SummaryThe Arabidopsis gene ORE9/MAX2 encodes an F-box leucine-rich repeat protein. F-box proteins function as the substrate-recruiting subunit of SCF-type ubiquitin E3 ligases in protein ubiquitination. One of several phenotypes of max2 mutants, the highly branched shoot, is identical to mutants at three other MAX loci. Reciprocal grafting, double mutant analysis and gene cloning suggest that all MAX genes act in a common pathway, where branching suppression depends on MAX2 activity in the shoot, in response to an acropetally mobile signal that requires MAX3, MAX4 and MAX1 for its production. Here, we further investigate the site and mode of action of MAX2 in branching. Transcript analysis and a translational MAX2-GUS fusion indicate that MAX2 is expressed throughout the plant, most highly in developing vasculature, and is nuclear-localized in many cell types. Analysis of cell autonomy shows that MAX2 acts locally, either in the axillary bud, or in adjacent stem or petiole tissue. Expression of MAX2 from the CaMV 35S promoter complements the max2 mutant, does not affect branching in a wild-type background and partially rescues increased branching in the max1, max3 and max4 backgrounds. Expression of mutant MAX2, lacking the F-box domain, under the CaMV 35S promoter does not complement max2, and dominant-negatively affects branching in the wild-type background. Myc-epitope-tagged MAX2 interacts with the core SCF subunits ASK1 and AtCUL1 in planta. We conclude that axillary shoot growth is controlled locally, at the node, by an SCF MAX2 , the action of which is enhanced by the mobile MAX signal.

Histone acetylation and deacetylation are connected with transcriptional activation and silencing in many eukaryotic organisms. Gene families for enzymes that accomplish these modifications show a surprising multiplicity in sequence and expression levels, suggesting a high specificity for different targets. We show that mutations in Arabidopsis (Arabidopsis thaliana) HDA6, a putative class I histone deacetylase gene, result in loss of transcriptional silencing from several repetitive transgenic and endogenous templates. Surprisingly, total levels of histone H4 acetylation are only slightly affected, whereas significant hyperacetylation is restricted to the nucleolus organizer regions that contain the rDNA repeats. This switch coincides with an increase of histone 3 methylation at Lys residue 4, a modified DNA methylation pattern, and a concomitant decondensation of the chromatin. These results indicate that HDA6 might play a role in regulating activity of rRNA genes, and this control might be functionally linked to silencing of other repetitive templates and to its previously assigned role in RNA-directed DNA methylation.

Genes introduced into higher plant genomes can become silent (gene silencing) and/or cause silencing of homologous genes at unlinked sites (homology-dependent gene silencing or HDG silencing). Mutations of the HOMOLOGY-DEPENDENT GENE SILENCING1 (HOG1) locus relieve transcriptional gene silencing and methylation-dependent HDG silencing and result in genome-wide demethylation. The hog1 mutant plants also grow slowly and have low fertility and reduced seed germination. Three independent mutants of HOG1 were each found to have point mutations at the 39 end of a gene coding for S-adenosyl-L-homocysteine (SAH) hydrolase, and hog1-1 plants show reduced SAH hydrolase activity. A transposon (hog1-4) and a T-DNA tag (hog1-5) in the HOG1 gene each behaved as zygotic embryo lethal mutants and could not be made homozygous. The results suggest that the homozygous hog1 point mutants are leaky and result in genome demethylation and poor growth and that homozygous insertion mutations result in zygotic lethality. Complementation of the hog1-1 point mutation with a T-DNA containing the gene coding for SAH hydrolase restored gene silencing, HDG silencing, DNA methylation, fast growth, and normal seed viability. The same T-DNA also complemented the zygotic embryo lethal phenotype of the hog1-4 tagged mutant. A model relating the HOG1 gene, DNA methylation, and methylation-dependent HDG silencing is presented.

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