Red Light-inhibited Mesocotyl Elongation in Maize Seedlings

Abstract

Red light-hibited mesocotyl elongation, which occurs in intact Zea mays L. seedlngs, was studied in excised segments which Included the coleoptile (or parts therefrom) and apical centimeter of the mesocotyl. Expriments took into account, first, the abiUty of the segments to regenerate auxin supply sites, and, second, that auxin uptake can be greatly reduced if there is no cut surface, apical to the ekngating cells, to act as a port of entry. In all cases, auxin completely reversed the inhibion of elongation by light. The results support the hypotbesis that lit regulates mesocotyl elongation by controing auxin supply from the coleoptHe. Sucrose concentration had no effect on auxin reversal of light-inhited elongation, but relatively high concentrations of gibbereHlic acid (10 PM) could substitute for auxin in this system. elongation in grass seedlings, however, has long been known to be a response to red light only (3, 10, 12, 29, 30). As part of a general characterization of light-inhibited elonga-tion, we performed experiments which tested van Overbeek's theory of light-inhibited growth. Certain artifactual results of past work were avoided by using excised segments which included just the coleoptile and the apical cm of mesocotyl (or parts therefrom), by taking into account the fact that auxin-producing centers can be regenerated in decapitated coleoptiles (5, 8, 25) and the recognizing that auxin uptake can be greatly reduced if there is no cut surface, apical to the elongating cells, to act as a port of entry (1). We have concluded that light-inhibited elongation can be anti-doted by exogenous auxin, consistent with the theory as originally presented by van Overbeek. The growth of a seedling germinating in the dark is affected by illumination in two important ways. One of these, phototropism, has been extensively studied and reviewed (e.g. 7). The second, light-inhibited shoot elongation, has been somewhat less studied, but is equally important in the economy of the plant. Such inhibition of shoot elongation has been known since 1894 (21). However, it was not until 1936, after the discovery of auxin, that a mechanism for this inhibition was proposed. Van Overbeek (28) suggested that light regulated mesocotyl elongation by controlling auxin supply from the coleoptile. He showed a significant reduction in production of diffusible auxin from coleoptile tips when the etiolated seedlings were exposed to 3 hr of room light. The cells containing the photoreceptor would thus be remote from the responding tissue. His proposal was supported by Inge and Loomis (11) who antidoted white light inhibition of mesocotyl elongation in corn seedlings by applying auxin paste to the cut surfaces of coleoptiles from which the tips were excised. Goodwin (10) suggested that the inhibition must be more complex, proposing an effect of light on cell division as well as cell elongation; and Schneider (23) obtained some light inhibition of isolated meso-cotyl sections. Mer (18) also failed to obtain evidence in support of the hypothessis, although he used white light and exposures of up to a week so the experiments are hardly comparable. Galston and Hand (9), working with etiolated peas stem sections, proposed that the inhibition of elongation did not involve auxin at all. A combination of primitive light sources and optical filters clearly limited the early work. With seedlings of dicots, both blue and red light may inhibit hypocotyl elongation (4, 17, 27) and the responses may be quite different and easily separable either in time (17) or by site of perception (4). Inhibition of mesocotyl ' C.) were sown dry on two layers of water-saturated absorbant-paper (Kimberly-Clarke, supercrepe Kimpack). Growth was in complete darkness at 30 C and 80%o relative humidity. At age 72 hr segments which included the coleoptile (or parts therefrom) and the apical cm of mesocotyl were excised in dim green light (500-560 nm, 530 maximum) and stored for not more than I hr in 1% sucrose on ice. Segments were floated in open Petri dishes on 10 mm KH2PO4 (pH 6) containing 30 mm sucrose, 77 AM chloramphenicol, and various concentrations of IAA. The mesocotyl portion of the segment was measured to the nearest 0.5 mm after 16 hr at 30 C in darkness or under red light. Irradiation with fluorescent tubes (Sylvania warm white) passed through red plastic (Rohm and Haas, No. 2423) gave approximately 1,750 ergs cm-2 sec-' at tissue level (590-730 nm, maximum-630 nm). When intact seedlings were grown in the light (Fig. 1) the identical light source was used with from 1,600 to 1,800 ergs cm-2 sec' at seed level. The mesocotyl was measured at 24-hr intervals. RESULTS AND DISCUSSION The mesocotyl of intact plants elongated rapidly when the corn seedlings were grown in the dark, but hardly at all when grown in red light. When seedlings were transferred from darkness to red light at 3 or 4 days the mesocotyl elongation rate decreased almost to zero (Fig. 1). This inhibition of mesocotyl elongation by red light also could be demonstrated in excised tissue (Fig. 2). The experiment presented in Figure 2 leads to several conclusions. (a) Elongation of the 1-cm mesocotyl segment (intact co-leoptile attached) was 56% inhibited by light. (b) Removing the 3-mm coleoptile tip, the site of auxin supply, did not completely eliminate the dark elongation increment. It is well known (5, 8, 25) that an actively exporting auxin supply site can be regenerated in decapitated coleoptiles; this regeneration process (or perhaps auxin export by, or transport from, the newly regenerated auxin supply site) may also be inhibited by light. (c) E*tn in the 534