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Targeted engineering of plant gene expression holds great promise for ensuring food security and for producing biopharmaceuticals in plants. However, this engineering requires thorough knowledge of cis-regulatory elements to precisely control either endogenous or introduced genes. To generate this knowledge, we used a massively parallel reporter assay to measure the activity of nearly complete sets of promoters from Arabidopsis, maize and sorghum. We demonstrate that core promoter elements—notably the TATA box—as well as promoter GC content and promoter-proximal transcription factor binding sites influence promoter strength. By performing the experiments in two assay systems, leaves of the dicot tobacco and protoplasts of the monocot maize, we detect species-specific differences in the contributions of GC content and transcription factors to promoter strength. Using these observations, we built computational models to predict promoter strength in both assay systems, allowing us to design highly active promoters comparable in activity to the viral 35S minimal promoter. Our results establish a promising experimental approach to optimize native promoter elements and generate synthetic ones with desirable features. A massively parallel reporter assay was used to measure the activity of nearly complete sets of promoters from Arabidopsis, maize and sorghum in two assay systems, uncovering the sequence features affecting promoter strength and facilitating promoter strength prediction and synthetic design.

The scarcity of accessible sites that are dynamic or cell type-specific in plants may be due in part to tissue heterogeneity in bulk studies. To assess the effects of tissue heterogeneity, we apply single-cell ATAC-seq to Arabidopsis thaliana roots and identify thousands of differentially accessible sites, sufficient to resolve all major cell types of the root. We find that the entirety of a cell’s regulatory landscape and its transcriptome independently capture cell type identity. We leverage this shared information on cell identity to integrate accessibility and transcriptome data to characterize developmental progression, endoreduplication and cell division. We further use the combined data to characterize cell type-specific motif enrichments of transcription factor families and link the expression of family members to changing accessibility at specific loci, resolving direct and indirect effects that shape expression. Our approach provides an analytical framework to infer the gene regulatory networks that execute plant development., Existing studies of the chromatin accessibility, the primary mark of regulatory DNA, in Arabidopsis are based mainly on bulk samples. Here, the authors report the regulatory landscape of Arabidopsis thaliana roots at single-cell resolution.

Genetic engineering of cis-regulatory elements in crop plants is a promising strategy to ensure food security. However, such engineering is currently hindered by our limited knowledge of plant cis-regulatory elements. Here, we adapted STARR-seq — a technology for the high-throughput identification of enhancers — for its use in transiently transformed tobacco leaves. We demonstrate that the optimal placement in the reporter construct of enhancer sequences from a plant virus, pea and wheat was just upstream of a minimal promoter, and that none of these four known enhancers was active in the 3′-UTR of the reporter gene. The optimized assay sensitively identified small DNA regions containing each of the four enhancers, including two whose activity was stimulated by light. Furthermore, we coupled the assay to saturation mutagenesis to pinpoint functional regions within an enhancer, which we recombined to create synthetic enhancers. Our results describe an approach to define enhancer properties that can be performed in potentially any plant species or tissue transformable by Agrobacterium and that can use regulatory DNA derived from any plant genome.One-sentence summaryWe developed a high-throughput assay in transiently transformed tobacco leaves that can identify enhancers, characterize their functional elements and detect condition-specific enhancer activity.

Background The 3′ untranslated region (UTR) plays critical roles in determining the level of gene expression through effects on activities such as mRNA stability and translation. Functional elements within this region have largely been identified through analyses of native genes, which contain multiple co-evolved sequence features. Results To explore the effects of 3′ UTR sequence elements outside of native sequence contexts, we analyze hundreds of thousands of random 50-mers inserted into the 3′ UTR of a reporter gene in the yeast Saccharomyces cerevisiae. We determine relative protein expression levels from the fitness of transformants in a growth selection. We find that the consensus 3′ UTR efficiency element significantly boosts expression, independent of sequence context; on the other hand, the consensus positioning element has only a small effect on expression. Some sequence motifs that are binding sites for Puf proteins substantially increase expression in the library, despite these proteins generally being associated with post-transcriptional downregulation of native mRNAs. Our measurements also allow a systematic examination of the effects of point mutations within efficiency element motifs across diverse sequence backgrounds. These mutational scans reveal the relative in vivo importance of individual bases in the efficiency element, which likely reflects their roles in binding the Hrp1 protein involved in cleavage and polyadenylation. Conclusions The regulatory effects of some 3′ UTR sequence features, like the efficiency element, are consistent regardless of sequence context. In contrast, the consequences of other 3′ UTR features appear to be strongly dependent on their evolved context within native genes.

