Biology subjects

Axtell, M. J.

Publications and source records attributed to Axtell, M. J..

9 recordsLinked to original sources

bioRxiv2025

The parasitic plant Cuscuta campestris selectively loads trans-species miRNAs onto host Argonautes, but not self Argonautes

The obligate parasitic plant Cuscuta campestris delivers trans-species microRNAs (miRNAs) into host plants to silence host mRNAs. Here, the genetic requirements for biogenesis, movement, and function of these miRNAs were investigated. Primary miRNA transcript accumulation precedes mature miRNA accumulation by 24 to 48 hours. Trans-species miRNAs accumulate in host tissues a short distance from the site of parasite attachment. Trans-species miRNAs require C. campestris but not host Dicer-Like 1 (DCL1) for accumulation. These miRNAs specifically avoid Argonaute (AGO) loading in C. campestris tissue where they instead accumulate as miRNA/miRNA* duplexes. After arrival and short-distance spreading in host tissues, they are selectively loaded into host AGO1. This study clarifies the transcription, dicing, delivery, and function of C. campestris trans-species miRNAs. We propose that selective avoidance of self AGO-loading is a mechanism to facilitate the delivery of these "export only" miRNAs to host tissues.

plant biology↗

bioRxiv2024

miRScore: a rapid and precise microRNA validation tool

MicroRNAs (miRNAs) are small non-protein-coding RNAs that regulate gene expression in many eukaryotes. Next-generation sequencing of small RNAs (small RNA-seq) has accelerated discovery and annotation of novel miRNAs. Newly discovered miRNAs are typically submitted to databases such as the miRBase microRNA registry following the publication of a peer-reviewed study. However, genome-wide scans using small RNA-seq data often yield high rates of false-positive miRNA annotations, highlighting the need for more robust validation methods. miRScore was developed as an independent and efficient tool for evaluating miRNA annotations using sRNA-seq data. miRScore combines structural and expression-based analyses to provide rapid and reliable validation of miRNA annotations. By providing users with detailed metrics and visualization, miRScore enhances the ability to assess confidence in novel and existing miRNA annotations. miRScore has the potential to advance the overall quality of miRNA annotations by improving accuracy of new submissions to miRNA databases and serving as a resource for re-evaluating existing annotations.

bioinformatics↗

bioRxiv2023

Small RNA Analysis of Virus-virus Interaction between Two Orthotospoviruses

Mixed infections of plant viruses are commonly found in natural patho-systems and pre-sent a valuable opportunity to understand how multiple viruses can co-infect the same host. Tomato spotted wilt orthotospovirus (TSWV) and impatiens necrotic spot orthotospovirus (INSV) are pre-sent in the same geographic areas and are closely related. More mixed infections of TSWV and INSV have been reported in recent years, and the INSV host range has been reported to be increasing. In a previous study, we have isolated one strain of INSV and one of TSWV and showed that they have an antagonistic relationship in their vectors, but we were unable to determine, the underlying mech-anisms governing their antagonism in planta and the contribution of the host to this. Here, we used small RNA sequencing to study TSWV-INSV antagonistic interaction and showed that INSV alters plant responses and the processing of TSWV.

plant biology↗

bioRxiv2022

Trans-species microRNA loci in the parasitic plant Cuscuta campestris have a U6-like snRNA promoter

