Turning Garlic into a Modern Crop: State of the Art and Perspectives

Garlic is cultivated worldwide for the value of its bulbs, but its cultivation is challenged by the infertility of commercial cultivars and the accumulation of pathogens over time, which occurs as a consequence of vegetative (clonal) propagation. In this review, we summarize the state of the art of garlic genetics and genomics, highlighting recent developments that will lead to its development as a modern crop, including the restoration of sexual reproduction in some garlic strains. The set of tools available to the breeder currently includes a chromosome-scale assembly of the garlic genome and multiple transcriptome assemblies that are furthering our understanding of the molecular processes underlying important traits like the infertility, the induction of flowering and bulbing, the organoleptic properties and resistance to various pathogens.

Complete Genome Sequence of an Isolate of Passiflora chlorosis virus from Passion Fruit (Passiflora edulis Sims)

We report the first complete genome sequence of an isolate of Passiflora chlorosis virus (PaCV), a member of the Potyviridae family, identified in passion fruit (Passiflora edulis Sims) plants grown in Israel. The assembled genome is 9672 nucleotides long and encodes a 3084 amino acids polyprotein that is predicted to be proteolytically cleaved into 10 mature peptides. Our analysis of the genome sequence shows that PaCV is a distinct species, sharing 68.5% nucleotide sequence identity and 71.5% amino acid sequence identity with isolates of the bean common mosaic necrosis virus (BCMNV), the most closely related virus classified within the genus Potyvirus. Using quantitative PCR, we detected the virus in RNA samples from leaves exhibiting symptoms of infection, with higher levels in clearly chlorotic leaves, but not in those from healthy leaves.

Members of the DEAL subfamily of the DUF1218 gene family are required for bilateral symmetry but not for dorsoventrality in Arabidopsis leaves

Most plant leaves exhibit bilateral symmetry, which has been hypothesized as an inevitable consequence of the existence of the proximodistal and dorsoventral axes. No gene has been described that affects leaf bilateral symmetry but not dorsoventrality in Arabidopsis thaliana. We screened for viable insertional mutations that affect leaf morphology, and out of more than 700 mutants found only one, desigual1-1 (deal1-1), that exhibited bilateral symmetry breaking but no obvious defects in dorsoventrality. We found that deal1-1 is an allele of VASCULATURE COMPLEXITY AND CONNECTIVITY (VCC). Several overlapping regulatory pathways establish the interspersed lobes and indentations along the margin of Arabidopsis thaliana leaves. These pathways involve feedback loops of auxin, the PIN-FORMED1 (PIN1) auxin efflux carrier, and the CUP-SHAPED COTYLEDON2 (CUC2) transcriptional regulator. Early vcc (deal1) leaf primordia fail to acquire bilateral symmetry and instead form ectopic lobes and sinuses. The vcc leaves show aberrant recruitment of marginal cells expressing properly polarized PIN1, resulting in misplaced auxin maxima. Normal PIN1 polarization requires CUC2 expression and CUC2 genetically interacts with VCC; VCC also affects CUC2 expression. VCC has a domain of unknown function, DUF1218, and localizes to the endoplasmic reticulum membrane. VCC acts partially redundantly with its two closest paralogs, DEAL2 and DEAL3, in early leaf margin patterning and is required for bilateral symmetry, but its loss of function does not visibly affect dorsoventrality.

Loss of function of Arabidopsis microRNA-machinery genes impairs fertility, and has effects on homologous recombination and meiotic chromatin dynamics

MicroRNAs (miRNAs) are ~22-nt single-stranded noncoding RNAs with regulatory roles in a wide range of cellular functions by repressing eukaryotic gene expression at a post-transcriptional level. Here, we analyzed the effects on meiosis and fertility of hypomorphic or null alleles of the HYL1, HEN1, DCL1, HST and AGO1 genes, which encode miRNA-machinery components in Arabidopsis. Reduced pollen and megaspore mother cell number and fertility were shown by the mutants analyzed. These mutants also exhibited a relaxed chromatin conformation in male meiocytes at the first meiotic division, and increased chiasma frequency, which is likely to be due to increased levels of mRNAs from key genes involved in homologous recombination. The hen1-13 mutant was found to be hypersensitive to gamma irradiation, which mainly causes double-strand breaks susceptible to be repaired by homologous recombination. Our findings uncover a role for miRNA-machinery components in Arabidopsis meiosis, as well as in the repression of key genes required for homologous recombination. These genes seem to be indirect miRNA targets.