Single cell transcriptomic profiling can map developmental trajectories and assess cell-type–specific changes during heat shock at single cell resolution., Single cell RNA sequencing can yield high-resolution cell-type–specific expression signatures that reveal new cell types and the developmental trajectories of cell lineages. Here, we apply this approach to Arabidopsis (Arabidopsis thaliana) root cells to capture gene expression in 3,121 root cells. We analyze these data with Monocle 3, which orders single cell transcriptomes in an unsupervised manner and uses machine learning to reconstruct single cell developmental trajectories along pseudotime. We identify hundreds of genes with cell-type–specific expression, with pseudotime analysis of several cell lineages revealing both known and novel genes that are expressed along a developmental trajectory. We identify transcription factor motifs that are enriched in early and late cells, together with the corresponding candidate transcription factors that likely drive the observed expression patterns. We assess and interpret changes in total RNA expression along developmental trajectories and show that trajectory branch points mark developmental decisions. Finally, by applying heat stress to whole seedlings, we address the longstanding question of possible heterogeneity among cell types in the response to an abiotic stress. Although the response of canonical heat-shock genes dominates expression across cell types, subtle but significant differences in other genes can be detected among cell types. Taken together, our results demonstrate that single cell transcriptomics holds promise for studying plant development and plant physiology with unprecedented resolution.

The 3′ untranslated region (UTR) plays critical roles in determining the level of gene expression, through effects on activities such as mRNA stability and translation. The underlying functional elements within this region have largely been identified through analyses of the limited number of native genes. To explore the effects of sequence elements when not present in biologically evolved sequence backgrounds, we analyzed hundreds of thousands of random 50-mers inserted into the 3′ UTR of a reporter gene in the yeast Saccharomyces cerevisiae. We determined relative protein expression levels from the fitness of a library of transformants in a growth selection. We find that the consensus 3′ UTR efficiency element significantly boosts expression, independent of sequence context; on the other hand, the consensus positioning element has only a small effect on expression. Some sequence motifs that are binding sites for Puf proteins substantially increase expression in this random library, despite these proteins generally being associated with post-transcriptional downregulation when bound to native mRNAs. Thus, the regulatory effects of 3′ UTR sequence features like the positioning element and Puf binding sites appear to be strongly dependent on their context within native genes, where they exist alongside co-evolved sequence features. Our measurements also allowed a systematic examination of the effects of point mutations within efficiency element motifs across diverse sequence backgrounds. These mutational scans reveal the relative in vivo importance of individual bases in the efficiency element, which likely reflects their roles in binding the Hrp1 protein involved in cleavage and polyadenylation.

Seoul virus (SEOV) is a zoonotic orthohantavirus carried by rats. In humans, SEOV can cause hemorrhagic fever with renal syndrome. Recent human SEOV cases described in the USA, United Kingdom, France and the Netherlands were associated with contact with pet or feeder rats. The prevalence of SEOV in these types of rats is unknown. We collected 175 pet and feeder rats (Rattus norvegicus) from private owners, ratteries and commercial breeders/traders in the Netherlands. Lung tissue of the rats was tested using a SEOV real-time RT-qPCR and heart fluid was tested for the presence of antibodies against SEOV. In all three investigated groups, RT-qPCR-positive rats were found: in 1/29 rats from private owners (3.6%), 2/56 rats from ratteries (3.4%) and 11/90 rats from commercial breeders (12.2%). The seroprevalence was largely similar to the prevalence calculated from RT-qPCR-positive rats. The SEOV sequences found were highly similar to sequences previously found in domesticated rats in Europe. In conclusion, SEOV is spread throughout different populations of domesticated rats.

Eukaryotes maintain hundreds of copies of ribosomal DNA (rDNA), many more than required for ribosome biogenesis, suggesting a yet undefined role for large rDNA arrays outside of ribosomal RNA synthesis. We demonstrate that reducing the Saccharomyces cerevisiae rDNA array to 35 copies, which is sufficient for ribosome function, shifts rDNA from being the latest replicating region in the genome to one of the earliest. This change in replication timing results in delayed genome-wide replication and classic replication defects. We present evidence that the requirement for rDNA to replicate late, which is conserved among eukaryotes, also coordinates the completion of genome replication with anaphase entry through the proper sequestration of the mitotic exit regulator Cdc14p in the rDNA-containing nucleolus. Our findings suggest that, instead of being a passive repetitive element, the large late-replicating rDNA array plays an active role in genome replication and cell cycle control.

