Streamlined spatial and environmental expression signatures characterize the minimalist duckweed Wolffia australiana.

Single-cell genomics permits a new resolution in the examination of molecular and cellular dynamics, allowing global, parallel assessments of cell types and cellular behaviors through development and in response to environmental circumstances, such as interaction with water and the light-dark cycle of the Earth. Here, we leverage the smallest, and possibly most structurally reduced, plant, the semiaquatic Wolffia australiana, to understand dynamics of cell expression in these contexts at the whole-plant level. We examined single-cell-resolution RNA-sequencing data and found Wolffia cells divide into four principal clusters representing the above- and below-water-situated parenchyma and epidermis. Although these tissues share transcriptomic similarity with model plants, they display distinct adaptations that Wolffia has made for the aquatic environment. Within this broad classification, discrete subspecializations are evident, with select cells showing unique transcriptomic signatures associated with developmental maturation and specialized physiologies. Assessing this simplified biological system temporally at two key time-of-day (TOD) transitions, we identify additional TOD-responsive genes previously overlooked in whole-plant transcriptomic approaches and demonstrate that the core circadian clock machinery and its downstream responses can vary in cell-specific manners, even in this simplified system. Distinctions between cell types and their responses to submergence and/or TOD are driven by expression changes of unexpectedly few genes, characterizing Wolffia as a highly streamlined organism with the majority of genes dedicated to fundamental cellular processes. Wolffia provides a unique opportunity to apply reductionist biology to elucidate signaling functions at the organismal level, for which this work provides a powerful resource.

The START domain potentiates HD-ZIPIII transcriptional activity.

The CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIPIII) transcription factors (TFs) were repeatedly deployed over 725 million years of evolution to regulate central developmental innovations. The START domain of this pivotal class of developmental regulators was recognized over 20 years ago, but its putative ligands and functional contributions remain unknown. Here, we demonstrate that the START domain promotes HD-ZIPIII TF homodimerization and increases transcriptional potency. Effects on transcriptional output can be ported onto heterologous TFs, consistent with principles of evolution via domain capture. We also show the START domain binds several species of phospholipids, and that mutations in conserved residues perturbing ligand binding and/or its downstream conformational readout abolish HD-ZIPIII DNA-binding competence. Our data present a model in which the START domain potentiates transcriptional activity and uses ligand-induced conformational change to render HD-ZIPIII dimers competent to bind DNA. These findings resolve a long-standing mystery in plant development and highlight the flexible and diverse regulatory potential coded within this widely distributed evolutionary module.

Genome-wide analysis of plant miRNA action clarifies levels of regulatory dynamics across developmental contexts.

Development of complex organisms requires the delicate and dynamic spatiotemporal regulation of gene expression. Central to this are microRNAs (miRNAs). These mobile small RNAs offer specificity in conveying positional information and versatility in patterning the outcomes of gene expression. However, the parameters that shape miRNA output during development are still to be clarified. Here, we address this question on a genome-wide scale, using the maize shoot apex as a model. We show that patterns and levels of miRNA accumulation are largely determined at the transcriptional level, but are finessed post-transcriptionally in a tissue-dependent manner. The stem cell environments of the shoot apical meristem and vasculature appear particularly liable to this. Tissue-specific effects are also apparent at the level of target repression, with target cleavage products in the vasculature exceeding those of other tissues. Our results argue against a clearance mode of regulation purely at the level of transcript cleavage, leading us to propose that transcript cleavage provides a baseline level of target repression, onto which miRNA-driven translational repression can act to toggle the mode of target regulation between clearance and rheostat. Our data show how the inherent complexities of miRNA pathways allow the accumulation and activity of these small RNAs to be tailored in space and time to bring about the gene expression versatility needed during development.

Distinct identities of leaf phloem cells revealed by single cell transcriptomics.