Arabidopsis MAS2, an Essential Gene That Encodes a Homolog of Animal NF-κ B Activating Protein, Is Involved in 45S Ribosomal DNA Silencing

Ribosome biogenesis requires stoichiometric amounts of ribosomal proteins and rRNAs. Synthesis of rRNAs consumes most of the transcriptional activity of eukaryotic cells, but its regulation remains largely unclear in plants. We conducted a screen for ethyl methanesulfonate-induced suppressors of Arabidopsis thaliana ago1-52, a hypomorphic allele of AGO1 (ARGONAUTE1), a key gene in microRNA pathways. We identified nine extragenic suppressors as alleles of MAS2 (MORPHOLOGY OF AGO1-52 SUPPRESSED2). Positional cloning showed that MAS2 encodes the putative ortholog of NKAP (NF-κ B activating protein), a conserved eukaryotic protein involved in transcriptional repression and splicing in animals. The mas2 point mutations behave as informational suppressors of ago1 alleles that cause missplicing. MAS2 is a single-copy gene whose insertional alleles are embryonic lethal. In yeast two-hybrid assays, MAS2 interacted with splicing and ribosome biogenesis proteins, and fluorescence in situ hybridization showed that MAS2 colocalizes with the 45S rDNA at the nucleolar organizer regions (NORs). The artificial microRNA amiR-MAS2 partially repressed MAS2 and caused hypomethylation of 45S rDNA promoters as well as partial NOR decondensation, indicating that MAS2 negatively regulates 45S rDNA expression. Our results thus reveal a key player in the regulation of rRNA synthesis in plants.

Multi-gene silencing in Arabidopsis: a collection of artificial microRNAs targeting groups of paralogs encoding transcription factors

Functional redundancy often hampers the analysis of gene families. To overcome this difficulty, we constructed Arabidopsis thaliana lines that expressed artificial microRNAs designed to simultaneously target two to six paralogous genes encoding members of transcription factor families. Of the 576 genes that we chose as targets, only 122 had already been functionally studied at some level. As a simple indicator of the inhibitory effects of our amiRNAs on their targets, we examined the amiRNA-expressing transgenic lines for morphological phenotypes at the rosette stage. Of 338 transgenes tested, 21 caused a visible morphological phenotype in leaves, a proportion that is much higher than that expected as a result of insertional mutagenesis. Also, our collection probably represents many other mutant phenotypes, not just those in leaves. This robust, versatile method enables functional examination of redundant transcription factor paralogs, and is particularly useful for genes that occur in tandem.

Leaf phenomics: a systematic reverse genetic screen for Arabidopsis leaf mutants

The study and eventual manipulation of leaf development in plants requires a thorough understanding of the genetic basis of leaf organogenesis. Forward genetic screens have identified hundreds of Arabidopsis mutants with altered leaf development, but the genome has not yet been saturated. To identify genes required for leaf development we are screening the Arabidopsis Salk Unimutant collection. We have identified 608 lines that exhibit a leaf phenotype with full penetrance and almost constant expressivity and 98 additional lines with segregating mutant phenotypes. To allow indexing and integration with other mutants, the mutant phenotypes were described using a custom leaf phenotype ontology. We found that the indexed mutation is present in the annotated locus for 78% of the 553 mutants genotyped, and that in half of these the annotated T-DNA is responsible for the phenotype. To quickly map non-annotated T-DNA insertions, we developed a reliable, cost-effective and easy method based on whole-genome sequencing. To enable comprehensive access to our data, we implemented a public web application named PhenoLeaf (http://genetics.umh.es/phenoleaf) that allows researchers to query the results of our screen, including text and visual phenotype information. We demonstrated how this new resource can facilitate gene function discovery by identifying and characterizing At1g77600, which we found to be required for proximal-distal cell cycle-driven leaf growth, and At3g62870, which encodes a ribosomal protein needed for cell proliferation and chloroplast function. This collection provides a valuable tool for the study of leaf development, characterization of biomass feedstocks and examination of other traits in this fundamental photosynthetic organ.

