Gene silencing in plants using artificial microRNAs and other small RNAs

AT-hook motif nuclear localized transcription factors function redundantly in promoting root growth through modulation of redox homeostasis

Maintaining an optimal redox status is essential for plant growth and development, particularly when the plants are under stress. AT-hook motif nuclear localized (AHL) proteins are evolutionarily conserved transcription factors in plants. Much of our understanding about this gene family has been derived from studies on clade A members. To elucidate the functions of clade B genes, we first analyzed their spatial expression patterns using transgenic plants expressing a nuclear localized GFP under the control of their promoter sequences. AHL1, 2, 6, 7, and 10 were further functionally characterized owing to their high expression in the root apical meristem. Through mutant analyses and transgenic studies, we showed that these genes have the ability to promote root growth. Using yeast one-hybrid and dual luciferase assays, we demonstrated that AHL1, 2, 6, 7, and 10 are transcription regulators and this activity is required for their roles in root growth. Although mutants for these genes did not showed obvious defects in root growth, transgenic plants expressing their fusion proteins with the SRDX repressor motif exhibited a short-root phenotype. Through transcriptome analysis, histochemical staining and molecular genetics experiments, we found that AHL10 maintains redox homeostasis via direct regulation of glutathione transferase (GST) genes. When the transcript level of GSTF2, a top-ranked target of AHL10, was reduced by RNAi, the short-root phenotype in the AHL10-SRDX expressing plant was largely rescued. These results together suggest that AHL genes function redundantly in promoting root growth through direct regulation of redox homeostasis.

The Arabidopsis U1 snRNP regulates mRNA 3'-end processing

The removal of introns by the spliceosome is a key gene regulatory mechanism in eukaryotes, with the U1 snRNP subunit playing a crucial role in the early stages of splicing. Studies in metazoans show that the U1 snRNP also conducts splicing-independent functions, but the lack of genetic tools and knowledge about U1 snRNP-associated proteins have limited the study of such splicing-independent functions in plants. Here we describe an RNA-centric approach that identified more than 200 proteins associated with the Arabidopsis U1 snRNP and revealed a tight link to mRNA cleavage and polyadenylation factors. Interestingly, we found that the U1 snRNP protects mRNAs against premature cleavage and polyadenylation within introns-a mechanism known as telescripting in metazoans-while also influencing alternative polyadenylation site selection in 3'-UTRs. Overall, our work provides a comprehensive view of U1 snRNP interactors and reveals novel functions in regulating mRNA 3'-end processing in Arabidopsis, laying the groundwork for understanding non-canonical functions of plant U1 snRNPs.

Upstream regulator of genomic imprinting in rice endosperm is a small RNA-associated chromatin remodeler

Genomic imprinting is observed in endosperm, a placenta-like seed tissue, where transposable elements (TEs) and repeat-derived small RNAs (sRNAs) mediate epigenetic changes in plants. In imprinting, uniparental gene expression arises due to parent-specific epigenetic marks on one allele but not on the other. The importance of sRNAs and their regulation in endosperm development or in imprinting is poorly understood in crops. Here we show that a previously uncharacterized CLASSY (CLSY)-family chromatin remodeler named OsCLSY3 is essential for rice endosperm development and imprinting, acting as an upstream player in the sRNA pathway. Comparative transcriptome and genetic analysis indicated its endosperm-preferred expression and its likely paternal imprinted nature. These important features are modulated by RNA-directed DNA methylation (RdDM) of tandemly arranged TEs in its promoter. Upon perturbation of OsCLSY3 in transgenic lines, we observe defects in endosperm development and a loss of around 70% of all sRNAs. Interestingly, well-conserved endosperm-specific sRNAs (siren) that are vital for reproductive fitness in angiosperms are also dependent on OsCLSY3. We observed that many imprinted genes and seed development-associated genes are under the control of OsCLSY3. These results support an essential role of OsCLSY3 in rice endosperm development and imprinting, and propose similar regulatory strategies involving CLSY3 homologs among other cereals.

Nα-acetyltransferase NAA50 mediates plant immunity independent of the Nα-acetyltransferase A complex

In humans and plants, 40% of the proteome is cotranslationally acetylated at the N-terminus by a single Nα-acetyltransferase (Nat) termed NatA. The core NatA complex is comprised of the catalytic subunit Nα-acetyltransferase 10 (NAA10) and the ribosome-anchoring subunit NAA15. The regulatory subunit Huntingtin Yeast Partner K (HYPK) and the acetyltransferase NAA50 join this complex in humans. Even though both are conserved in Arabidopsis (Arabidopsis thaliana), only AtHYPK is known to interact with AtNatA. Here we uncover the AtNAA50 interactome and provide evidence for the association of AtNAA50 with NatA at ribosomes. In agreement with the latter, a split-luciferase approach demonstrated close proximity of AtNAA50 and AtNatA in planta. Despite their interaction, AtNatA/HYPK and AtNAA50 exerted different functions in vivo. Unlike NatA/HYPK, AtNAA50 did not modulate drought tolerance or promote protein stability. Instead, transcriptome and proteome analyses of a novel AtNAA50-depleted mutant (amiNAA50) implied that AtNAA50 negatively regulates plant immunity. Indeed, amiNAA50 plants exhibited enhanced resistance to oomycetes and bacterial pathogens. In contrast to what was observed in NatA-depleted mutants, this resistance was independent of an accumulation of salicylic acid prior to pathogen exposure. Our study dissects the in vivo function of the NatA interactors HYPK and NAA50 and uncovers NatA-independent roles for NAA50 in plants.

