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what is the role of gibberellin in rosette plants?
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Long day (LD) exposure of rosette plants causes rapid stem/petiole elongation, a more vertical growth habit, and flowering; all changes are suggestive of a role for the gibberellin (GA) plant growth regulators. For Arabidopsis (Arabidopsis thaliana) L. (Heynh), we show that enhancement of petiole elongation by a far-red (FR)-rich LD is mimicked by a brief (10 min) end-of-day (EOD) FR exposure in short day (SD). The EOD response shows red (R)/FR photoreversibility and is not affected in a phytochrome (PHY) A mutant so it is mediated by PHYB and related PHYs. FR photoconversion of PHYB to an inactive form activates a signaling pathway, leading to increased GA biosynthesis. Of 10 GA biosynthetic genes, expression of the 20-oxidase, AtGA20ox2, responded most to FR (up to a 40-fold increase within 3 h). AtGA20ox1 also responded but to a lesser extent. Stimulation of petiole elongation by EOD FR is reduced in a transgenic AtGA20ox2 hairpin gene silencing line. By contrast, it was only in SD that a T-DNA insertional mutant of AtGA20ox1 (ga5-3) showed reduced response. Circadian entrainment to a daytime pattern provides an explanation for the SD expression of AtGA20ox1. Conversely, the strong EOD/LD FR responses of AtGA20ox2 may reflect its independence of circadian regulation. While FR acting via PHYB increases expression of AtGA20ox2, other GA biosynthetic genes are known to respond to R rather than FR light and/or to other PHYs. Thus, there must be different signal transduction pathways, one at least showing a positive response to active PHYB and another showing a negative response.
The light environment of plants, particularly its quality and daily duration, regulates many aspects of plant development including seed germination, shoot elongation, shoot architecture, and flowering. Of the plant photoreceptors involved in such light responses, the phytochromes (PHYs), cryptochromes, and phototropins are most important (see Quail, 2002).
Typically, PHY acts by regulating the synthesis of the GA class of plant growth regulators. For example, a brief (5–10 min) exposure to PHY-active far-red (FR) light at the end-of-day (EOD) increases bioactive GA content in isolated, elongating epicotyls of cowpea (Vigna unguiculata; Martínez-García et al., 2000), a system in which applied GA stimulates elongation, as is also known for a number of rosette dicot species (for review, see Smith, 1995; García-Martínez and Gil, 2002). Similarly, a prolonged (>6 h) FR-rich long day (LD) exposure causes petiole and stem elongation and an increase in GA content in spinach (Spinacia oleracea; Wu et al., 1996), bean (Phaseolus vulgaris; Beall et al., 1996), and Arabidopsis (Arabidopsis thaliana; Xu et al., 1997; Gocal et al., 2001). In addition, FR-rich LD exposures increase the activity of a key group of GA biosynthetic enzymes, the 20-oxidases (Gilmour et al., 1986), and there are matching increases in expression of 20-oxidase mRNAs (Wu et al., 1996; Xu et al., 1997; Lee and Zeevaart, 2002).
In contrast to the FR stimulation of 20-oxidase expression in shoots, in germinating seed (Toyomasu et al., 1993; Yamaguchi et al., 1998), and during seedling deetiolation (Reid et al., 2002), red (R) light acting via PHY may regulate expression of 3-oxidases, another class of GA biosynthetic gene. In germinating lettuce (Lactuca sativa) seed, for example, GA 3-oxidase expression and its biosynthetic product, GA1, increase within 4 h of a brief R exposure (Toyomasu et al., 1993, 1998). Similarly, during Arabidopsis seed germination, expression of two 3-oxidase genes increases rapidly on exposure to R light (Yamaguchi et al., 1998). In a parallel manner, during deetiolation in pea, inhibition of stem elongation by R light involves a reduction in GA content and down-regulation of the expression of various GA biosynthetic genes (Reid et al., 2002). Interestingly, in germinating lettuce seed, R promotes the expression of a 3-oxidase during the early hours of germination but also inhibits the expression of two GA 20-oxidase genes, one of these showing an increase in FR (Toyomasu et al., 1998). Thus, depending on the physiological response and the wavelengths of light used, expression of any GA biosynthetic gene might increase or decrease.