The leaf vasculature plays a key role in solute translocation. Veins consist of at least seven distinct cell types, with specific roles in transport, metabolism, and signaling. Little is known about leaf vascular cells, in particular the phloem parenchyma (PP). PP effluxes sucrose into the apoplasm as a basis for phloem loading, yet PP has been characterized only microscopically. Here, we enriched vascular cells from Arabidopsis leaves to generate a single-cell transcriptome atlas of leaf vasculature. We identified at least 19 cell clusters, encompassing epidermis, guard cells, hydathodes, mesophyll, and all vascular cell types, and used metabolic pathway analysis to define their roles. Clusters comprising PP cells were enriched for transporters, including SWEET11 and SWEET12 sucrose and UmamiT amino acid efflux carriers. We provide evidence that PP development occurs independently from ALTERED PHLOEM DEVELOPMENT, a transcription factor required for phloem differentiation. PP cells have a unique pattern of amino acid metabolism activity distinct from companion cells (CCs), explaining differential distribution/metabolism of amino acids in veins. The kinship relation of the vascular clusters is strikingly similar to the vein morphology, except for a clear separation of CC from the other vascular cells including PP. In summary, our single-cell RNA-sequencing analysis provides a wide range of information into the leaf vasculature and the role and relationship of the leaf cell types.

Sponging of glutamate at the outer plasma membrane surface reveals roles for glutamate in development.

Plants use electrical and chemical signals for systemic communication. Herbivory, for instance, appears to trigger local apoplasmic glutamate accumulation, systemic electrical signals, and calcium waves that travel to report insect damage to neighboring leaves and initiate defense. To monitor extra- and intracellular glutamate concentrations in plants, we generated Arabidopsis lines expressing genetically encoded fluorescent glutamate sensors. In contrast to cytosolically localized sensors, extracellularly displayed variants inhibited plant growth and proper development. Phenotypic analyses of high-affinity display sensor lines revealed that root meristem development, particularly the quiescent center, number of lateral roots, vegetative growth, and floral architecture were impacted. Notably, the severity of the phenotypes was positively correlated with the affinity of the display sensors, intimating that their ability to sequester glutamate at the surface of the plasma membrane was responsible for the defects. Root growth defects were suppressed by supplementing culture media with low levels of glutamate. Together, the data indicate that sequestration of glutamate at the cell surface either disrupts the supply of glutamate to meristematic cells and/or impairs localized glutamatergic signaling important for developmental processes.

A high-resolution gene expression atlas links dedicated meristem genes to key architectural traits.

The shoot apical meristem (SAM) orchestrates the balance between stem cell proliferation and organ initiation essential for postembryonic shoot growth. Meristems show a striking diversity in shape and size. How this morphological diversity relates to variation in plant architecture and the molecular circuitries driving it are unclear. By generating a high-resolution gene expression atlas of the vegetative maize shoot apex, we show here that distinct sets of genes govern the regulation and identity of stem cells in maize versus Arabidopsis. Cell identities in the maize SAM reflect the combinatorial activity of transcription factors (TFs) that drive the preferential, differential expression of individual members within gene families functioning in a plethora of cellular processes. Subfunctionalization thus emerges as a fundamental feature underlying cell identity. Moreover, we show that adult plant characters are, to a significant degree, regulated by gene circuitries acting in the SAM, with natural variation modulating agronomically important architectural traits enriched specifically near dynamically expressed SAM genes and the TFs that regulate them. Besides unique mechanisms of maize stem cell regulation, our atlas thus identifies key new targets for crop improvement.

Correction: Mendelian and Non-Mendelian Regulation of Gene Expression in Maize.

[This corrects the article DOI: 10.1371/journal.pgen.1003202.].

The ASYMMETRIC LEAVES complex maintains repression of KNOX homeobox genes via direct recruitment of Polycomb-repressive complex2.

Polycomb-repressive complexes (PRCs) ensure the correct spatiotemporal expression of numerous key developmental regulators. Despite their pivotal role, how PRCs are recruited to specific targets remains largely unsolved, particularly in plants. Here we show that the Arabidopsis ASYMMETRIC LEAVES complex physically interacts with PRC2 and recruits this complex to the homeobox genes BREVIPEDICELLUS and KNAT2 to stably silence these stem cell regulators in differentiating leaves. The recruitment mechanism resembles the Polycomb response element-based recruitment of PRC2 originally defined in flies and provides the first such example in plants. Combined with recent studies in mammals, our findings reveal a conserved paradigm to epigenetically regulate homeobox gene expression during development.