Wound-induced small-peptide-mediated signaling cascade, regulated by OsPSKR, dictates balance between growth and defense in rice

Wounding is a general stress in plants that results from various pest and pathogenic infections in addition to environment-induced mechanical damages. Plants have sophisticated molecular mechanisms to recognize and respond to wounding, with those of monocots being distinct from dicots. Here, we show the involvement of two distinct categories of temporally separated, endogenously derived peptides, namely, plant elicitor peptides (PEPs) and phytosulfokine (PSK), mediating wound responses in rice. These peptides trigger a dynamic signal relay in which a receptor kinase involved in PSK perception named OsPSKR plays a major role. Perturbation of OsPSKR expression in rice leads to compromised development and constitutive autoimmune phenotypes. OsPSKR regulates the transitioning of defense to growth signals upon wounding. OsPSKR displays mutual antagonism with the OsPEPR1 receptor involved in PEP perception. Collectively, our work indicates the presence of a stepwise peptide-mediated signal relay that regulates the transition from defense to growth upon wounding in monocots.

Reduced expression of bZIP19 and bZIP23 increases zinc and cadmium accumulation in Arabidopsis halleri

Zinc is an essential micronutrient for all living organisms. When challenged by zinc-limiting conditions, Arabidopsis thaliana plants use a strategy centered on two transcription factors, bZIP19 and bZIP23, to enhance the expression of several zinc transporters to improve their zinc uptake capacity. In the zinc and cadmium hyperaccumulator plant Arabidopsis halleri, highly efficient root-to-shoot zinc translocation results in constitutive local zinc deficiency in roots and in constitutive high expression of zinc deficiency-responsive ZIP genes, supposedly boosting zinc uptake and accumulation. Here, to disrupt this process and to analyze the functions of AhbZIP19, AhbZIP23 and their target genes in hyperaccumulation, the genes encoding both transcriptional factors were knocked down using artificial microRNAs (amiRNA). Although AhbZIP19, AhbZIP23, and their ZIP target genes were downregulated, amiRNA lines surprisingly accumulated more zinc and cadmium compared to control lines in both roots and shoot driving to shoot toxicity symptoms. These observations suggested the existence of a substitute metal uptake machinery in A. halleri to maintain hyperaccumulation. We propose that the iron uptake transporter AhIRT1 participates in this alternative pathway in A. halleri.

Plant genome information facilitates plant functional genomics

MAIN CONCLUSION

In this review, we give an overview of plant sequencing efforts and how this impacts plant functional genomics research. Plant genome sequence information greatly facilitates the studies of plant biology, functional genomics, evolution of genomes and genes, domestication processes, phylogenetic relationships, among many others. More than two decades of sequencing efforts have boosted the number of available sequenced plant genomes. The first plant genome, of Arabidopsis, was published in the year 2000 and currently, 4604 plant genomes from 1482 plant species have been published. Various large sequence initiatives are running, which are planning to produce tens of thousands of sequenced plant genomes in the near future. In this review, we give an overview on the status of sequenced plant genomes and on the use of genome information in different research areas.

Stem-cell-expressed DEVIL-like small peptides maintain root growth under abiotic stress via abscisic acid signaling

Stem cells are essential to plant growth and development. Through data mining, we identified five DEVIL-like (DVL) small peptide genes that are preferentially expressed in the quiescent center of Arabidopsis (Arabidopsis thaliana) root but whose functions are unknown. When overexpressed, these genes caused a dramatic decrease in root length and pleiotropic phenotypes in the shoot. No root-growth defect was observed in the single-gene mutants, but the quintuple mutant exhibited slightly longer roots than the wild type (WT). Through transcriptome analysis with DVL20-overexpressing plants, we found that many genes involved in abscisic acid (ABA) signaling were regulated by these peptides. Consistent with this finding, we demonstrated that, relative to the WT, DVL20-overexpressing plants were more tolerant whereas the quintuple mutant was more sensitive to ABA. Using RT-qPCR, we showed that ABA signaling-associated genes were affected in an opposite manner when the plants were grown in normal or ABA-containing medium. Strikingly, ectopic expression of ABA signaling genes such as PYRABACTIN RESISTANCE 1-LIKE (PYL) 4, 5, or 6 or suppression of HIGHLY ABA-INDUCED 2 (HAI2) and MITOGEN-ACTIVATED PROTEIN KINASE KINASE KINASE 18 (MAPKKK18) not only largely rescued the root growth defects in DVL20-overexpressing plants in normal growth condition but also conferred tolerance to ABA. Based on these results, we propose that DVL1, 2, 5, 8 and 20 function redundantly in root stem-cell maintenance under abiotic stress, and this role is achieved via ABA signaling.

Enhancement of specialized metabolites using CRISPR/Cas gene editing technology in medicinal plants

Plants are the richest source of specialized metabolites. The specialized metabolites offer a variety of physiological benefits and many adaptive evolutionary advantages and frequently linked to plant defense mechanisms. Medicinal plants are a vital source of nutrition and active pharmaceutical agents. The production of valuable specialized metabolites and bioactive compounds has increased with the improvement of transgenic techniques like gene silencing and gene overexpression. These techniques are beneficial for decreasing production costs and increasing nutritional value. Utilizing biotechnological applications to enhance specialized metabolites in medicinal plants needs characterization and identification of genes within an elucidated pathway. The breakthrough and advancement of CRISPR/Cas-based gene editing in improving the production of specific metabolites in medicinal plants have gained significant importance in contemporary times. This article imparts a comprehensive recapitulation of the latest advancements made in the implementation of CRISPR-gene editing techniques for the purpose of augmenting specific metabolites in medicinal plants. We also provide further insights and perspectives for improving metabolic engineering scenarios in medicinal plants.

Role of microRNAs and their putative mechanism in regulating potato (Solanum tuberosum L.) life cycle and response to various environmental stresses

The exponentially increasing population and the demand for food is inextricably linked. This has shifted global attention to improving crop plant traits to meet global food demands. Potato (Solanum tuberosum L.) is a major non-grain food crop that is grown all over the world. Currently, some of the major global potato research work focuses on the significance of microRNAs (miRNAs) in potato. miRNAs are a type of non-coding RNAs that regulate the gene expression of their target mRNA genes by cleavage and/or their translational inhibition. This suggests an essential role of miRNAs in a multitude of plant biological processes, including maintenance of genome integrity, plant growth, development and maturation, and initiation of responses to various stress conditions. Therefore, engineering miRNAs to generate stress-resistant varieties of potato may result in high yield and improved nutritional qualities. In this review, we discuss the potato miRNAs specifically known to play an essential role in the various stages of the potato life cycle, conferring stress-resistant characteristics, and modifying gene expression. This review highlights the significance of the miRNA machinery in plants, especially potato, encouraging further research into engineering miRNAs to boost crop yields and tolerance towards stress.

