Second Positive Phototropism in the Avena Coleoptile

Abstract

Abstract. A method has been developed whereby the second positive phototropism can be observed separately from the first positive and negative phototropic responses w-hioh also oocur in oat coleoptiles. Although the second positive phototropic response has often been referred to as the base response, photoreception for it is shown to occur mainly in the apical 3 mm of the coleoptile. The Bunsen-Roscoe reciprocity law, so typical of first positive phototropism, does not apply to the second positive responses, and the amount of curvature increa.ses linearly with the duration of the stimulus. However, although this linear proportionality between stimulus duration and response is the major factor determining response at all intensities tested, the inten,sity of the stimulus does influence the Tesponse somewhat. The action spectrum for the response shows no activity above 510 nm and 'has peaks at 375 and 450 nm. In all but one particular it closely resembles the aotion spectrum ifor the first positive phototropism, and it is conoluded that the same, or similar, pigments may well be the photoreceptors for both types of response. The identity of this blue light absorbing pigment is not known.De-spite the availability of action spectra for the first positive phototropism in oats (15,17). the photoreceptor for this response has not been definitely characterized (9,17). Furthermore, the earlx experiments on phototropism demonstrated that in addition to first positive phototropic curvatures, oat coleoptiles also exhibit negative and second positive types of response (1, 4). From a study of th-e doseresponse curves for phototropism in Avena and of theoretical models for these data, Zimmerman and Briggs (19,20) suggested that Avena coleoptiles possess 3 different photoreceptor systems for the 3 types of phototropic response-first positive, negative, and second positive. If this be the case, then the photoreceptive pigment for the second positive type of curvature would he di'fferent from that for the first positive phototropism and might be identified from an action spectrum for the secolnd positive response. For this reason, and in order to characterize the little-known second positive response more clearly, the present experiments on second positive p)hototropism in coleoptiles of Are(nMa wer-e undertaken.Although the second positive phototropic response, which occurs with large stimulus energies or long stimulus durations, has not beeli extensively studied, there are strong indications that the B3unsen-Roscoe reciprocity law, which holds for first positive phototropism, is not applicable to it (2, 5. 17 ((20) subtracted the responses predicted by their theor-etical models for first positive and negative l)hototropism from their phototropic dose-response curves, the remaining responses, assume(d to be of the second positive type, increased linearly xvith stimulus duration and were not affected by a 10-fol(d vtariation in intensity. Unfortunately the overlap of first positive responces bv second positive otnes when low i...

Exposure of Oat Seedlings to Blue Light Results in Amplified Phosphorylation of the Putative Photoreceptor for Phototropism and in Higher Sensitivity of the Plants to Phototropic Stimulation

“…6A) when compared with nonpreirradiated plants. About 70% of the investigated plants showed average degrees of curvature of 20" when phototropic stimulation was performed with blue light dosages known to induce neither FPPC nor SPPC (Everett and Thimann, 1968) in control plants. The observed bending reaction is obviously not the result of desensitization, i.e.…”

“…Therefore, photosensing for SPPC can occur along the entire length of the coleoptile and does not predominantly take place in the apex. The BunsenRoscoe reciprocity law is valid for FPPC and FNPC, but does not hold for phototropic reactions of the secondpositive type (Briggs, 1960;Zimmermann and Briggs, 1963;Everett and Thimann, 1968). Despite the great number of physiological studies, we know little about the nature of the photoreceptor and the particular steps within the phototropic signaling pathway.…”

“…The most extensive studies on phototropic-curvature reactions were done on the coleoptile of etiolated oat (Avena sativa L.) seedlings, a classic object of phototropism research. From the fluenceresponse curves obtained by unilateral irradiation of oat coleoptiles with blue or white light at least three different types of bending reactions have been distinguished with respect to the light-stimulus intensity and duration (Briggs, 1960;Zimmermann and Briggs, 1963;Everett and Thimann, 1968;Meyer, 1969aMeyer, , 1969bIino, 1988a). Pulse irradiations at low fluences result in FPPC, whereas longer irradiation times are needed to induce SPPC.…”

“…The main characteristic of this response can be summarized as follows: Increasing the time of irradiation results in a continual shift of the reaction toward lower total fluences, and SPPC, therefore, must be considered as a time-dependent rather than a fluence-dependent response. Although different physiological approaches have been used to investigate this phenomenon (Zimmermann and Briggs, 1963;Everett and Thimann, 1968;Steinitz et al, 1988), nothing is known about the possible biochemical and molecular processes accounting for the observed time dependence of SPPC.…”

“…FPPC is mainly restricted to the extreme tip of the coleoptile, the most photosensitive tissue. Since SPPC can be induced when only the apical part of the coleoptile is phototropically stimulated, it has been postulated that the tip may also be the site of photoperception for this type of curvature, and that the subsequent differential growth occurring along the entire coleoptile base is mainly controlled by the apex (Zimmermann and Briggs, 1963;Everett and Thimann, 1968). However, more recent studies clearly demonstrated that, at least for oat, each zone of the base can perceive and respond to unilateral blue light irradiation (Macleod et al, 1984).…”