated. Chlorotetracycline (CTC) was used to visualize membrane-associated calcium gradients in the tip cell of moss caulonemal filaments. A tip-to-base Ca2+gradient was observed. The ionophore monensin rapidly inhibited the growth of the tip cell and abolished the CTC fluorescence. Six hours after transferring to inhibitor-free medium, protonemal growth resumed and reached the normal growth rate within 12 h. The growth was accompanied by a reappearance of the CTC-fluorescence gradient. Unilateral irradiation given during the monensin treatment or after the treatment during the period when growth inhibition persisted led, with the re-initiation of growth, to a typical positive phototropic bending in complete darkness. Far-red light applied just before the growth response started, or during growth inhibition, abolished the phototropic response. The phytochrome-mediated signal was qualitatively (position) and quantitatively (degree of bending) memorized. Signal perception and response could be separated temporally. This result indicates that at least under some circumstances, e.g. under the influence of monensin, the phytochrome-mediated signal can be stored for several hours in darkness. Calcium seems to be essential for the processing of polar growth only. A specific function (second messenger) in phytochrome-dependent signal transduction could not be confirmed.
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