SummaryBackground-Regeneration, a remarkable example of developmental plasticity displayed by both plants and animals, involves successive developmental events driven in response to environmental cues. Despite decades of study on the ability of the plant tissues to regenerate complete fertile shoot system after inductive cues, the mechanisms by which cells acquire pluripotency and subsequently regenerate complete organs remain unknown.

Leaves are determinate organs that arise from the flanks of the shoot apical meristem as polar structures with distinct adaxial (dorsal) and abaxial (ventral) sides. Opposing regulatory interactions between genes specifying adaxial or abaxial fates function to maintain dorsoventral polarity. One component of this regulatory network is the Myb-domain transcription factor gene ASYMMETRIC LEAVES1 (AS1). The contribution of AS1 to leaf polarity varies across different plant species; however, in Arabidopsis, as1 mutants have only mild defects in leaf polarity, suggesting that alternate pathways exist for leaf patterning. Here, we describe three genes, PIGGYBACK1 (PGY1), PGY2 and PGY3, which alter leaf patterning in the absence of AS1. All three pgy mutants develop dramatic ectopic lamina outgrowths on the adaxial side of the leaf in an as1 mutant background. This leaf-patterning defect is enhanced by mutations in the adaxial HD-ZIPIII gene REVOLUTA (REV), and is suppressed by mutations in abaxial KANADI genes. Thus, PGY genes influence leaf development via genetic interactions with the HD-ZIPIII-KANADI pathway. PGY1, PGY2 and PGY3 encode cytoplasmic large subunit ribosomal proteins, L10a, L9 and L5, respectively. Our results suggest a role for translation in leaf dorsoventral patterning and indicate that ribosomes are regulators of key patterning events in plant development. Development 135, 1315Development 135, -1324Development 135, (2008 DEVELOPMENT 1316 from the Arabidopsis Biological Resource Centre (ABRC). kan1-2 and kan2-1 were obtained from John Bowman. All genetic interactions were in a Ler background. Plants were grown either in soil or on Murashige and Skoog media at 22°C with a day length of 16 hours. KEY WORDS: Ribosomal protein, Leaf polarity, ASYMMETRIC LEAVES1, PIGGYBACK, Arabidopsis Geneticspgy genes were cloned using Ler ϫ Columbia F2 mapping populations. For complementation a 2.1 kb genomic fragment encompassing At2g27530, a 5 kb genomic fragment encompassing At1g33140 and a 3.5 kb genomic fragment encompassing At3g25520 were cloned into the binary vector pMDC123 (Curtis and Grossniklaus, 2003) and transformed into pgy1-1/pgy1-1 as1/+, pgy2-1/pgy2-1 as1/+ and pgy3-1/pgy3-1 as1/+ plants, respectively, using standard agrobacterium-mediated transformation (Clough and Bent, 1998). For each complementation construct, basta resistant plants with an as1 phenotype were confirmed as as1 pgy homozygotes.as1-1 rev-6 was analysed in the F3 generation of the cross as1-1 ϫ rev-6. In the F2 generation of this cross as1-1 rev-6 segregated at 1:15. pgy1-1 rev-6 were obtained from the F3 generation of the cross pgy1-1 ϫ rev-6. Progeny from pgy1-1 rev-6/+ individuals segregated 1:3 pgy1-1 rev-6 mutants. as1-1 pgy1-1 rev-6 triple mutants were analysed in the F4 generation of the cross as1-1 pgy1-1 ϫ as1-1 rev-6, after selfing as1-1 pgy1-1 rev-6/+ F3 plants. Segregation of as1-1 pgy1-1 rev-6 in this F4 generation was 1:3. as1-1 kan1-2 and pgy1-1 kan1-2 were obtained from the F3 generation of the respective crosses...