Genomic prediction of maize microphenotypes provides insights for optimizing selection and mining diversity.

Effective evaluation of millions of crop genetic stocks is an essential component of exploiting genetic diversity to achieve global food security. By leveraging genomics and data analytics, genomic prediction is a promising strategy to efficiently explore the potential of these gene banks by starting with phenotyping a small designed subset. Reliable genomic predictions have enhanced selection of many macroscopic phenotypes in plants and animals. However, the use of genomicprediction strategies for analysis of microscopic phenotypes is limited. Here, we exploited the power of genomic prediction for eight maize traits related to the shoot apical meristem (SAM), the microscopic stem cell niche that generates all the above-ground organs of the plant. With 435 713 genomewide single-nucleotide polymorphisms (SNPs), we predicted SAM morphology traits for 2687 diverse maize inbreds based on a model trained from 369 inbreds. An empirical validation experiment with 488 inbreds obtained a prediction accuracy of 0.37-0.57 across eight traits. In addition, we show that a significantly higher prediction accuracy was achieved by leveraging the U value (upper bound for reliability) that quantifies the genomic relationships of the validation set with the training set. Our findings suggest that double selection considering both prediction and reliability can be implemented in choosing selection candidates for phenotyping when exploring new diversity is desired. In this case, individuals with less extreme predicted values and moderate reliability values can be considered. Our study expands the turbocharging gene banks via genomic prediction from the macrophenotypes into the microphenotypic space.

Intragenic Meiotic Crossovers Generate Novel Alleles with Transgressive Expression Levels.

Meiotic recombination is an evolutionary force that generates new genetic diversity upon which selection can act. Whereas multiple studies have assessed genome-wide patterns of recombination and specific cases of intragenic recombination, few studies have assessed intragenic recombination genome-wide in higher eukaryotes. We identified recombination events within or near genes in a population of maize recombinant inbred lines (RILs) using RNA-sequencing data. Our results are consistent with case studies that have shown that intragenic crossovers cluster at the 5' ends of some genes. Further, we identified cases of intragenic crossovers that generate transgressive transcript accumulation patterns, that is, recombinant alleles displayed higher or lower levels of expression than did nonrecombinant alleles in any of ∼100 RILs, implicating intragenic recombination in the generation of new variants upon which selection can act. Thousands of apparent gene conversion events were identified, allowing us to estimate the genome-wide rate of gene conversion at SNP sites (4.9 × 10-5). The density of syntenic genes (i.e., those conserved at the same genomic locations since the divergence of maize and sorghum) exhibits a substantial correlation with crossover frequency, whereas the density of nonsyntenic genes (i.e., those which have transposed or been lost subsequent to the divergence of maize and sorghum) shows little correlation, suggesting that crossovers occur at higher rates in syntenic genes than in nonsyntenic genes. Increased rates of crossovers in syntenic genes could be either a consequence of the evolutionary conservation of synteny or a biological process that helps to maintain synteny.

The Sussex signal: insights into leaf dorsiventrality.

The differentiation of a leaf - from its inception as a semicircular bulge on the surface of the shoot apical meristem into a flattened structure with specialized upper and lower surfaces - is one of the most intensely studied processes in plant developmental biology. The large body of contemporary data on leaf dorsiventrality has its origin in the pioneering experiments of Ian Sussex, who carried out these studies as a PhD student in the early 1950s. Here, we review his original experiments in their historical context and describe our current understanding of this surprisingly complex process. Finally, we postulate possible candidates for the 'Sussex signal' - the elusive meristem-derived factor that first ignited interest in this important developmental problem.

In Planta Single-Molecule Pull-Down Reveals Tetrameric Stoichiometry of HD-ZIPIII:LITTLE ZIPPER Complexes.