Unraveling the regulatory network of miRNA expression in Potato Y virus-infected of Nicotiana benthamiana using integrated small RNA and transcriptome sequencing

Potato virus Y (PVY) disease is a global problem that causes significant damage to crop quality and yield. As traditional chemical control methods are ineffective against PVY, it is crucial to explore new control strategies. MicroRNAs (miRNAs) play a crucial role in plant and animal defense responses to biotic and abiotic stresses. These endogenous miRNAs act as a link between antiviral gene pathways and host immunity. Several miRNAs target plant immune genes and are involved in the virus infection process. In this study, we conducted small RNA sequencing and transcriptome sequencing on healthy and PVY-infected N. benthamiana tissues (roots, stems, and leaves). Through bioinformatics analysis, we predicted potential targets of differentially expressed miRNAs using the N. benthamiana reference genome and the PVY genome. We then compared the identified differentially expressed mRNAs with the predicted target genes to uncover the complex relationships between miRNAs and their targets. This study successfully constructed a miRNA-mRNA network through the joint analysis of Small RNA sequencing and transcriptome sequencing, which unveiled potential miRNA targets and identified potential binding sites of miRNAs on the PVY genome. This miRNA-mRNA regulatory network suggests the involvement of miRNAs in the virus infection process.

Processes for regulating genetically modified and gene edited plants

Innovation in agriculture has been essential in improving productivity of crops and forages to support a growing population, improving living standards while contributing toward maintaining environment integrity, human health, and wellbeing through provision of more nutritious, varied, and abundant food sources. A crucial part of that innovation has involved a range of techniques for both expanding and exploiting the genetic potential of plants. However, some techniques used for generating new variation for plant breeders to exploit are deemed higher risk than others despite end products of both processes at times being for all intents and purposes identical for the benefits they provide. As a result, public concerns often triggered by poor communication from innovators, resulting in mistrust and suspicion has, in turn, caused the development of a range of regulatory systems. The logic and motivations for modes of regulation used are reviewed and how the benefits from use of these technologies can be delivered more efficiently and effectively is discussed.

Salicylic acid accelerates carbon starvation-induced leaf senescence in Arabidopsis thaliana by inhibiting autophagy through Nonexpressor of pathogenesis-related genes 1

In plants, leaf senescence is regulated by several factors, including age and carbon starvation. The molecular mechanism of age-regulated developmental leaf senescence differs from that of carbon starvation-induced senescence. Salicylic acid (SA) and Nonexpressor of pathogenesis-related genes 1 (NPR1) play important roles in promoting developmental leaf senescence. However, the relationship between SA signaling and carbon starvation-induced leaf senescence is not currently well understood. Here, we used Arabidopsis thaliana as material and found that carbon starvation-induced leaf senescence was accelerated in the SA dihydroxylase mutants s3hs5h compared to the Columbia ecotype (Col). Exogenous SA treatment significantly promoted carbon starvation-induced leaf senescence, especially in NPR1-GFP. Increasing the endogenous SA and overexpression of NPR1 inhibited carbon starvation-induced autophagy. However, mutation of NPR1 delayed carbon starvation-induced leaf senescence, increased autophagosome production and accelerated autophagic degradation of the Neighbor of BRCA1 gene 1 (NBR1). In conclusion, SA promotes carbon starvation-induced leaf senescence by inhibiting autophagy via NPR1.

Efficient artificial microRNA-mediated resistance against zucchini yellow mosaic virus in zucchini via agroinfiltration

Zucchini yellow mosaic virus (ZYMV) infects cucurbits causing yellow mosaic in leaves, malformations in fruits, and degradation of the product quality. RNA interference (RNAi) is a cellular mechanism in eukaryotes and it is exploited to protect them against viruses. The artificial micro RNA (amiRNA) mediated approach was employed to develop resistance against ZYMV. Four amiRNAs, amiZYMV_HC-115s and amiZYMV_HC-1162s (sense), amiZYMV_HC-182as and amiZYMV_HC-196as (antisense), were computationally designed and introduced into the AtMIR390a backbone. At four days post agroinfiltration (dpa) of zucchini cotyledons the corresponding pre- and the mature amiRNAs were identified in local tissue. Upon ZYMV inoculation of zucchini, ZYMV titer was significantly lower where amiZYMV_HCs were applied in relation to control starting at two days post inoculation (dpi). Control zucchini plants exhibited symptoms at 5-8 dpi, whereas the amiZYMV_HC-treated zucchini had symptoms at 14 dpi; at 21 dpi treated zucchini exhibited a 16 %, 19 %, 32 %, and 42.5 % protection, respectively. For luffa, we observed a lower protection (0 %, 17 %, 22.5 %, and 31 % at 21 dpi). Nicotiana benthamiana DCL4 knock-down mutants were infected by ZYMV, whereas when the amiZYMV_HC-196as was agroinfiltrated ZYMV was not detected by RT-PCR. These results indicate that amiRNA-mediated resistance could be applied against ZYMV in zucchini.

Transgene-free, virus-based gene silencing in plants by artificial microRNAs derived from minimal precursors

Artificial microRNAs (amiRNAs) are highly specific, 21-nucleotide (nt) small RNAs designed to silence target transcripts. In plants, their application as biotechnological tools for functional genomics or crop improvement is limited by the need of transgenically expressing long primary miRNA (pri-miRNA) precursors to produce the amiRNAs in vivo. Here, we analyzed the minimal structural and sequence requirements for producing effective amiRNAs from the widely used, 521-nt long AtMIR390a pri-miRNA from Arabidopsis thaliana. We functionally screened in Nicotiana benthamiana a large collection of constructs transiently expressing amiRNAs against endogenous genes and from artificially shortened MIR390-based precursors and concluded that highly effective and accurately processed amiRNAs can be produced from a chimeric precursor of only 89 nt. This minimal precursor was further validated in A. thaliana transgenic plants expressing amiRNAs against endogenous genes. Remarkably, minimal but not full-length precursors produce authentic amiRNAs and induce widespread gene silencing in N. benthamiana when expressed from an RNA virus, which can be applied into leaves by spraying infectious crude extracts. Our results reveal that the length of amiRNA precursors can be shortened without affecting silencing efficacy, and that viral vectors including minimal amiRNA precursors can be applied in a transgene-free manner to induce whole-plant gene silencing.

