Plants absolutely do "see" green light. It's how they "know" that they are being overcrowded, and causes stem elongation.
Plant Physiology
American Society of Plant Biologists
Green Light Stimulates Early Stem Elongation, Antagonizing Light-Mediated Growth Inhibition1
Kevin M. Folta
Additional article information
ABSTRACT
During the transition from darkness to light, the rate of hypocotyl elongation is determined from the integration of light signals sensed through the phototropin, cryptochrome, and phytochrome signaling pathways. In all light conditions studied, from UV to far-red, early hypocotyl growth is rapidly and robustly suppressed within minutes of illumination in a manner dependent upon light quality and quantity. In this study, it is shown that green light (GL) irradiation leads to a rapid increase in the growth rate of etiolated Arabidopsis seedlings. GL-mediated growth promotion was detected in response to constant irradiation or a short, single pulse of light with a similar time course. The response has a threshold between 10−1 and 100
μmol m−2, is saturated before 102
μmol m−2 and obeys reciprocity. Genetic analyses indicate that the cryptochrome or phototropin photoreceptors do not participate in the response. The major phytochrome receptors influence the normal amplitude and timing of the GL response, yet the GL response is normal in seedlings grown for hours under constant dim-red light. Therefore, phytochrome activation enhances, but is not required for, the GL response. Seedlings grown under green, red, and blue light together are longer than those grown under red and blue alone. These data indicate that a novel GL-activated light sensor promotes early stem elongation that antagonizes growth inhibition.
The first sensing of light transitions the etiolated seedling into a developmental program that prepares the plant for autotrophy. This process, photomorphogenesis, is typified by changes at the biochemical, molecular, and physiological levels that guide early plant morphology during establishment. One of the most conspicuous changes to occur during photomorphogenic development is an inhibition of hypocotyl (stem) growth rate. Ultraviolet, blue, red, and far-red light each rapidly inhibit stem growth within minutes of irradiation (
Meijer, 1968;
Gaba et al., 1984;
Spalding and Cosgrove, 1989), making this rapid response an excellent reporter of light sensing and signal integration.
High-resolution imaging techniques have allowed monitoring of the growth inhibition process with high temporal resolution in the miniscule Arabidopsis seedling. These methods facilitated genetic tests to describe two critical parameters of the growth inhibition response: first, which photosensors mediate early growth inhibition, and second, precisely when specific photosensors contribute to this rapid response. These studies demonstrated that growth inhibition is dependent upon contributions from phytochromes, phototropins, and cryptochromes, often acting in a sequential and orchestrated manner (
Parks et al., 2001a). In red light, growth inhibition is first imparted through phytochrome A (phyA) activation for 3 h before phytochrome B (phyB) exerts its influence and the effect of phyA wanes (
Parks and Spalding, 1999). In response to blue light, inhibition occurs in at least two distinct phases that can be separated genetically, as well as by time course and fluence response (
Folta et al., 2003b). The primary phase of growth inhibition is mediated by phototropin 1 (phot1;
Folta and Spalding, 2001a), the autophosphorylating Ser-Thr kinase that mediates phototropism (
Huala et al., 1997;
Christie et al., 1998). The second phase of growth inhibition requires cryptochrome 1 (cry1) and cryptochrome 2 (cry2) as well as phyA and initiates after 30 min of continuous irradiation of 100
μmol m−2 s−1 (
Parks et al., 1998;
Folta and Spalding, 2001a,
2001b). In all cases studied, irradiation with monochromatic light induces growth inhibition. The timing of inhibition coincides closely with the translocation of phyA and phyB to the nucleus in red or far-red light (
Hisada et al., 2000;
Kircher et al., 2002), phototropin and cryptochrome phosphorylation in blue light (
Reymond et al., 1992;
Shalitin et al., 2002), as well as alterations in the global gene expression (
Ma et al., 2001;
Tepperman et al., 2001;
Folta et al., 2003a).
Monochromatic green light (GL) has been shown to act as a signal in regulating specific facets of plant physiology, inhibiting seedling mass, plant cell culture growth, and light-induced gravitropic root elongation (Klein, 1992). Recently it has been shown that GL can reverse blue light-induced stomatal opening (Frechilla et al., 2000; Talbott et al., 2002, 2003; Eisinger et al., 2003). The GL response is mediated through a yet-to-be-defined photosensor, and genetic analyses suggest the response to be zeaxanthin based (Frechilla et al., 1999; Zeiger, 2000). Plant responses to GL may be initiated through known light sensors. Phytochromes and cryptochromes absorb GL and possibly influence light-induced events (Mandoli and Briggs, 1981;Lin et al., 1995b; Liscum and Briggs, 1995; Swartz et al., 2001). However, the action/response spectra for GL-induced responses exhibit a peak between 540 to 550 nm (Klein, 1964,1979; Steinitz et al., 1985; Reymond et al., 1992; Frechilla et al., 2000) and thus are incongruous with the absorption spectra for phytochromes, cryptochromes, and phototropins and the action spectra for the responses they govern (Christie et al., 1998; Ahmad et al., 2002). GL signals may also be a consequence of low-level coactivation of multiple sensory systems that together guide atypical physiological outcomes (Pepper et al., 2001).
In this report, high-resolution analyses of early growth kinetics have identified that GL irradiation causes a rapid increase in early stem elongation rate, a response that is contrary to that induced by all other light conditions studied. The transient growth promotion is evident within 15 min of irradiation, its magnitude is regulated in a dose-dependent manner, and it cannot be completely attributed genetically or photophysiologically to the described action of known photoreceptors. This report presents photophysiological and genetic characterization of a novel response to narrow-bandwidth GL.