Deciphering complex biological processes markedly benefits from approaches that directly assess the underlying biomolecular interactions. Most commonly used approaches to monitor protein-protein interactions typically provide nonquantitative readouts that lack statistical power and do not yield information on the heterogeneity or stoichiometry of protein complexes. Single-molecule pull-down (SiMPull) uses single-molecule fluorescence detection to mitigate these disadvantages and can quantitatively interrogate interactions between proteins and other compounds, such as nucleic acids, small molecule ligands, and lipids. Here, we establish SiMPull in plants using the HOMEODOMAIN LEUCINE ZIPPER III (HD-ZIPIII) and LITTLE ZIPPER (ZPR) interaction as proof-of-principle. Colocalization analysis of fluorophore-tagged HD-ZIPIII and ZPR proteins provides strong statistical evidence of complex formation. In addition, we use SiMPull to directly quantify YFP and mCherry maturation probabilities, showing these differ substantially from values obtained in mammalian systems. Leveraging these probabilities, in conjunction with fluorophore photobleaching assays on over 2000 individual complexes, we determined HD-ZIPIII:ZPR stoichiometry. Intriguingly, these complexes appear as heterotetramers, comprising two HD-ZIPIII and two ZPR molecules, rather than heterodimers as described in the current model. This surprising result raises new questions about the regulation of these key developmental factors and is illustrative of the unique contribution SiMPull is poised to make to in planta protein interaction studies.

The ASYMMETRIC LEAVES Complex Employs Multiple Modes of Regulation to Affect Adaxial-Abaxial Patterning and Leaf Complexity.

Flattened leaf architecture is not a default state but depends on positional information to precisely coordinate patterns of cell division in the growing primordium. This information is provided, in part, by the boundary between the adaxial (top) and abaxial (bottom) domains of the leaf, which are specified via an intricate gene regulatory network whose precise circuitry remains poorly defined. Here, we examined the contribution of the ASYMMETRIC LEAVES (AS) pathway to adaxial-abaxial patterning in Arabidopsis thaliana and demonstrate that AS1-AS2 affects this process via multiple, distinct regulatory mechanisms. AS1-AS2 uses Polycomb-dependent and -independent mechanisms to directly repress the abaxial determinants MIR166A, YABBY5, and AUXIN RESPONSE FACTOR3 (ARF3), as well as a nonrepressive mechanism in the regulation of the adaxial determinant TAS3A. These regulatory interactions, together with data from prior studies, lead to a model in which the sequential polarization of determinants, including AS1-AS2, explains the establishment and maintenance of adaxial-abaxial leaf polarity. Moreover, our analyses show that the shared repression of ARF3 by the AS and trans-acting small interfering RNA (ta-siRNA) pathways intersects with additional AS1-AS2 targets to affect multiple nodes in leaf development, impacting polarity as well as leaf complexity. These data illustrate the surprisingly multifaceted contribution of AS1-AS2 to leaf development showing that, in conjunction with the ta-siRNA pathway, AS1-AS2 keeps the Arabidopsis leaf both flat and simple.

Novel DICER-LIKE1 siRNAs Bypass the Requirement for DICER-LIKE4 in Maize Development.

Dicer enzymes function at the core of RNA silencing to defend against exogenous RNA or to regulate endogenous genes. Plant DICER-LIKE4 (DCL4) performs dual functions, acting in antiviral defense and in development via the biogenesis of trans-acting short-interfering RNAs (siRNAs) termed tasiR-ARFs. These small RNAs play an essential role in the grasses, spatially defining the expression domain of AUXIN RESPONSE FACTOR3 (ARF3) transcription factors. However, contrary to tasiR-ARFs' essential function in development, DCL4 proteins exhibit strong evidence of recurrent adaptation typical of host factors involved in antiviral immunity. Here, we address how DCL4 balances its role in development with pressures to diversify in response to viral attack. We show that, in contrast to other tasiR-ARF biogenesis mutants, dcl4 null alleles have an uncharacteristically mild phenotype, correlated with normal expression of select arf3 targets. Loss of DCL4 activity yields a class of 22-nucleotide tasiR-ARF variants associated with the processing of arf3 transcripts into 22-nucleotide secondary siRNAs by DCL1. Our findings reveal a DCL1-dependent siRNA pathway that bypasses the otherwise adverse developmental effects of mutations in DCL4. This pathway is predicted to have important implications for DCL4's role in antiviral defense by reducing the selective constraints on DCL4 and allowing it to diversify in response to viral suppressors.