Reduction in PLANT DEFENSIN 1 expression in Arabidopsis thaliana results in increased resistance to pathogens and zinc toxicity

Ectopic expression of defensins in plants correlates with their increased capacity to withstand abiotic and biotic stresses. This applies to Arabidopsis thaliana, where some of the seven members of the PLANT DEFENSIN 1 family (AtPDF1) are recognised to improve plant responses to necrotrophic pathogens and increase seedling tolerance to excess zinc (Zn). However, few studies have explored the effects of decreased endogenous defensin expression on these stress responses. Here, we carried out an extensive physiological and biochemical comparative characterization of (i) novel artificial microRNA (amiRNA) lines silenced for the five most similar AtPDF1s, and (ii) a double null mutant for the two most distant AtPDF1s. Silencing of five AtPDF1 genes was specifically associated with increased aboveground dry mass production in mature plants under excess Zn conditions, and with increased plant tolerance to different pathogens - a fungus, an oomycete and a bacterium, while the double mutant behaved similarly to the wild type. These unexpected results challenge the current paradigm describing the role of PDFs in plant stress responses. Additional roles of endogenous plant defensins are discussed, opening new perspectives for their functions.

A high-efficiency gene silencing in plants using two-hit asymmetrical artificial MicroRNAs

MicroRNAs (miRNAs) are small non-coding RNA molecules that play a crucial role in gene regulation. They are produced through an enzyme-guided process called dicing and have an asymmetrical structure with two nucleotide overhangs at the 3' ends. Artificial microRNAs (amiRNAs or amiRs) are designed to mimic the structure of miRNAs and can be used to silence specific genes of interest. Traditionally, amiRNAs are designed based on an endogenous miRNA precursor with certain mismatches at specific positions to increase their efficiency. In this study, the authors modified the highly expressed miR168a in Arabidopsis thaliana by replacing the single miR168 stem-loop/duplex with tandem asymmetrical amiRNA duplexes that follow the statistical rules of miRNA secondary structures. These tandem amiRNA duplexes, called "two-hit" amiRNAs, were shown to have a higher efficiency in silencing GFP and endogenous PDS reporter genes compared to traditional "one-hit" amiRNAs. The authors also demonstrated the effectiveness of "two-hit" amiRNAs in silencing genes involved in miRNA, tasiRNA, and hormone signalling pathways, individually or in families. Importantly, "two-hit" amiRNAs were also able to over-express endogenous miRNAs for their functions. The authors compare "two-hit" amiRNA technology with CRISPR/Cas9 and provide a web-based amiRNA designer for easy design and wide application in plants and even animals.

SNF1-RELATED KINASE 1 and TARGET OF RAPAMYCIN control light-responsive splicing events and developmental characteristics in etiolated Arabidopsis seedlings

The kinases SNF1-RELATED KINASE 1 (SnRK1) and TARGET OF RAPAMYCIN (TOR) are central sensors of the energy status, linking this information via diverse regulatory mechanisms to plant development and stress responses. Despite the well-studied functions of SnRK1 and TOR under conditions of limited or ample energy availability, respectively, little is known about the extent to which the 2 sensor systems function and how they are integrated in the same molecular process or physiological context. Here, we demonstrate that both SnRK1 and TOR are required for proper skotomorphogenesis in etiolated Arabidopsis (Arabidopsis thaliana) seedlings, light-induced cotyledon opening, and regular development in light. Furthermore, we identify SnRK1 and TOR as signaling components acting upstream of light- and sugar-regulated alternative splicing events, expanding the known action spectra for these 2 key players in energy signaling. Our findings imply that concurring SnRK1 and TOR activities are required throughout various phases of plant development. Based on the current knowledge and our findings, we hypothesize that turning points in the activities of these sensor kinases, as expected to occur upon illumination of etiolated seedlings, instead of signaling thresholds reflecting the nutritional status may modulate developmental programs in response to altered energy availability.

Genetic modification strategies for enhancing plant resilience to abiotic stresses in the context of climate change

Enhancing the resilience of plants to abiotic stresses, such as drought, salinity, heat, and cold, is crucial for ensuring global food security challenge in the context of climate change. The adverse effects of climate change, characterized by rising temperatures, shifting rainfall patterns, and increased frequency of extreme weather events, pose significant threats to agricultural systems worldwide. Genetic modification strategies offer promising approaches to develop crops with improved abiotic stress tolerance. This review article provides a comprehensive overview of various genetic modification techniques employed to enhance plant resilience. These strategies include the introduction of stress-responsive genes, transcription factors, and regulatory elements to enhance stress signaling pathways. Additionally, the manipulation of hormone signaling pathways, osmoprotectant accumulation, and antioxidant defense mechanisms is discussed. The use of genome editing tools, such as CRISPR-Cas9, for precise modification of target genes related to stress tolerance is also explored. Furthermore, the challenges and future prospects of genetic modification for abiotic stress tolerance are highlighted. Understanding and harnessing the potential of genetic modification strategies can contribute to the development of resilient crop varieties capable of withstanding adverse environmental conditions caused by climate change, thereby ensuring sustainable agricultural productivity and food security.

