Physiological Mechanism of the Auxin-Induced Increase in Light Sensitivity of Phytochrome-Mediated Growth Responses in Avena Coleoptile Sections

Abstract

The physiology of the auxin-induced 10,000-fold increase in light sensitivity of a phytochrome-mediated growth response (Shinkle and Briggs, 1984 Proc Nati Acad Sci USA 81: 3742-3746) has been characterized in subapical coleoptile sections from dark-grown oat (Avena satita L. cv Lodi) seedlings. Six micromolar indole-3-acetic acid (IAA) must be present for 1 hour before to 2 hour after irradiation in order to confer maximal sensitivity to light. The direct effect of IAA on growth can be separated from its effect on light sensitivity. Several classes of synthetic auxins will substitute for IAA in inducing an increase in sensitivity to light, as will both the phytotoxin fusicoccin and treatment of sections with pH 4.5 buffer. The increase in sensitivity to light induced by 6 micromolar IAA is completely inhibited by buffering the sections at pH 5.9 with 30 millimolar 24N-morpholino)ethanesulfonic acid. These findings suggest that the capacity to respond to very low fluences of light is regulated by extracellular pH.Between 10 and 15 millimolar K will inhibit the induction of the increased sensitivity to light, independent of the mechanism of induction. The effect of K appears to be specific to the process-by which the sections respond to very low levels of light.

Photobiology of Phytochrome-Mediated Growth Responses in Sections of Stem Tissue from Etiolated Oats and Corn

“…For both panels, the R fluence used was sufficient to saturate the LF response. In Figure 2A, the treatment with pH 4.5 evokes a VLF response (14), causing FR to induce 50% ofthe maximum capacity for light-induced elongation. The reversal of the Rinduced growth by FR is to the level of elongation induced by FR alone.…”

“…Figure 1 shows the fluence-response data for the reversal of the R-induced increase in coleoptile section length by FR. The incubation conditions (pH 5.9, no IAA) were such that Rinduced growth is potentiated almost exclusively by the LF response ( 14). The R fluence used was a fluence which saturates the LF response.…”

“…The fluorescent light source previously referred to as 'B' (14) was used for the R irradiations in the FR reversal experiments, and was also used for the VLF and LF R irradiations, using the protocols described. For irradiations with FR, a custom-built 1000 W light source was used.…”

“…A major finding was that alterations in the physiological state of the coleoptile cells can alter phytochrome signal transduction processes considerably. Decreasing the extracellular pH in section tissue caused the sensitivity of the sections to R3 to increase by as much as a factor of 10,000 (13,14). Certain assumptions about the responses seen have been made in interpreting the results, and assessing their significance.…”

“…In previous papers, the physiology of phytochrome-mediated stimulation of coleoptile elongation induced by red light, both in terms of the nature of the change in elongation rate (15), and the nature of the coupling of the phytochrome signal to the response (13,14) were described. A major finding was that alterations in the physiological state of the coleoptile cells can alter phytochrome signal transduction processes considerably.…”