Mutations in the microRNA complementarity site of the INCURVATA4 gene perturb meristem function and adaxialize lateral organs in arabidopsis

Here, we describe how the semidominant, gain-of-function icu4-1 and icu4-2 alleles of the INCURVATA4 (ICU4) gene alter leaf phyllotaxis and cell organization in the root apical meristem, reduce root length, and cause xylem overgrowth in the stem. The ICU4 gene was positionally cloned and found to encode the ATHB15 transcription factor, a class III homeodomain/leucine zipper family member, recently named CORONA. The icu4-1 and icu4-2 alleles bear the same point mutation that affects the microRNA complementarity site of ICU4 and is identical to those of several semidominant alleles of the class III homeodomain/leucine zipper family members PHABULOSA and PHAVOLUTA. The icu4-1 and icu4-2 mutations significantly increase leaf transcript levels of the ICU4 gene. The null hst-1 allele of the HASTY gene, which encodes a nucleocytoplasmic transporter, synergistically interacts with icu4-1, the double mutant displaying partial adaxialization of rosette leaves and carpels. Our results suggest that the ICU4 gene has an adaxializing function and that it is down-regulated by microRNAs that require the HASTY protein for their biogenesis.

Plant microRNAs and development

MicroRNAs (miRNAs) act as negative regulators of gene expression in eukaryotes, a discovery that has opened an expanding field of biological research. Plant miRNAs are known to repress gene expression posttranscriptionally, mainly by guiding cleavage but also by attenuating the translation of target transcripts. In addition, it has been shown that plant miRNAs can also act at the transcriptional level by directing the methylation of target chromosomal loci. Genetic and biochemical approaches are quickly broadening our knowledge of the biogenesis and function of plant miRNAs. Computational approaches have uncovered an unexpectedly large number of miRNAs and their targets in plants. The targets of plant miRNAs often belong to families of transcription factors involved in the control of developmental processes. We review the status of research in this dynamic field, summarizing recent advances in our understanding of the biogenesis and mechanism of action of plant miRNAs, as well as in the developmental processes they regulate.

The rotunda2 mutants identify a role for the LEUNIG gene in vegetative leaf morphogenesis

Leaf development in Arabidopsis thaliana is considered to be a two-step process. In the first step, a leaf primordium is formed that involves a switch from indeterminate to leaf developmental fate in the shoot apical meristem cells. The second step, known as leaf morphogenesis, consists of post-initiation developmental events such as patterned cell proliferation, cell expansion, and cell differentiation. The results are presented of the molecular and genetic analyses of the rotunda2 (ron2) mutants of Arabidopsis, which were isolated based on their wide and serrated vegetative leaf lamina. The RON2 gene was positionally cloned and was identical to LEUNIG (LUG); it encodes a transcriptional co-repressor that has been described to affect flower development. Morphological and histological analyses of expanded leaves indicated that RON2 (LUG) acts at later stages of leaf development by restricting cell expansion during leaf growth. Real-time reverse-transcription polymerase chain reaction was used to quantify the expression of KNOX, WUSCHEL, YABBY3, LEAFY, ASYMMETRIC LEAVES, and GIBBERELLIN OXIDASE genes in expanding and fully expanded rosette leaf laminas of the wild type and ron2 and lug mutants. SHOOTMERISTEMLESS was expressed in wild-type leaves and down-regulated in the mutants. The results indicate that RON2 (LUG) has a function in later stages of leaf development.