Cas13a-based multiplex RNA targeting for potato virus Y

MAIN CONCLUSION

The Cas13a-based multiplex RNA targeting system can be engineered to confer resistance to RNA viruses, whereas the number and expression levels of gRNAs have no significant effect on viral interference. The CRISPR-Cas systems provide adaptive immunity to bacterial and archaeal species against invading phages and foreign plasmids. The class 2 type VI CRISPR/Cas effector Cas13a has been harnessed to confer the protection against RNA viruses in diverse eukaryotic species. However, whether the number and expression levels of guide RNAs (gRNAs) have effects on the efficiency of RNA virus inhibition is unknown. Here, we repurpose CRISPR/Cas13a in combination with an endogenous tRNA-processing system (polycistronic tRNA-gRNA) to target four genes of potato virus Y (PVY) with varying expression levels. We expressed Cas13a and four different gRNAs in potato lines, and the transgenic plants expressing multiple gRNAs displayed similar suppression of PVY accumulation and reduced disease symptoms as those expressing a single gRNA. Moreover, PTG/Cas13a-transformed plants with different expression levels of multiple gRNAs displayed similar resistance to PVY strains. Collectively, this study suggests that the Cas13a-based multiplex RNA targeting system can be utilized to engineer resistance to RNA viruses in plants, whereas the number and expression levels of gRNAs have no significant effect on CRISPR/Cas13a-mediated viral interference in plants.

Gold nanoparticles (AuNPs) can rapidly deliver artificial microRNA (AmiRNA)-ATG6 to silence ATG6 expression in Arabidopsis

KEY MESSAGE

We establish a fast and efficient transient silencing system that facilitates functional studies of some genes, whose knockout leads to plant lethality. In plants, the generation of loss-of-function mutants is crucial for studying gene function. Artificial microRNA (AmiRNA) technology is a more targeted and effective tool for gene silencing. Gold nanoparticles (AuNPs) can bind nucleic acids and deliver them into animal cells. Here, AuNPs are used in combination with AmiRNA technology in plants. We found that AmiRNA-autophagy-related proteins (ATG6) can be delivered to cells by AuNPs to achieve the effect of ATG6 silencing. It is worth noting that on the 10th day there is still a silencing effect. Similar to the atg5 lines, silencing of ATG6 significantly reduced plant resistance to Pseudomonas syringae pv.maculicola (Psm) ES4326/AvrRpt2. Interestingly, ATG6 silencing and ATG5 mutation in NPR1-GFP (nonexpressor of pathogenesis-related genes) lines significantly reduced plant resistance to Psm ES4326/AvrRpt2, suggesting that autophagy is also involved in NPR1-regulated plant immune responses. In summary, we establish a fast and efficient transient silencing system that facilitates functional studies of some genes, whose knockout leads to plant lethality.

Plant polymerase IV sensitizes chromatin through histone modifications to preclude spread of silencing into protein-coding domains

Across eukaryotes, gene regulation is manifested via chromatin states roughly distinguished as heterochromatin and euchromatin. The establishment, maintenance, and modulation of the chromatin states is mediated using several factors including chromatin modifiers. However, factors that avoid the intrusion of silencing signals into protein-coding genes are poorly understood. Here we show that a plant specific paralog of RNA polymerase (Pol) II, named Pol IV, is involved in avoidance of facultative heterochromatic marks in protein-coding genes, in addition to its well-established functions in silencing repeats and transposons. In its absence, H3K27 trimethylation (me3) mark intruded the protein-coding genes, more profoundly in genes embedded with repeats. In a subset of genes, spurious transcriptional activity resulted in small(s) RNA production, leading to post-transcriptional gene silencing. We show that such effects are significantly pronounced in rice, a plant with a larger genome with distributed heterochromatin compared with Arabidopsis Our results indicate the division of labor among plant-specific polymerases, not just in establishing effective silencing via sRNAs and DNA methylation but also in influencing chromatin boundaries.

ABCB-mediated shootward auxin transport feeds into the root clock

Although strongly influenced by environmental conditions, lateral root (LR) positioning along the primary root appears to follow obediently an internal spacing mechanism dictated by auxin oscillations that prepattern the primary root, referred to as the root clock. Surprisingly, none of the hitherto characterized PIN- and ABCB-type auxin transporters seem to be involved in this LR prepatterning mechanism. Here, we characterize ABCB15, 16, 17, 18, and 22 (ABCB15-22) as novel auxin-transporting ABCBs. Knock-down and genome editing of this genetically linked group of ABCBs caused strongly reduced LR densities. These phenotypes were correlated with reduced amplitude, but not reduced frequency of the root clock oscillation. High-resolution auxin transport assays and tissue-specific silencing revealed contributions of ABCB15-22 to shootward auxin transport in the lateral root cap (LRC) and epidermis, thereby explaining the reduced auxin oscillation. Jointly, these data support a model in which LRC-derived auxin contributes to the root clock amplitude.

Artificial microRNA-mediated resistance against Oman strain of tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV) is a global spreading begomovirus that is exerting a major restraint on global tomato production. In this transgenic approach, an RNA interference (RNAi)-based construct consisting of sequences of an artificial microRNA (amiRNA), a group of small RNA molecules necessary for plant cell development, signal transduction, and stimulus to biotic and abiotic disease was engineered targeting the AC1/Rep gene of the Oman strain of TYLCV-OM. The Rep-amiRNA constructs presented an effective approach in regulating the expression of the Rep gene against TYLCV as a silencing target to create transgenic Solanum lycopersicum L. plant tolerance against TYLCV infection. Molecular diagnosis by PCR followed by a Southern hybridization analysis were performed to confirm the effectiveness of agrobacterium-mediated transformation in T0/T1-transformed plants. A substantial decrease in virus replication was observed when T1 transgenic tomato plants were challenged with the TYLCV-OM infectious construct. Although natural resistance options against TYLCV infection are not accessible, the current study proposes that genetically transformed tomato plants expressing amiRNA could be a potential approach for engineering tolerance in plants against TYLCV infection and conceivably for the inhibition of viral diseases against different strains of whitefly-transmitted begomoviruses in Oman.