Small regulatory RNAs can move between organisms during pathogenic interactions and regulate gene expression in the recipient. If and how such "trans-species" small RNAs are distinguished from normal small RNAs is not known. The parasitic plant Cuscuta campestris produces a number of microRNAs that specifically accumulate at the interface between parasite and host, several of which have been demonstrated to have trans-species activity. We find that induction of C. campestris interface-induced microRNAs was similar regardless of host species, and can be replicated in haustoria stimulated to develop in the complete absence of a host. We also find that the loci encoding C. campestris interface-induced microRNAs are distinguished by a common 10 base-pair cis-regulatory element. This element is identical to a previously described upstream sequence element used by all plant small nuclear RNA loci. The sequence context of this element strongly suggests U6-like transcription by RNA polymerase III. The element promotes accumulation of interface-induced miRNAs in a heterologous system. This common promoter element distinguishes C. campestris interface-induced microRNA loci from other plant small RNAs; other plant small RNA loci are transcribed by polymerases II or IV, and lack any common promoter motifs. Our data suggest that C. campestris interface-induced miRNAs are produced in a manner distinct from canonical miRNAs. All confirmed C. campestris microRNAs with confirmed trans-species activity are interface-induced and possess these features. We speculate that this distinct production may allow these miRNAs to be exported to hosts.

plant biology↗

bioRxiv2021

Expression and processing of polycistronic artificial microRNAs and trans-acting siRNAs in Solanum lycopersicum and Nicotiana benthamiana

Targeted gene silencing using small regulatory RNAs is a widely used technique for genetic studies in plants. Artificial microRNAs are one common approach; they have the advantage of producing just a single functional small RNA which can be designed for high target specificity and low off-target effects. Simultaneous silencing of multiple targets with artificial microRNAs can be achieved by producing polycistronic microRNA precursors. Alternatively, specialized trans-acting short interfering RNA (tasiRNA) precursors can be designed to produce several specific tasiRNAs at once. Here we tested several artificial microRNA- and tasiRNA-based methods for multiplexed gene silencing in Solanum lycopersicum (tomato) and Nicotiana benthamiana. Small RNA sequencing analyses revealed that many previously described approaches resulted in poor small RNA processing. The 5-most microRNA precursor hairpins on polycistronic artificial microRNA precursors were generally processed more accurately than precursors at the 3 end. Polycistronic artificial microRNAs where the hairpin precursors were separated by transfer RNAs had the best processing precision. Strikingly, artificial tasiRNA precursors failed to be processed in the expected phased manner in our system. These results highlight the need for further development of multiplexed artificial microRNA and tasiRNA strategies. The importance of small RNA sequencing, as opposed to single-target assays such as RNA blots or real-time PCR, is also discussed. Significance statementSeveral strategies for multiplexed gene silencing using artificial microRNAs or tasiRNAs have been described. We find that many result in imprecise processing, and thus low accumulation of the intended small RNAs. Our findings highlight the importance of small RNA sequencing to fully analyze gene silencing experiments, and also the need for continued methodological development of these methods.

plant biology↗

bioRxiv2019

Integrated annotations and analyses of small RNA-producing loci from 47 diverse plants

Plant endogenous small RNAs (sRNAs) are important regulators of gene expression. There are two broad categories of plant sRNAs: microRNAs (miRNAs) and endogenous short interfering RNAs (siRNAs). MicroRNA loci are relatively well-annotated but comprise only a small minority of the total sRNA pool; siRNA locus annotations have lagged far behind. Here, we used a large dataset of published and newly generated sRNA sequencing data (1,333 sRNA-seq libraries containing over 20 billion reads) and a uniform bioinformatic pipeline to produce comprehensive sRNA locus annotations of 47 diverse plants, yielding over 2.7 million sRNA loci. The two most numerous classes of siRNA loci produced mainly 24 nucleotide and 21 nucleotide siRNAs, respectively. 24 nucleotide-dominated siRNA loci usually occurred in intergenic regions, especially at the 5-flanking regions of protein-coding genes. In contrast, 21 nucleotide-dominated siRNA loci were most often derived from double-stranded RNA precursors copied from spliced mRNAs. Genic 21 nucleotide-dominated loci were especially common from disease resistance genes, including from a large number of monocots. Individual siRNA sequences of all types showed very little conservation across species, while mature miRNAs were more likely to be conserved. We developed a web server where our data and several search and analysis tools are freely accessible at http://plantsmallrnagenes.science.psu.edu.

plant biology↗

bioRxiv2019

Compensatory sequence variation between trans-species small RNAs and their target sites