Genome-wide analysis of leafbladeless1-regulated and phased small RNAs underscores the importance of the TAS3 ta-siRNA pathway to maize development.

Maize leafbladeless1 (lbl1) encodes a key component in the trans-acting short-interfering RNA (ta-siRNA) biogenesis pathway. Correlated with a great diversity in ta-siRNAs and the targets they regulate, the phenotypes conditioned by mutants perturbing this small RNA pathway vary extensively across species. Mutations in lbl1 result in severe developmental defects, giving rise to plants with radial, abaxialized leaves. To investigate the basis for this phenotype, we compared the small RNA content between wild-type and lbl1 seedling apices. We show that LBL1 affects the accumulation of small RNAs in all major classes, and reveal unexpected crosstalk between ta-siRNA biogenesis and other small RNA pathways regulating transposons. Interestingly, in contrast to data from other plant species, we found no evidence for the existence of phased siRNAs generated via the one-hit model. Our analysis identified nine TAS loci, all belonging to the conserved TAS3 family. Information from RNA deep sequencing and PARE analyses identified the tasiR-ARFs as the major functional ta-siRNAs in the maize vegetative apex where they regulate expression of AUXIN RESPONSE FACTOR3 (ARF3) homologs. Plants expressing a tasiR-ARF insensitive arf3a transgene recapitulate the phenotype of lbl1, providing direct evidence that deregulation of ARF3 transcription factors underlies the developmental defects of maize ta-siRNA biogenesis mutants. The phenotypes of Arabidopsis and Medicago ta-siRNA mutants, while strikingly different, likewise result from misexpression of the tasiR-ARF target ARF3. Our data indicate that diversity in TAS pathways and their targets cannot fully account for the phenotypic differences conditioned by ta-siRNA biogenesis mutants across plant species. Instead, we propose that divergence in the gene networks downstream of the ARF3 transcription factors or the spatiotemporal pattern during leaf development in which these proteins act constitute key factors underlying the distinct contributions of the ta-siRNA pathway to development in maize, Arabidopsis, and possibly other plant species as well.

Signals and prepatterns: new insights into organ polarity in plants.

The flattening of leaves results from the interaction between upper (adaxial) and lower (abaxial) domains in the developing primordium. These domains are specified by conserved, overlapping genetic pathways involving several distinct transcription factor families and small regulatory RNAs. Polarity determinants employ a series of antagonistic interactions to produce mutually exclusive cell fates whose positioning is likely refined by signaling across the adaxial-abaxial boundary. Signaling candidates include a mobile small RNA-the first positional signal described in adaxial-abaxial polarity. Possible mechanisms to polarize the incipient primordium are discussed, including meristem-derived signaling and a model in which a polarized organogenic zone prepatterns the adaxial-abaxial axis.

Pattern formation via small RNA mobility.

MicroRNAs and trans-acting siRNAs (ta-siRNAs) have important regulatory roles in development. Unlike other developmentally important regulatory molecules, small RNAs are not known to act as mobile signals during development. Here, we show that low-abundant, conserved ta-siRNAs, termed tasiR-ARFs, move intercellularly from their defined source of biogenesis on the upper (adaxial) side of leaves to the lower (abaxial) side to create a gradient of small RNAs that patterns the abaxial determinant AUXIN RESPONSE FACTOR3. Our observations have important ramifications for the function of small RNAs and suggest they can serve as mobile, instructive signals during development.

Small RNAs are on the move.

A key feature of RNA interference is its ability to spread from cell to cell. Such non-cell-autonomous gene silencing has been characterized extensively in both plants and animals, but the identity of the mobile silencing signal has remained elusive. Several recent studies now shed light on the identity of this signal in plants, and indicate that small RNA molecules-from short-interfering RNAs to microRNAs-are capable of moving between cells and through the vasculature. The movement of small, 21-24-nucleotide RNA species has implications for biological processes ranging from developmental patterning and stress responses to epigenetic inheritance.