The transcriptional regulator JAZ8 interacts with the C2 protein from geminiviruses and limits the geminiviral infection in Arabidopsis

Jasmonates (JAs) are phytohormones that finely regulate critical biological processes, including plant development and defense. JASMONATE ZIM-DOMAIN (JAZ) proteins are crucial transcriptional regulators that keep JA-responsive genes in a repressed state. In the presence of JA-Ile, JAZ repressors are ubiquitinated and targeted for degradation by the ubiquitin/proteasome system, allowing the activation of downstream transcription factors and, consequently, the induction of JA-responsive genes. A growing body of evidence has shown that JA signaling is crucial in defending against plant viruses and their insect vectors. Here, we describe the interaction of C2 proteins from two tomato-infecting geminiviruses from the genus Begomovirus, tomato yellow leaf curl virus (TYLCV) and tomato yellow curl Sardinia virus (TYLCSaV), with the transcriptional repressor JAZ8 from Arabidopsis thaliana and its closest orthologue in tomato, SlJAZ9. Both JAZ and C2 proteins colocalize in the nucleus, forming discrete nuclear speckles. Overexpression of JAZ8 did not lead to altered responses to TYLCV infection in Arabidopsis; however, knock-down of JAZ8 favors geminiviral infection. Low levels of JAZ8 likely affect the viral infection specifically, since JAZ8-silenced plants neither display obvious developmental phenotypes nor present differences in their interaction with the viral insect vector. In summary, our results show that the geminivirus-encoded C2 interacts with JAZ8 in the nucleus, and suggest that this plant protein exerts an anti-geminiviral effect.

Multiple ubiquitin E3 ligase genes antagonistically regulate chloroplast-associated protein degradation

The chloroplast is the most prominent member of a diverse group of plant organelles called the plastids, and it is characterized by its vital role in photosynthesis. ^1,2,3 Most of the ∼3,000 different proteins in chloroplasts are synthesized in the cytosol in precursor (preprotein) form, each with a cleavable transit peptide. ^4,5,6,7,8 Preproteins are imported via translocons in the outer and inner envelope membranes of the chloroplast, termed TOC and TIC, respectively. ^{9,10,11,12,13} Discovery of the chloroplast-localized ubiquitin E3 ligase SUPPRESSOR OF PPI1 LOCUS1 (SP1) demonstrated that the nucleocytosolic ubiquitin-proteasome system (UPS) targets the TOC apparatus to dynamically control protein import and chloroplast biogenesis in response to developmental and environmental cues. The relevant UPS pathway is termed chloroplast-associated protein degradation (CHLORAD). ^14,15,16 Two homologs of SP1 exist, SP1-like1 (SPL1) and SPL2, but their roles have remained obscure. Here, we show that SP1 is ubiquitous in the Viridiplantae and that SPL2 and SPL1 appeared early during the evolution of the Viridiplantae and land plants, respectively. Through genetic and biochemical analysis, we reveal that SPL1 functions as a negative regulator of SP1, potentially by interfering with its ability to catalyze ubiquitination. In contrast, SPL2, the more distantly related SP1 homolog, displays partial functional redundancy with SP1. Both SPL1 and SPL2 modify the extent of leaf senescence, like SP1, but do so in diametrically opposite ways. Thus, SPL1 and SPL2 are bona fide CHLORAD system components with negative and positive regulatory functions that allow for nuanced control of this vital proteolytic pathway.

The battle of crops against drought: Genetic dissection and improvement

With ongoing global climate change, water scarcity-induced drought stress remains a major threat to agricultural productivity. Plants undergo a series of physiological and morphological changes to cope with drought stress, including stomatal closure to reduce transpiration and changes in root architecture to optimize water uptake. Combined phenotypic and multi-omics studies have recently identified a number of drought-related genetic resources in different crop species. The functional dissection of these genes using molecular techniques has enriched our understanding of drought responses in crops and has provided genetic targets for enhancing resistance to drought. Here, we review recent advances in the cloning and functional analysis of drought resistance genes and the development of technologies to mitigate the threat of drought to crop production.

The lifetime of the oxygen-evolving complex subunit PSBO depends on light intensity and carbon availability in Chlamydomonas

PSBO is essential for the assembly of the oxygen-evolving complex in plants and green algae. Despite its importance, we lack essential information on its lifetime and how it depends on the environmental conditions. We have generated nitrate-inducible PSBO amiRNA lines in the green alga Chlamydomonas reinhardtii. Transgenic strains grew normally under non-inducing conditions, and their photosynthetic performance was comparable to the control strain. Upon induction of the PSBO amiRNA constructs, cell division halted. In acetate-containing medium, cellular PSBO protein levels decreased by 60% within 24 h in the dark, by 75% in moderate light, and in high light, the protein completely degraded. Consequently, the photosynthetic apparatus became strongly damaged, probably due to 'donor-side-induced photoinhibition', and cellular ultrastructure was also severely affected. However, in the absence of acetate during induction, PSBO was remarkably stable at all light intensities and less substantial changes occurred in photosynthesis. Our results demonstrate that the lifetime of PSBO strongly depends on the light intensity and carbon availability, and thus, on the metabolic status of the cells. We also confirm that PSBO is required for photosystem II stability in C. reinhardtii and demonstrate that its specific loss also entails substantial changes in cell morphology and cell cycle.

Common evolutionary trajectory of short life-cycle in Brassicaceae ruderal weeds

Weed species are detrimental to crop yield. An understanding of how weeds originate and adapt to field environments is needed for successful crop management and reduction of herbicide use. Although early flowering is one of the weed trait syndromes that enable ruderal weeds to overcome frequent disturbances, the underlying genetic basis is poorly understood. Here, we establish Cardamine occulta as a model to study weed ruderality. By genome assembly and QTL mapping, we identify impairment of the vernalization response regulator gene FLC and a subsequent dominant mutation in the blue-light receptor gene CRY2 as genetic drivers for the establishment of short life cycle in ruderal weeds. Population genomics study further suggests that the mutations in these two genes enable individuals to overcome human disturbances through early deposition of seeds into the soil seed bank and quickly dominate local populations, thereby facilitating their spread in East China. Notably, functionally equivalent dominant mutations in CRY2 are shared by another weed species, Rorippa palustris, suggesting a common evolutionary trajectory of early flowering in ruderal weeds in Brassicaceae.