With growing populations and pressing environmental problems, future economies will be increasingly plant-based. Now is the time to reimagine plant science as a critical component of fundamental science, agriculture, environmental stewardship, energy, technology and healthcare. This effort requires a conceptual and technological framework to identify and map all cell types, and to comprehensively annotate the localization and organization of molecules at cellular and tissue levels. This framework, called the Plant Cell Atlas (PCA), will be critical for understanding and engineering plant development, physiology and environmental responses. A workshop was convened to discuss the purpose and utility of such an initiative, resulting in a roadmap that acknowledges the current knowledge gaps and technical challenges, and underscores how the PCA initiative can help to overcome them., National Science Foundation 1916797 David W Ehrhardt, Kenneth D Birnbaum, Seung Yon Rhee; National Science Foundation 2052590 Seung Yon Rhee

Root architecture is a major determinant of plant fitness and is under constant modification in response to favorable and unfavorable environmental stimuli. Beyond impacts on the primary root, the environment can alter the position, spacing, density, and length of secondary or lateral roots. Lateral root development is among the best-studied examples of plant organogenesis, yet there are still many unanswered questions about its earliest steps. Among the challenges faced in capturing these first molecular events is the fact that this process occurs in a small number of cells with unpredictable timing. Single-cell sequencing methods afford the opportunity to isolate the specific transcriptional changes occurring in cells undergoing this fate transition. Using this approach, we successfully captured the transcriptomes of initiating lateral root primordia in Arabidopsis thaliana and discovered many upregulated genes associated with this process. We developed a method to selectively repress target gene transcription in the xylem pole pericycle cells where lateral roots originate and demonstrated that the expression of several of these targets is required for normal root development. We also discovered subpopulations of cells in the pericycle and endodermal cell files that respond to lateral root initiation, highlighting the coordination across cell files required for this fate transition.

Root architecture is a major determinant of fitness, and is under constant modification in response to favorable and unfavorable environmental stimuli. Beyond impacts on the primary root, the environment can alter the position, spacing, density and length of secondary or lateral roots. Lateral root development is among the best-studied examples of plant organogenesis, yet there are still many unanswered questions about its earliest steps. Among the challenges faced in capturing these first molecular events is the fact that this process occurs in a small number of cells with unpredictable timing. Single-cell sequencing methods afford the opportunity to isolate the specific transcriptional changes occurring in cells undergoing this fate transition. Using this approach, we successfully captured the transcriptomes of initiating lateral root primordia, and discovered many previously unreported upregulated genes associated with this process. We developed a method to selectively repress target gene transcription in the xylem pole pericycle cells where lateral roots originate, and demonstrated that expression of several of these targets was required for normal root development. We also discovered novel subpopulations of cells in the pericycle and endodermal cell files that respond to lateral root initiation, highlighting the coordination across cell files required for this fate transition.One sentence summarySingle cell RNA sequencing reveals new molecular details about lateral root initiation, including the transcriptional impacts of the primordia on bordering cells.

In plants, chromatin accessibility – the primary mark of regulatory DNA – is relatively static across tissues and conditions. This scarcity of accessible sites that are dynamic or tissue-specific may be due in part to tissue heterogeneity in previous bulk studies. To assess the effects of tissue heterogeneity, we apply single-cell ATAC-seq to A. thaliana roots and identify thousands of differentially accessible sites, sufficient to resolve all major cell types of the root. However, even this vast increase relative to bulk studies in the number of dynamic sites does not resolve the poor correlation at individual loci between accessibility and expression. Instead, we find that the entirety of a cell’s regulatory landscape and its transcriptome each capture cell type identity independently. We leverage this shared information on cell identity to integrate accessibility and transcriptome data in order to characterize developmental progression, endoreduplication and cell division in the root. We further use the combined data to characterize cell type-specific motif enrichments of large transcription factor families and to link the expression of individual family members to changing accessibility at specific loci, taking the first steps toward resolving the direct and indirect effects that shape gene expression. Our approach provides an analytical framework to infer the gene regulatory networks that execute plant development.

