Gibberellin and ethylene control endoreduplication levels in the Arabidopsis thaliana hypocotyl.

We have previously shown that endoreduplication levels in hypocotyls of Arabidopsis thaliana (L.) Heynh. are under negative control of phytochromes. In this study, the hormonal regulation of this process was analysed using a collection of A. thaliana mutants. The results show that two hormones in particular, gibberellin (GA) and ethylene, play distinct roles. Hypocotyl cells of the GA-deficient mutant ga1-11 grown in the dark did not elongate and showed a greatly reduced endoreduplication. Normal endoreduplication could be restored by supplying 10(-9) M of the gibberellin GA(4+7), whereas the restoration of normal cell growth required 100-fold higher concentrations. The GA-insensitive mutant gai showed reduced cell elongation but normal ploidy levels. We conclude that (i) GA(4+7) has a global positive effect on endoreduplication and (ii) that endoreduplication is more sensitive to GA(4+7) than cell elongation. Ethylene had a completely different effect. It induced an extra round of endoreduplication both in light- and dark-grown seedlings and acted mainly on discrete steps rather than having a global effect on endoreduplication. The genes EIN2 and CTR1, components of the ethylene signal transduction pathway were both involved in this process.

Interaction of light and gravitropism with nutation of hypocotyls of Arabidopsis thaliana seedlings.

Etiolated seedlings of Arabidopsis thaliana nutated under conditions of physiological darkness while about ten percent of monitored individuals exhibited regular elliptical nutation, circumnutation. Pre-irradiation with red light prevented occurrence of circumnutation without having an effect on the average rate of the nutational movement. Phototropic response of seedlings to unilateral blue light appeared to be superimposed over nutation. Throughout gravitropism, some seedlings continued to exhibit nutation suggesting that these two processes are independently controlled. Based on these results, we suggest that nutation in Arabidopsis probably is not controlled by the mechanism predicted by the theory of gravitropic overshoots.

Procuste1 mutants identify two distinct genetic pathways controlling hypocotyl cell elongation, respectively in dark- and light-grown Arabidopsis seedlings.

Plant morphogenesis is dependent on a tight control of cell division and expansion. Cell elongation during post-embryonic hypocotyl growth is under the control of a light-regulated developmental switch. Light is generally believed to exert its effects on hypocotyl elongation through a phytochrome-and blue-light receptor-mediated inhibitory action on a so far unknown cell elongation mechanism. We describe here a new class of allelic mutants in Arabidopsis, at the locus PROCUSTE1 (prc1-1 to -4), which have a hypocotyl elongation defect specifically associated with the dark-grown development program. Normal hypocotyl elongation is restored in plants grown in white, blue or red light. In agreement with this, the constitutive photomorphogenic mutation cop1-6, which induces a de-etiolated phenotype in the dark, is epistatic to prc1-2 for the hypocotyl phenotype. Epistasis analyses in red and blue light respectively, indicate that phytochrome B but not the blue light receptor HY4, is required for the switch from PRC1-dependent to PRC1-independent elongation. The conditional hypocotyl growth defect is associated with a deformation of the hypocotyl surface due to an uncontrolled swelling of epidermal, cortical or endodermal cells, suggesting a defect in the structure of the expanding cell wall. A similar phenotype was observed in elongating roots, which was however, independent of the light conditions. The aerial part of mature mutant plants grown in the light was indistinguishable from the wild type. prc1 mutants provide a means of distinguishing, for the first time, two genetic pathways regulating hypocotyl cell elongation respectively in dark- and light-grown seedlings, whereby light not only inhibits hypocotyl growth, but also activates a PRC1-independent cell elongation program.

Characterization of the auxin-inducible SAUR-AC1 gene for use as a molecular genetic tool in Arabidopsis.

The small auxin up RNA (SAUR) genes were originally characterized in soybean, where they encode a set of unstable transcripts that are rapidly induced by auxin. In this report, the isolation of a SAUR gene, designated SAUR-AC1, from Arabidopsis thaliana (L.) Heynh. ecotype Columbia is described. The promoter of the SAUR-AC1 gene contains putative regulatory motifs conserved among soybean SAUR promoters, as well as sequences implicated in the regulation of other genes in response to auxin. The transcribed region is approximately 500 bp in length and contains no introns. Highly conserved sequences located within the SAUR-AC1 transcript include the central portion of the coding region and a putative mRNA instability sequence (DST) located in the 3' untranslated region. Accumulation of SAUR-AC1 mRNA is readily induced by natural and synthetic auxins and by the translational inhibitor cycloheximide. Moreover, several auxin- and gravity-response mutants of Arabidopsis exhibit decreased accumulation of the SAUR-AC1 mRNA in elongating etiolated seedlings. In particular, in the axr2-1 mutant the SAUR-AC1 transcript accumulates to less than 5% of wild-type levels. These studies indicate that SAUR-AC1 will be a useful probe of auxin-induced gene expression in Arabidopsis and will facilitate the functional analysis of both transcriptional and posttranscriptional regulatory elements.

Growth Distribution during Phototropism of Arabidopsis thaliana Seedlings.

The elongation rates of two opposite sides of hypocotyls of Arabidopsis thaliana seedlings were measured during phototropism by using an infrared imaging system. In first positive phototropism, second positive phototropism, and red light-enhanced first positive phototropism, curvature toward the light source was the result of an increase in the rate of elongation of the shaded side and a decrease in the rate of elongation of the lighted side of the seedlings. The phase of straightening that followed maximum curvature resulted from a decrease in the elongation rate of the shaded side and an increase in the elongation rate of the lighted side. These data for the three types of blue light-induced phototropism tested in this study and for the phase of straightening are all clearly consistent with the growth rate changes predicted by the Cholodny-Went theory.

Kinetics for phototropic curvature by etiolated seedlings of Arabidopsis thaliana.

An infrared-imaging system has been used to study the influence of gravity on the kinetics of first positive phototropism. The development of phototropic curvature of etiolated seedlings of Arabidopsis thaliana was measured in the absence of visible radiation. Following a pulse of blue light, stationary seedlings curved to a maximum of approximately 16 degrees about 80 minutes after stimulation. The seedlings then curved upward again or straightened by about 6 degrees during the subsequent 100 minutes. Seedlings rotated on a clinostat reached a similar maximum curvature following photostimulation. These seedlings maintained that curvature for 30 to 40 minutes before subsequently straightening to the same extent as the stationary seedlings. It is concluded that straightening is not a consequence of gravitropism, although gravity has some effect on the phototropism kinetics.