Breeding for Higher Yields of Wheat and Rice through Modifying Nitrogen Metabolism

Wheat and rice produce nutritious grains that provide 32% of the protein in the human diet globally. Here, we examine how genetic modifications to improve assimilation of the inorganic nitrogen forms ammonium and nitrate into protein influence grain yield of these crops. Successful breeding for modified nitrogen metabolism has focused on genes that coordinate nitrogen and carbon metabolism, including those that regulate tillering, heading date, and ammonium assimilation. Gaps in our current understanding include (1) species differences among candidate genes in nitrogen metabolism pathways, (2) the extent to which relative abundance of these nitrogen forms across natural soil environments shape crop responses, and (3) natural variation and genetic architecture of nitrogen-mediated yield improvement. Despite extensive research on the genetics of nitrogen metabolism since the rise of synthetic fertilizers, only a few projects targeting nitrogen pathways have resulted in development of cultivars with higher yields. To continue improving grain yield and quality, breeding strategies need to focus concurrently on both carbon and nitrogen assimilation and consider manipulating genes with smaller effects or that underlie regulatory networks as well as genes directly associated with nitrogen metabolism.

A comprehensive map of preferentially located motifs reveals distinct proximal cis-regulatory sequences in plants

Identification of cis-regulatory sequences controlling gene expression is an arduous challenge that is being actively explored to discover key genetic factors responsible for traits of agronomic interest. Here, we used a genome-wide de novo approach to investigate preferentially located motifs (PLMs) in the proximal cis-regulatory landscape of Arabidopsis thaliana and Zea mays. We report three groups of PLMs in both the 5'- and 3'-gene-proximal regions and emphasize conserved PLMs in both species, particularly in the 3'-gene-proximal region. Comparison with resources from transcription factor and microRNA binding sites shows that 79% of the identified PLMs are unassigned, although some are supported by MNase-defined cistrome occupancy analysis. Enrichment analyses further reveal that unassigned PLMs provide functional predictions that differ from those derived from transcription factor and microRNA binding sites. Our study provides a comprehensive map of PLMs and demonstrates their potential utility for future characterization of orphan genes in plants.

Endoplasmic reticulum oxidoreductin provides resilience against reductive stress and hypoxic conditions by mediating luminal redox dynamics

Oxidative protein folding in the endoplasmic reticulum (ER) depends on the coordinated action of protein disulfide isomerases and ER oxidoreductins (EROs). Strict dependence of ERO activity on molecular oxygen as the final electron acceptor implies that oxidative protein folding and other ER processes are severely compromised under hypoxia. Here, we isolated viable Arabidopsis thaliana ero1 ero2 double mutants that are highly sensitive to reductive stress and hypoxia. To elucidate the specific redox dynamics in the ER in vivo, we expressed the glutathione redox potential (EGSH) sensor Grx1-roGFP2iL-HDEL with a midpoint potential of -240 mV in the ER of Arabidopsis plants. We found EGSH values of -241 mV in wild-type plants, which is less oxidizing than previously estimated. In the ero1 ero2 mutants, luminal EGSH was reduced further to -253 mV. Recovery to reductive ER stress induced by dithiothreitol was delayed in ero1 ero2. The characteristic signature of EGSH dynamics in the ER lumen triggered by hypoxia was affected in ero1 ero2 reflecting a disrupted balance of reductive and oxidizing inputs, including nascent polypeptides and glutathione entry. The ER redox dynamics can now be dissected in vivo, revealing a central role of EROs as major redox integrators to promote luminal redox homeostasis.

The StBBX24 protein affects the floral induction and mediates salt tolerance in Solanum tuberosum

The transition from vegetative growth to reproductive development is a critical developmental switch in flowering plants to ensure a successful life cycle. However, while the genes controlling flowering are well-known in model plants, they are less well-understood in crops. In this work, we generated potato lines both silenced and overexpressed for the expression of StBBX24, a clock-controlled gene encoding a B-box protein located in the cytosol and nuclear chromatin fraction. We revealed that Solanum tuberosum lines silenced for StBBX24 expression displayed much earlier flowering than wild-type plants. Conversely, plants overexpressing StBBX24 mostly did not produce flower buds other than wild-type plants. In addition, RT-qPCR analyses of transgenic silenced lines revealed substantial modifications in the expression of genes functioning in flowering. Furthermore, S. tuberosum lines silenced for StBBX24 expression displayed susceptibility to high salinity with a lower capacity of the antioxidant system and strongly decreased expression of genes encoding Na+ transporters that mediate salt tolerance, contrary to the plants with StBBX24 overexpression. Altogether, these data reveal that StBBX24 participates in potato flowering repression and is involved in salt stress response.

Fatty acid export (FAX) proteins contribute to oil production in the green microalga Chlamydomonas reinhardtii

In algae and land plants, transport of fatty acids (FAs) from their site of synthesis in the plastid stroma to the endoplasmic reticulum (ER) for assembly into acyl lipids is crucial for cellular lipid homeostasis, including the biosynthesis of triacylglycerol (TAG) for energy storage. In the unicellular green alga Chlamydomonas reinhardtii, understanding and engineering of these processes is of particular interest for microalga-based biofuel and biomaterial production. Whereas in the model plant Arabidopsis thaliana, FAX (fatty acid export) proteins have been associated with a function in plastid FA-export and hence TAG synthesis in the ER, the knowledge on the function and subcellular localization of this protein family in Chlamydomonas is still scarce. Among the four FAX proteins encoded in the Chlamydomonas genome, we found Cr-FAX1 and Cr-FAX5 to be involved in TAG production by functioning in chloroplast and ER membranes, respectively. By in situ immunolocalization, we show that Cr-FAX1 inserts into the chloroplast envelope, while Cr-FAX5 is located in ER membranes. Severe reduction of Cr-FAX1 or Cr-FAX5 proteins by an artificial microRNA approach results in a strong decrease of the TAG content in the mutant strains. Further, overexpression of chloroplast Cr-FAX1, but not of ER-intrinsic Cr-FAX5, doubled the content of TAG in Chlamydomonas cells. We therefore propose that Cr-FAX1 in chloroplast envelopes and Cr-FAX5 in ER membranes represent a basic set of FAX proteins to ensure shuttling of FAs from chloroplasts to the ER and are crucial for oil production in Chlamydomonas.

