Summary: Arabidopsis thalianareticulate mutants exhibit differential pigmentation of the veinal and interveinal leaf regions, a visible phenotype that often indicates impaired mesophyll development. We performed a metabolomic analysis of oneven6(venosa6) and threeven3reticulate mutants that revealed altered levels of arginine precursors, namely increased ornithine and reduced citrulline levels. In addition, the mutants were more sensitive than the wild‐type to exogenous ornithine, and leaf reticulation and mesophyll defects of these mutants were completely rescued by exogenous citrulline. Taken together, these results indicate thatven3andven6mutants experience a blockage of the conversion of ornithine into citrulline in the arginine pathway. Consistent with the participation of VEN3 and VEN6 in the same pathway, the morphological phenotype ofven3 ven6double mutants was synergistic. Map‐based cloning showed that theVEN3andVEN6genes encode subunits of Arabidopsis carbamoyl phosphate synthetase (CPS), which is assumed to be required for the conversion of ornithine into citrulline in arginine biosynthesis. Heterologous expression of the ArabidopsisVEN3andVEN6genes in a CPS‐deficientEscherichia colistrain fully restored bacterial growth in minimal medium, demonstrating the enzymatic activity of the VEN3 and VEN6 proteins, and indicating a conserved role for CPS in these distinct and distant species. Detailed study of the reticulate leaf phenotype in theven3andven6mutants revealed that mesophyll development is highly sensitive to impaired arginine biosynthesis.

The Arabidopsis rugosa1 (rug1) mutant has irregularly shaped leaves and reduced growth. In the absence of pathogens, leaves of rug1 plants have spontaneous lesions reminiscent of those seen in lesion-mimic mutants; rug1 plants also express cytological and molecular markers associated with defence against pathogens. These rug1 phenotypes are made stronger by dark/light transitions. The rug1 mutant also has delayed flowering time, upregulation of the floral repressor FLOWERING LOCUS C (FLC) and downregulation of the flowering promoters FT and SOC1/AGL20. Vernalization suppresses the late flowering phenotype of rug1 by repressing FLC. Microarray analysis revealed that 280 nuclear genes are differentially expressed between rug1 and wild type; almost a quarter of these genes are involved in plant defence. In rug1, the auxin response is also affected and several auxin-responsive genes are downregulated. We identified the RUG1 gene by map-based cloning and found that it encodes porphobilinogen deaminase (PBGD), also known as hydroxymethylbilane synthase, an enzyme of the tetrapyrrole biosynthesis pathway, which produces chlorophyll, heme, siroheme and phytochromobilin in plants. PBGD activity is reduced in rug1 plants, which accumulate porphobilinogen. Our results indicate that Arabidopsis PBGD deficiency impairs the porphyrin pathway and triggers constitutive activation of plant defence mechanisms leading to leaf lesions and affecting vegetative and reproductive development.

Dear Editor, The PHYTOCHROME B ( PHYB ) gene, encoding the apoprotein of the major red/far-red photoreceptor phyB, has been one of the most extensively studied genes in Arabidopsis ( Arabidopsis thaliana ) for the past 2 decades ([Casal, 2013][1]). During these research efforts, the phyB-9 allele (

Ribosome biogenesis is fundamental to growth and development in eukaryotes and is linked to human diseases and cancer. Arabidopsis thaliana MORPHOLOGY OF ARGONAUTE1-52 SUPPRESSED 2 (MAS2) participates in splicing and 45S ribosomal DNA (rDNA) expression. In a screen for MAS2 interactors, we identified RIBOSOMAL RNA PROCESSING 7 (RRP7), an ortholog of yeast ribosomal RNA processing protein 7 (Rrp7), which is required for 18S ribosomal RNA (rRNA) maturation. Arabidopsis rrp7 mutants exhibit a pleiotropic phenotype including slow growth, altered shoot phyllotaxy, aberrant venation in lateral organs, partial infertility, and abscisic acid hypersensitivity in seedlings. In Arabidopsis, RRP7 localizes mainly to the nucleolus, the site of the 45S rDNA transcription that produces a 45S pre-rRNA primary transcript, precursor of the 25S, 18S and 5.8S rRNAs. Lack of RRP7 function perturbs 18S rRNA maturation, causes nucleolar hypertrophy, and results in an increased 25S/18S rRNA ratio. Arabidopsis contains hundreds of 45S rDNA genes whose expression is epigenetically regulated, and deregulated in rrp7 mutants. Double mutant analysis revealed synergistic interactions between RRP7 alleles and alleles of MAS2, NUCLEOLIN1 (NUC1) and HISTONE DEACETYLASE 6 (HDA6), which encode epigenetic regulators of 45S rDNA transcription. Our results reveal the evolutionarily conserved but divergent roles of RRP7 as a ribosome biogenesis factor.