The pattern of plant organ initiation at the shoot apical meristem (SAM), termed phyllotaxis, displays regularities that have long intrigued botanists and mathematicians alike. In the SAM, the central zone (CZ) contains a population of stem cells that replenish the surrounding peripheral zone (PZ), where organs are generated in regular patterns. These patterns differ between species and may change in response to developmental or environmental cues [1]. Expression analysis of auxin efflux facilitators of the PIN-FORMED (PIN) family combined with modeling of auxin transport has indicated that organ initiation is associated with intracellular polarization of PIN proteins and auxin accumulation [2-10]. However, regulators that modulate PIN activity to determine phyllotactic patterns have hitherto been unknown. Here we reveal that three redundantly acting PLETHORA (PLT)-like AP2 domain transcription factors control shoot organ positioning in the model plant Arabidopsis thaliana. Loss of PLT3, PLT5, and PLT7 function leads to nonrandom, metastable changes in phyllotaxis. Phyllotactic changes in plt3plt5plt7 mutants are largely attributable to misregulation of PIN1 and can be recapitulated by reducing PIN1 dosage, revealing that PLT proteins are key regulators of PIN1 activity in control of phyllotaxis.

Lateral organ distribution at the shoot apical meristem defines specific and robust phyllotaxis patterns that have intrigued biologists and mathematicians for centuries. In silico studies have revealed that this self-organizing process can be recapitulated by modeling the polar transport of the phytohormone auxin. Phyllotactic patterns change between species and developmental stages, but the processes behind these variations have remained unknown. Here we use regional complementation experiments to reveal that phyllotactic switches in Arabidopsis shoots can be mediated by PLETHORA-dependent control of local auxin biosynthesis.pattern formation | plant

Intriguingly PLT 3, PLT5, and PLT7 not only control the positioning of organs at the shoot meristem but also in the root; a striking observation that raises many evolutionary questions.

SUMMARYHistone methylation is a major component in numerous processes such as determination of flowering time, which is fine-tuned by multiple genetic pathways that integrate both endogenous and environmental signals. Previous studies identified SET DOMAIN GROUP 26 (SDG26) as a histone methyltransferase involved in the activation of flowering, as loss of function of SDG26 caused a late-flowering phenotype in Arabidopsis thaliana. However, the SDG26 function and the underlying molecular mechanism remain largely unknown. In this study, we undertook a genetic analysis by combining the sdg26 mutant with mutants of other histone methylation enzymes, including the methyltransferase mutants Arabidopsis trithorax1 (atx1), sdg25 and curly leaf (clf), as well as the demethylase double mutant lsd1-like1 lsd1-like2 (ldl1 ldl2). We found that the early-flowering mutants sdg25, atx1 and clf interact antagonistically with the late-flowering mutant sdg26, whereas the late-flowering mutant ldl1 ldl2 interacts synergistically with sdg26. Based on microarray analysis, we observed weak overlaps in the genes that were differentially expressed between sdg26 and the other mutants. Our analyses of the chromatin of flowering genes revealed that the SDG26 protein binds at the key flowering integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1/AGAMOUS-LIKE 20 (SOC1/AGL20), and is required for histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 36 trimethylation (H3K36me3) at this locus. Together, our results indicate that SDG26 promotes flowering time through a distinctive genetic pathway, and that loss of function of SDG26 causes a decrease in H3K4me3 and H3K36me3 at its target gene SOC1, leading to repression of this gene and the late-flowering phenotype.