Single-cell genomic approaches have the potential to revolutionize the study of plant systems. Here, we highlight newly developed techniques to analyze transcriptomes at single-cell resolution. We focus on the rigorous standards necessary to generate and compare these data sets introducing analysis methods that can be applied to interpret their results. Lastly, we discuss the inherent limitations of single-cell studies and address future directions for plant single-cell genomics.

Single-cell RNA-seq can yield high-resolution cell-type-specific expression signatures that reveal new cell types and the developmental trajectories of cell lineages. Here, we apply this approach toA. thalianaroot cells to capture gene expression in 3,121 root cells. We analyze these data with Monocle 3, which orders single cell transcriptomes in an unsupervised manner and uses machine learning to reconstruct single-cell developmental trajectories along pseudotime. We identify hundreds of genes with cell-type-specific expression, with pseudotime analysis of several cell lineages revealing both known and novel genes that are expressed along a developmental trajectory. We identify transcription factor motifs that are enriched in early and late cells, together with the corresponding candidate transcription factors that likely drive the observed expression patterns. We assess and interpret changes in total RNA expression along developmental trajectories and show that trajectory branch points mark developmental decisions. Finally, by applying heat stress to whole seedlings, we address the longstanding question of possible heterogeneity among cell types in the response to an abiotic stress. Although the response of canonical heat shock genes dominates expression across cell types, subtle but significant differences in other genes can be detected among cell types. Taken together, our results demonstrate that single-cell transcriptomics holds promise for studying plant development and plant physiology with unprecedented resolution.

Few mechanisms are known that explain how transcription factors can adjust phenotypic outputs to accommodate differing environments. In Saccharomyces cerevisiae, the decision to mate or invade relies on environmental cues that converge on a shared transcription factor, Ste12. Specificity toward invasion occurs via Ste12 binding cooperatively with the cofactor Tec1. Here, we determine the range of phenotypic outputs (mating vs. invasion) of thousands of DNA-binding domain variants in Ste12 to understand how preference for invasion may arise. We find that single amino acid changes in the DNA-binding domain can shift the preference of yeast toward either mating or invasion. These mutations define two distinct regions of this domain, suggesting alternative modes of DNA binding for each trait. We characterize the DNA-binding specificity of wild-type Ste12 to identify a strong preference for spacing and orientation of both homodimeric and heterodimeric sites. Ste12 mutants that promote hyperinvasion in a Tec1-independent manner fail to bind cooperative sites with Tec1 and bind to unusual dimeric Ste12 sites composed of one near-perfect and one highly degenerate site. We propose a model in which Ste12 alone may have evolved to activate invasion genes, which could explain how preference for invasion arose in the many fungal pathogens that lack Tec1.

Our ability to predict protein expression from DNA sequence alone remains poor, reflecting our limited understanding of cis-regulatory grammar and hampering the design of engineered genes for synthetic biology applications. Here, we generate a model that predicts the protein expression of the 5′ untranslated region (UTR) of mRNAs in the yeast Saccharomyces cerevisiae. We constructed a library of half a million 50-nucleotide-long random 5′ UTRs and assayed their activity in a massively parallel growth selection experiment. The resulting data allow us to quantify the impact on protein expression of Kozak sequence composition, upstream open reading frames (uORFs), and secondary structure. We trained a convolutional neural network (CNN) on the random library and showed that it performs well at predicting the protein expression of both a held-out set of the random 5′ UTRs as well as native S. cerevisiae 5′ UTRs. The model additionally was used to computationally evolve highly active 5′ UTRs. We confirmed experimentally that the great majority of the evolved sequences led to higher protein expression rates than the starting sequences, demonstrating the predictive power of this model.

In Saccharomyces cerevisiae, the decision to mate or invade relies on environmental cues that converge on a shared transcription factor, Ste12. Specificity toward invasion occurs via Ste12 binding cooperatively with the co-factor Tec1. Here, we characterize the in vitro binding preferences of Ste12 to identify a defined spacing and orientation of dimeric sites, one that is common in pheromone-regulated genes. We find that single amino acid changes in the DNA-binding domain of Ste12 can shift the preference of yeast toward either mating or invasion. These mutations define two distinct regions of this domain, suggesting alternative modes of DNA binding for each trait. Some exceptional Ste12 mutants promote hyperinvasion in a Tec1-independent manner; these fail to bind cooperative sites with Tec1 and bind to unusual dimeric Ste12 sites that contain one highly degenerate half site. We propose a model for how activation of invasion genes could have evolved with Ste12 alone.