Trans-species small regulatory RNAs (sRNAs) are delivered to host plants from diverse pathogens and parasites and can target host mRNAs. How trans-species sRNAs can be effective on diverse hosts has been unclear. Multiple species of the parasitic plant Cuscuta produce trans-species sRNAs that collectively target many host mRNAs. Confirmed target sites are nearly always in highly conserved, protein-coding regions of host mRNAs. Cuscuta trans-species sRNAs can be grouped into superfamilies that have variation in a three-nucleotide period. These variants compensate for synonymous-site variation in host mRNAs. By targeting host mRNAs at highly conserved protein-coding sites, and simultaneously expressing multiple variants to cover synonymous-site variation, Cuscuta trans-species sRNAs may be able to successfully target homologous mRNAs from diverse hosts.\n\nOne Sentence SummaryThe parasitic plant Cuscuta produces a diverse set of sRNAs that compensate for sequence variation in mRNA targets in diverse hosts.

plant biology↗

bioRxiv2018

Several phased siRNA annotation methods can produce frequent false-positives for 24 nucleotide RNA-dominated loci in plants

Small RNAs regulate key physiological functions in land plants. Small RNAs can be divided into two categories: microRNAs (miRNAs) and short interfering RNAs (siRNAs); siRNAs are further sub-divided into transposon/repetitive region-localized heterochromatic siRNAs and phased siRNAs (phasiRNAs). PhasiRNAs are produced from the miRNA-mediated cleavage of a Pol II RNA transcript; the miRNA cleavage site provides a defined starting point from which phasiRNAs are produced in a distinctly phased pattern. 21-22 nucleotide (nt)-dominated phasiRNA-producing loci (PHAS) are well represented in all land plants to date. In contrast, 24 nt-dominated PHAS loci are known to be encoded only in monocots and are generally restricted to male reproductive tissues. Currently, only one miRNA (miR2275) is known to trigger the production of these 24 nt-dominated PHAS loci. In this study, we use stringent methodologies in order to examine whether or not 24 nt-dominated PHAS loci also exist in Arabidopsis thaliana. We find that highly expressed heterochromatic siRNAs were consistently mis-identified as 24 nt-dominated PHAS loci using multiple PHAS-detecting algorithms. We also find that MIR2275 is not found in A. thaliana, and it seems to have been lost in the last common ancestor of Brassicales. Altogether, our research highlights the potential issues with widely used PHAS-detecting algorithms which may lead to false positives when trying to annotate new PHAS, especially 24 nt-dominated loci.

plant biology↗

bioRxiv2017

Revisiting criteria for plant miRNA annotation in the era of big data

MicroRNAs (miRNAs) are ~21 nucleotide-long regulatory RNAs that arise from endonucleolytic processing of hairpin precursors. Many function as essential post-transcriptional regulators of target mRNAs and long non-coding RNAs. Alongside miRNAs, plants also produce large numbers of short interfering RNAs (siRNAs), which are distinguished from miRNAs primarily by their biogenesis (typically processed from long double-stranded RNA instead of single-stranded hairpins) and functions (typically via roles in transcriptional regulation instead of post-transcriptional regulation). Next-generation DNA sequencing methods have yielded extensive datasets of plant small RNAs, resulting in many miRNA annotations, occasionally inaccurately curated. The sheer number of endogenous siRNAs compared to miRNAs has been a major factor in the erroneous annotation of siRNAs as miRNAs. Here, we provide updated criteria for the confident annotation of plant miRNAs, suitable for the era of \"big data\" from DNA sequencing. The updated criteria emphasize replication, the minimization of false positives, and they require next-generation sequencing of small RNAs. We argue that improved annotation systems are needed for miRNAs and all other classes of plant small RNAs. Finally, to illustrate the complexities of miRNA and siRNA annotation, we review the evolution and functions of miRNAs and siRNAs in plants.

plant biology↗