Biochemical and molecular characterization of the biotin biosynthetic pathway in plants has dealt primarily with biotin synthase. This enzyme catalyzing the last step of the pathway is localized in mitochondria. Other enzymes of the pathway are however largely unknown. In this study, a genomic-based approach allowed us to clone an Arabidopsis (Arabidopsis thaliana) cDNA coding 7-keto-8-aminopelargonic acid (KAPA) synthase, the first committed enzyme of the biotin synthesis pathway, which we named AtbioF. The function of the enzyme was demonstrated by functional complementation of an Escherichia coli mutant deficient in KAPA synthase reaction, and by measuring in vitro activity. Overproduction and purification of recombinant AtbioF protein enabled a thorough characterization of the kinetic properties of the enzyme and a spectroscopic study of the enzyme interaction with its substrates and product. This is the first characterization of a KAPA synthase reaction in eukaryotes. Finally, both green fluorescent protein-targeting experiments and western-blot analyses showed that the Arabidopsis KAPA synthase is present in cytosol, thus revealing a unique compartmentation of the plant biotin synthesis, split between cytosol and mitochondria. The significance of the complex compartmentation of biotin synthesis and utilization in the plant cell and its potential importance in the regulation of biotin metabolism are also discussed.Biotin is an essential, water-soluble vitamin found in all living cells (Dakshinamurti and Cauhan, 1989). Plants, like most microorganisms, have the ability to synthesize biotin. In contrast, other multicellular eukaryotic organisms are biotin auxotroph. Biotin acts as a cofactor for a set of enzymes that catalyze carboxylation, decarboxylation, and transcarboxylation reactions in a number of crucial metabolic processes (Knowles, 1989).Biotin biosynthesis has been well characterized in bacteria such as Escherichia coli, Bacillus subtilis, and Bacillus sphaericus, by combined biochemical and genetic studies (for review, see Marquet et al., 2001). In all known microbes, the cofactor is synthesized from pimeloyl-CoA through four enzymatic steps comprising 7-keto-8-aminopelargonic acid (KAPA) synthase, 7,8-diaminopelargonic acid (DAPA) aminotransferase, dethiobiotin synthase, and biotin synthase coded by bioF, bioA, bioD, and bioB genes, respectively. Enzymes required for biotin synthesis in E. coli and B. sphaericus have been purified and their activities characterized in vitro (for review, see Streit and Entcheva, 2003). KAPA synthase, the first enzyme of this pathway, catalyzes the decarboxylative condensation of pimeloyl-CoA and L-Ala to produce KAPA, CoASH, and carbon dioxide (Fig. 1). The structure and reaction mechanism of KAPA synthase places it in the subfamily of a-oxoamine synthases, a small group of pyridoxal 5#-phosphate (PLP)-dependent enzymes of the a-family (Ploux and Marquet, 1996;Alexeev et al., 1998;Webster et al., 2000).In plants, the biosynthetic pathway beginning with pimeloyl-Co...

Epigenetic reprogramming occurring during reproduction is crucial for both animal and plant development. Histone H3 Lys 4 trimethylation (H3K4me3) is an evolutionarily conserved epigenetic mark of transcriptional active euchromatin. While much has been learned in somatic cells, H3K4me3 deposition and function in gametophyte is poorly studied. Here, we demonstrate that SET DOMAIN GROUP2 (SDG2)-mediated H3K4me3 deposition participates in epigenetic reprogramming during Arabidopsis male gametogenesis. We show that loss of SDG2 barely affects meiosis and cell fate establishment of haploid cells. However, we found that SDG2 is critical for postmeiotic microspore development. Mitotic cell division progression is partly impaired in the loss-of-function sdg2-1 mutant, particularly at the second mitosis setting up the two sperm cells. We demonstrate that SDG2 is involved in promoting chromatin decondensation in the pollen vegetative nucleus, likely through its role in H3K4me3 deposition, which prevents ectopic heterochromatic H3K9me2 speckle formation. Moreover, we found that derepression of the LTR retrotransposon ATLANTYS1 is compromised in the vegetative cell of the sdg2-1 mutant pollen. Consistent with chromatin condensation and compromised transcription activity, pollen germination and pollen tube elongation, representing the key function of the vegetative cell in transporting sperm cells during fertilization, are inhibited in the sdg2-1 mutant. Taken together, we conclude that SDG2-mediated H3K4me3 is an essential epigenetic mark of the gametophyte chromatin landscape, playing critical roles in gamete mitotic cell cycle progression and pollen vegetative cell function during male gametogenesis and beyond.