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.