[EN] In RNA-directed silencing pathways, ternary complexes result from small RNA-guided ARGONAUTE (AGO) associating with target transcripts. Target transcripts are often silenced through direct cleavage (slicing), destabilization through slicer-independent turnover mechanisms, and translational repression. Here, wild-type and active-site defective forms of several Arabidopsis thaliana AGO proteins involved in posttranscriptional silencing were used to examine several AGO functions, including small RNA binding, interaction with target RNA, slicing or destabilization of target RNA, secondary small interfering RNA formation, and antiviral activity. Complementation analyses in ago mutant plants revealed that the catalytic residues of AGO1, AGO2, and AGO7 are required to restore the defects of Arabidopsis ago1-25, ago2-1, and zip-1 (AGO7-defective) mutants, respectively. AGO2 had slicer activity in transient assays but could not trigger secondary small interfering RNA biogenesis, and catalytically active AGO2 was necessary for local and systemic antiviral activity against Turnip mosaic virus. Slicer-defective AGOs associated with miRNAs and stabilized AGO-miRNA-target RNA ternary complexes in individual target coimmunoprecipitation assays. In genome-wide AGO-miRNA-target RNA coimmunoprecipitation experiments, slicer-defective AGO1-miRNA associated with target RNA more effectively than did wild-type AGO1-miRNA. These data not only reveal functional roles for AGO1, AGO2, and AGO7 slicer activity, but also indicate an approach to capture ternary complexes more efficiently for genome-wide analyses., We thank Goretti Nguyen for excellent technical assistance. A. C. was supported by a postdoctoral fellowship from the Ministerio de Ciencia e Innovacion (BMC-2008-0188). H.G.-R. was the recipient of a Helen Hay Whitney Postdoctoral fellowship (F-972). This work was supported by grants from the National Science Foundation (MCB-1231726), the National Institutes of Health (AI043288), and Monsanto Corporation.

De novo DNa methylation in Arabidopsis thaliana is catalyzed by the methyltransferase DRM2, a homolog of the mammalian de novo methyltransferase DNMT3. DRM2 is targeted to DN a by small interfering RNas ( siRNas ) in a process known as RNa- directed DNa Methylation (RdDM). While several components of the RdDM pathway are known, a functional understanding of the underlying mechanism is far from complete. We employed both forward and reverse genetic approaches to identify factors involved in de novo methylation. We utilized the FWA transgene, which is methylated and silenced when transformed into wild-type plants, but unmethylated and expressed when transformed into de novo methylation mutants. Expression of FWA is marked by a late-flowering phenotype, which is easily scored in mutant versus wild-type plants. By reverse genetics we discovered the requirement for known RdDM effectors aG O6 and NRpE 5a for efficient de novo methylation. a forward genetic approach uncovered alleles of several components of the RdDM pathway, including alleles of clsy1 , ktf1 and nrpd/e2, which have not been previously shown to be required for the initial establishment of DN a methylation. Mutations were mapped and genes cloned by both traditional and whole genome sequencing approaches. The methodologies and the mutant alleles discovered will be instrumental in further studies of de novo DNa methylation.

Transacting siRNA (tasiRNA) biogenesis in Arabidopsis is initiated by microRNA (miRNA) –guided cleavage of primary transcripts. In the case of TAS3 tasiRNA formation, ARGONAUTE7 (AGO7)–miR390 complexes interact with primary transcripts at two sites, resulting in recruitment of RNA-DEPENDENT RNA POLYMERASE6 for dsRNA biosynthesis. An extensive screen for Arabidopsis mutants with specific defects in TAS3 tasiRNA biogenesis or function was done. This yielded numerous ago7 mutants, one dcl4 mutant, and two mutants that accumulated low levels of miR390. A direct genome sequencing-based approach to both map and rapidly identify one of the latter mutant alleles was developed. This revealed a G-to-A point mutation ( mir390a-1 ) that was calculated to stabilize a relatively nonpaired region near the base of the MIR390a foldback, resulting in misprocessing of the miR390/miR390* duplex and subsequent reduced TAS3 tasiRNA levels. Directed substitutions, as well as analysis of variation at paralogous miR390-generating loci ( MIR390a and MIR390b ), indicated that base pair properties and nucleotide identity within a region 4–6 bases below the miR390/miR390* duplex region contributed to the efficiency and accuracy of precursor processing.

