Indole-3-acetic acid sensitization of phytochrome-controlled growth of coleoptile sections

Abstract

Addition of 6 ,iM indole-3-acetic acid (IAA) to incubation buffer increases the sensitivity of coleoptile sections cut from dark-grown Avena sativa L. cv. Lodi to red light by a factor of 10,000, relative to the response in the absence of added IAA, without changing the maximum amount of lightinduced growth. From 0.03 to 4 IuM IAA sections show at least a 100-fold increase in sensitivity to red light relative to the response in the absence of added IAA. In this IAA concentration range, the light-induced increase in elongation shows two phases of response to red-light fluence, which are separated by a plateau. The biphasic fluence-response curve is also characteristic of the red-light-induced stimulation of coleoptile growth in intact dark-grown seedlings. The effect of IAA on the sensitivity of the phytochrome-mediated growth response appears to be on some step in the transduction of the phytochrome signal, rather than on the growth response itself.Brief irradiation with red light stimulates coleoptile elongation and inhibits mesocotyl elongation in etiolated oat (Avena sativa) seedlings (1, 2). Phytochrome control of such photomorphogenetic growth responses in etiolated cereal seedlings has been studied extensively in whole plants (see ref. 1 for review). Consistent and reproducible results for red-light-induced growth responses have been obtained by performing experiments on intact etiolated oat seedlings grown and handled in complete darkness without the use of green "safelights" (1, 2).Auxins have long been thought to mediate the red-lightinduced changes in growth in etiolated cereal seedlings (3)(4)(5). Cut sections of etiolated tissue have been used to study the phytochrome/auxin interaction, but findings have been inconclusive. In one such study (3), it was reported that indole-3-acetic acid (IAA), a naturally occurring auxin, must be present in the incubation buffer for red-light-stimulated growth and reversal by far-red light to be seen. This finding has not been replicated in subsequent studies (4, 6).There have also been difficulties in comparing the response of sections to the responses to red light seen in intact plants. Studies of the fluence dependence of the red-lightinduced increase in growth in coleoptile sections cut from etiolated seedlings (4, 6) do not report the biphasic fluenceresponse curve characteristic of the whole tissue responses (1, 2). In one of these studies (6), only the less sensitive phase of the red-light response found in whole plants was reported. In another study (4), only the more sensitive phase of the red-light response was detected. The complete lack of the less sensitive response generally seen (1, 2, 6) is unique to this study (4). Neither of the studies of section growth responses to red light (4, 6) included comparable experiments with intact plants.To investigate some of the possibilities suggested by earlier studies, we have adapted the techniques of Mandoli and Briggs (1) for the study of oat coleoptile sections cut in the dark from seedlings grown in co...

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

“…Although the variability ofthe growth responses in the presence of IAA was somewhat high, the pattern shown was seen in all four replicates. The induction of a VLF response by IAA is very similar to the effect of IAA in increasing the light sensitivity of Zea coleoptile sections (13).…”

“…The photobiology of the effects of FR on phytochrome responses has been used to examine similar assumptions to those mentioned above (1-3, 5, 6). The initial study of the FR reversibility of R-induced elongation (13) merely confirmed that the LF response was FR reversible in this system, and that growth in response to R in the VLF range was not reversible by FR. The sensitivity of the LF response to FR under different conditions was not examined, and the kinetics of the escape of the LF R-induced growth from FR reversibility were not obtained.…”

“…Action spectroscopy studies of two such responses (1,16,17) suggest that the VLF responses may be phytochrome-mediated. The previous finding that the VLF and LF responses share control of the light-induced growth in a reciprocal manner (13) also suggests that the VLF may be phytochrome-mediated, and a similar relationship between VLF and LF responses has been found for phytochrome regulation of photodormancy in seeds (20).…”

“…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.…”