Similarity between Cytokinin and Blue Light Inhibition of Cucumber Hypocotyl Elongation

Abstract

The cytokinin benzyladenine inhibited endogenous hypocotyl elongation in intact etiolated seedlings of cucumber (Cucumis sativus L.). In hypocotyl segments, the inhibitory effect of benzyladenine on growth was clearly detectable in the presence of indoleacetic acid. Fusicoccin-induced elongation was unaffected by the presence of cytokinin. The effect of cytokinin on elongation of the segments was determined by measuring changes in fresh weight, a linear function of extension growth. The effect of benzyladenine on hypocotyl growth was at least as large in segments prepared from red-light-grown seedlings as in those from seedlings grown in total darkness. A comparison was made between the inhibitory effects of cytokinin and blue light. The use of the calcium chelator ethyleneglycol-bis(jB-aminoethyl ether)-N, N'-tetraacetic acid indicated that calcium ions are required for manifestation of benzyladenine-induced inhibition.on cucumber cotyledon expansion, and on inhibition of greening, than either inducer alone (4). In the same system, red light or cytokinin alone caused maximal acceleration of greening (4). The presence of cytokinin in the incubation medium was found to inhibit auxin-promoted extension growth of segments in several different species (7,18,24,26).We have studied the effects of BA3, a synthetic cytokinin, on stem elongation in dark-grown cucumber seedlings. We investigated whether BA would inhibit IAA-induced extension growth of hypocotyl segments. In addition, we compared suppression of growth by cytokinin and by blue light. Furthermore, we examined whether the cytokinin-induced inhibition of hypocotyl elongation depends on calcium, as reported for inhibition by blue light (23). In all these studies we adopted the technically easy and accurate method developed for assaying the effect of IAA on elongation based on cucumber hypocotyl weight increase (10). MATERIALS AND METHODSThe suggestion that plant hormones might serve as second messengers in the signal response cascade is supported by the fact that many light-regulated plant growth responses are reported either to involve changes in hormone level or to be mimicked by hormone application (for review, see 6,15,17). Photocontrol of stem elongation has been widely explored and documented (see refs. 6, 13). The effect of cytokinins on shoot elongation is less well known. Photoinhibition of stem elongation operates through at least two photoreceptors, phytochrome and cryptochrome(s) (1,5,11,12,14

Cytokinin, Acting through Ethylene, Restores Gravitropism to Arabidopsis Seedlings Grown under Red Light

“…Under white light gravitropism is normal; phototropism or other effects of blue light apparently counteract the disorientation that occurs in red light (Liscum and Hangarter, 1993). Because cytokinins are known to mimic the effect of light, inhibiting hypocotyl elongation (Chory et al, 1991;Cohen et al, 1991;Gary et al, 1995;Su and Howell, 1995), we tested whether the synthetic cytokinin BA would modulate the randomization of gravitropism under red light.…”

“…Chloroplast development, which normally depends both on the inductive actions of light and on chlorophyll synthesis, proceeds at least partially in the dark when cytokinins are applied (Kasemir and Mohr, 1982;Chory et al, 1991). Cytokinin also inhibits hypocotyl elongation and acts inde-pendently and additively with light, in the sense that neither factor can cause further inhibition under conditions in which the other factor is saturating (in cucumber, Cohen et al, 1991;in Arabidopsis, Cary et al, 1995;Su and Howell, 1995). Recently, it was shown that cytokinin inhibits the elongation of Arabidopsis hypocotyls by increasing ethylene levels (Cary et al, 1995;Su and Howell, 1995).…”

Cytokinin affects circadian‐clock oscillation in a phytochrome B‐ and Arabidopsis response regulator 4‐dependent manner

“…Light and cytokinin are known to coregulate many developmental processes (Miller 1956, Huff and Ross 1975, Cohen et al 1991, Chory et al 1991, 1994, Thomas et al 1997). The best‐known example is the observation that the photomorphogenic process is also regulated by cytokinin.…”

“…The best‐known example is the observation that the photomorphogenic process is also regulated by cytokinin. When germinated in the presence of cytokinin in the dark, wild‐type (WT) Arabidopsis seedlings display a de‐etiolated phenotype similar to that of constitutive photomorphogenic mutants cop/det/fus (Cohen et al 1991, Chory et al 1994, Su and Howell 1995). As no alterations of cytokinin levels were found in det1 and det2 (Chory et al 1994), this phenotypic similarity is presumably due to a crosstalk between these two pathways, in which cytokinin is presumably a positive regulator of photomorphogenesis.…”

The Effects of Cytokinin and Light on Hypocotyl Elongation in Arabidopsis Seedlings Are Independent and Additive

“…The interaction between cytokinin and light responses was first reported by Miller (1956), who found that cytokinin (kinetin) in the dark could elicit several responses that were induced by red Iight, such as leaf disc expansion in bean and seed germination in lettuce. Other investigators have described the interaction between light and cytokinin responses in the context of processes such as betacyanin synthesis (Koehler, 1972), anthocyanin accumulation (Kasemir and Mohr, 1982), hypocotyl elongation (Cohen et al, 1991), chloroplast development, and gene expression (Feierabend and de Boer, 1978;Tobin, 1986, 1988;Longo et al, 1990;Abdelghani et al, 1991;Reski et al, 1991;Smart et al, 1991;Smigocki, 1991;Kusnetov et al, 1994). However, it is not known at what leve1 cytokinin and light responses interact.…”