SummaryTrans-acting siRNA form through a refined RNAi mechanism in plants. miRNA-guided cleavage triggers entry of precursor transcripts into an RNA-DEPENDENT RNA POLYMERASE6 pathway, and sets the register for phased tasiRNA formation by DICER-LIKE4. Here, we show that miR390-ARGONAUTE7 complexes function in distinct cleavage or noncleavage modes at two target sites in TAS3a transcripts. The AGO7 cleavage, but not the noncleavage, function could be provided by AGO1, the dominant miRNA-associated AGO, but only when AGO1 was guided to a modified target site through an alternate miRNA. AGO7 was highly selective for interaction with miR390, and miR390 in turn was excluded from association with AGO1 due entirely to an incompatible 5′ adenosine. Analysis of AGO1, AGO2, and AGO7 revealed a potent 5′ nucleotide discrimination function for some, although not all, ARGONAUTEs. miR390 and AGO7, therefore, evolved as a highly specific miRNA guide/effector protein pair to function at two distinct tasiRNA biogenesis steps.

Strategies to optimize a metabolic pathway often involve building a large collection of strains, each containing different versions of sequences that regulate the expression of pathway genes. Here, we develop reagents and methods to carry out this process at high efficiency in the yeast Saccharomyces cerevisiae. We identify variants of the Escherichia coli tet operator (tetO) sequence that bind a TetR-VP16 activator with differential affinity and therefore result in different TetR-VP16 activator-driven expression. By recombining these variants upstream of the genes of a pathway, we generate unique combinations of expression levels. Here, we built a tetO toolkit, which includes the I-OnuI homing endonuclease to create double-strand breaks, which increases homologous recombination by 10(5); a plasmid carrying six variant tetO sequences flanked by I-OnuI sites, uncoupling transformation and recombination steps; an S. cerevisiae-optimized TetR-VP16 activator; and a vector to integrate constructs into the yeast genome. We introduce into the S. cerevisiae genome the three crt genes from Erwinia herbicola required for yeast to synthesize lycopene and carry out the recombination process to produce a population of cells with permutations of tetO variants regulating the three genes. We identify 0.7% of this population as making detectable lycopene, of which the vast majority have undergone recombination at all three crt genes. We estimate a rate of ∼20% recombination per targeted site, much higher than that obtained in other studies. Application of this toolkit to medically or industrially important end products could reduce the time and labor required to optimize the expression of a set of metabolic genes.

Agrobacterium tumefaciens and Agrobacterium rhizogenes transfer plasmid-encoded genes and virulence (Vir) proteins into plant cells. The transferred DNA (T-DNA) is stably inherited and expressed in plant cells, causing crown gall or hairy root disease. DNA transfer from A. tumefaciens into plant cells resembles plasmid conjugation; single-stranded DNA (ssDNA) is exported from the bacteria via a type IV secretion system comprised of VirB1 through VirB11 and VirD4. Bacteria also secrete certain Vir proteins into plant cells via this pore. One of these, VirE2, is an ssDNA-binding protein crucial for efficient T-DNA transfer and integration. VirE2 binds incoming ssT-DNA and helps target it into the nucleus. Some strains of A. rhizogenes lack VirE2, but they still transfer T-DNA efficiently. We isolated a novel gene from A. rhizogenes that restored pathogenicity to virE2 mutant A. tumefaciens . The GALLS gene was essential for pathogenicity of A. rhizogenes . Unlike VirE2, GALLS contains a nucleoside triphosphate binding motif similar to one in TraA, a strand transferase conjugation protein. Despite their lack of similarity, GALLS substituted for VirE2.

Agrobacterium tumefaciens oncogenes cause transformed plant cells to overproduce auxin and cytokinin. Two oncogenes encode enzymes that convert tryptophan to indole-3-acetic acid (auxin): iaaM (tryptophan mono-oxygenase) and iaaH (indole-3-acetamide hydrolase). A third oncogene (ipt) encodes AMP isopentenyl transferase, which produces cytokinin (isopentenyl-AMP). Inactivation of ipt and iaaM (or iaaH) abolishes tumorigenesis. Because adequate means do not exist to control crown gall, we created resistant plants by introducing transgenes designed to elicit posttranscriptional gene silencing (PTGS) of iaaM and ipt. Transgenes that elicit silencing trigger sequence-specific destruction of the inducing RNA and messenger RNAs with related sequences. Although PTGS has proven effective against a variety of target genes, we found that a much higher percentage of transgenic lines silenced iaaM than ipt, suggesting that transgene sequences influenced the effectiveness of PTGS. Sequences required for oncogene silencing included a translation start site. A transgene encoding a translatable sense-strand RNA from the 5′ end of iaaM silenced the iaaM oncogene, but deletion of the translation start site abolished the ability of the transgene to silence iaaM. Silencing A. tumefaciens T-DNA oncogenes is a new and effective method to produce plants resistant to crown gall disease.

MicroRNAs (miRNAs) are small regulatory RNAs found in diverse eukaryotic lineages. In plants, a minority of annotated MIRNA gene families are conserved between plant families, while the majority are family- or species-specific, suggesting that most known MIRNA genes arose relatively recently in evolutionary time. Given the high proportion of young MIRNA genes in plant species, new MIRNA families are likely spawned and then lost frequently. Unlike highly conserved, ancient miRNAs, young miRNAs are often weakly expressed, processed imprecisely, lack targets, and display patterns of neutral variation, suggesting that young MIRNA loci tend to evolve neutrally. Genome-wide analyses from several plant species have revealed that variation in miRNA foldback expression, structure, processing efficiency, and miRNA size have resulted in the unique functionality of MIRNA loci and resulting miRNAs. Additionally, some miRNAs have evolved specific properties and functions that regulate other transcriptional or posttranscriptional silencing pathways. The evolution of miRNA processing and functional diversity underscores the dynamic nature of miRNA-based regulation in complex regulatory networks.

[EN] NA interference pathways can involve amplification of secondary siRNAs by RNA-dependent RNA polymerases. In plants, RDR6-dependent secondary siRNAs arise from transcripts targeted by some microRNAs (miRNAs). Here, Arabidopsis thaliana secondary siRNAs from mRNA as well as trans-acting siRNAs are shown to be triggered through initial targeting by a 22-nucleotide (nt) miRNA that associates with AGO1. In contrast to canonical 21-nt miRNAs, 22-nt miRNAs primarily arise from foldback precursors containing asymmetric bulges. Using artificial miRNA constructs, conversion of asymmetric foldbacks to symmetric foldbacks resulted in the production of 21-nt forms of miR173, miR472 and miR828. Both 21- and 22-nt forms associated with AGO 1 and guided accurate slicer activity, but only 22-nt forms were competent to trigger RDR6-dependent siRNA production from target RNA. These data suggest that AGO 1 functions differentially with 21- and 22-nt miRNAs to engage the RDR6-associated amplification apparatus., A. C. was supported by a postdoctoral fellowship from the Spanish Ministerio de Ciencia e Innovacion (BMC-2008-0188). This work was supported by grants from the US National Science Foundation (MCB-0618433 and MCB-0956526), the US National Institutes of Health (AI43288) and Monsanto Corporation.

In eukaryotes, RNA silencing pathways utilize 20-30-nucleotide small RNAs to regulate gene expression, specify and maintain chromatin structure, and repress viruses and mobile genetic elements. RNA silencing was likely present in the common ancestor of modern eukaryotes, but most research has focused on plant and animal RNA silencing systems. Phytophthora species belong to a phylogenetically distinct group of economically important plant pathogens that cause billions of dollars in yield losses annually as well as ecologically devastating outbreaks. We analyzed the small RNA-generating components of the genomes of P. infestans, P. sojae and P. ramorum using bioinformatics, genetic, phylogenetic and high-throughput sequencing-based methods. Each species produces two distinct populations of small RNAs that are predominantly 21- or 25-nucleotides long. The 25-nucleotide small RNAs were primarily derived from loci encoding transposable elements and we propose that these small RNAs define a pathway of short-interfering RNAs that silence repetitive genetic elements. The 21-nucleotide small RNAs were primarily derived from inverted repeats, including a novel microRNA family that is conserved among the three species, and several gene families, including Crinkler effectors and type III fibronectins. The Phytophthora microRNA is predicted to target a family of amino acid/auxin permeases, and we propose that 21-nucleotide small RNAs function at the post-transcriptional level. The functional significance of microRNA-guided regulation of amino acid/auxin permeases and the association of 21-nucleotide small RNAs with Crinkler effectors remains unclear, but this work provides a framework for testing the role of small RNAs in Phytophthora biology and pathogenesis in future work.