Silencing P25, HC-Pro and Brp1 of Potato Virus (Viroid) Using Artificial microRNA Confers Resistance to PVX, PVY and PSTVd in Transgenic Potato

Virus infection is the key constraint to potato cultivation worldwide. Especially, coinfection by multiple viruses could exacerbate the yield loss. Transgenic plants expressing artificial microRNAs (amiRNAs) have been shown to confer specific resistance to viruses. In this study, three amiRNAs containing Arabidopsis miR159 as a backbone, expressing genes targeting P25, HC-Pro and Brp1 of potato virus X (PVX), potato virus Y (PVY) and potato spindle tuber viroid (PSTVd), were constructed. amiR-159P25, amiR-159HCPro and amiR-159Brp1 were cloned into the plant expression vector pCAMBIA1301 with a CaMV35S promoter, producing the p1301-pre-amiRP25-HCPro-Brp1 vector. Twenty-three transgenic plants (Solanum tuberosum cv. 'Youjin') were obtained by Agrobacterium tumefaciens-mediated transformation, and ten PCR-positive transplants were chosen for further analysis. Quantitative real-time PCR results indicated that 10 transgenic plants could express amiRNAs successfully. Southern blotting hybridization proved that amiR-159P25-HCPro-Brp1 had integrated into potato genome in transgenic lines. Viral (viroid) challenge assays revealed that these transgenic plants demonstrated resistance against PVX, PVY and PSTVd coinfection simultaneously, whereas the untransformed controls developed severe symptoms. This study demonstrates a novel amiRNA-based mechanism that may have the potential to develop multiple viral resistance strategies in potato.

Simple Webserver-Facilitated Method to Design and Synthesize Artificial miRNA Gene and Its Application in Engineering Viral Resistance

Plant viruses impose serious threats on crop production. Artificial miRNAs can mediate specific and effective gene silencing in plants and are widely used in plant gene function studies and to engineer plant viral resistance. To facilitate the design of artificial miRNA genes, we developed a webserver, AMIRdesigner, which can be used to design oligos for artificial miRNA synthesis using wild-type and permutated MIR171 and MIR164 backbones. The artificial miRNA genes designed by AMIRdesigner can be easily assembled into miRNA clusters for multiple target sites. To validate the server functionality, we designed four artificial miRNA genes targeting four conserved regions in the potato leafroll virus genome using AMIRdesigner. These genes were synthesized with the server-designed oligos and further assembled into a quadruple miRNA cluster, which was cloned into an overexpression vector and transformed into potato plants. Small RNA Northern blot and virus inoculation analyses showed that a high level of artificial miRNA expression and good viral resistance were achieved in some of the transgenic lines. These results demonstrate the utility of our webserver AMIRdesigner for engineering crop viral resistance.

Environmental fate and behaviour of antibiotic resistance genes and small interference RNAs released from genetically modified crops

Rising global populations have amplified food scarcity across the world and ushered in the development of genetically modified (GM) crops to overcome these challenges. Cultivation of major crops such as corn and soy has favoured GM crops over conventional varieties to meet crop production and resilience needs. Modern GM crops containing small interference RNA molecules and antibiotic resistance genes have become increasingly common in the United States. However, the use of these crops remains controversial due to the uncertainty regarding the unintended release of its genetic material into the environment and possible downstream effects on human and environmental health. DNA or RNA transgenes may be exuded from crop tissues during cultivation or released during plant decomposition and adsorbed by soil. This can contribute to the persistence and bioavailability in soil or water environment and possible uptake by soil microbial communities and further passing of this information to neighbouring bacteria, disrupting microbial ecosystem services such as nutrient cycling and soil fertility. In this review, transgene mechanisms of action, uses in crops, and knowledge regarding their environmental fate and impact to microbes are evaluated. This aims to encapsulate the current knowledge and promote further research regarding unintended effects transgenes may cause.

Artificial microRNA guide strand selection from duplexes with no mismatches shows a purine-rich preference for virus- and non-virus-based expression vectors in plants

Artificial microRNA (amiRNA) technology has allowed researchers to direct efficient silencing of specific transcripts using as few as 21 nucleotides (nt). However, not all the artificially designed amiRNA constructs result in selection of the intended ~21-nt guide strand amiRNA. Selection of the miRNA guide strand from the mature miRNA duplex has been studied in detail in human and insect systems, but not so much for plants. Here, we compared a nuclear-replicating DNA viral vector (tomato mottle virus, ToMoV, based), a cytoplasmic-replicating RNA viral vector (tobacco mosaic virus, TMV, based), and a non-viral binary vector to express amiRNAs in plants. We then used deep sequencing and mutational analysis and show that when the structural factors caused by base mismatches in the mature amiRNA duplex were excluded, the nucleotide composition of the mature amiRNA region determined the guide strand selection. We found that the strand with excess purines was preferentially selected as the guide strand and the artificial miRNAs that had no mismatches in the amiRNA duplex were predominantly loaded into AGO2 instead of loading into AGO1 like the majority of the plant endogenous miRNAs. By performing assays for target effects, we also showed that only when the intended strand was selected as the guide strand and showed AGO loading, the amiRNA could provide the expected RNAi effects. Thus, by removing mismatches in the mature amiRNA duplex and designing the intended guide strand to contain excess purines provide better control of the guide strand selection of amiRNAs for functional RNAi effects.

Systemic silencing of an endogenous plant gene by two classes of mobile 21-nucleotide artificial small RNAs

Artificial small RNAs (art-sRNAs) are 21-nucleotide small RNAs (sRNAs) computationally designed to silence plant genes or pathogenic RNAs with high efficacy and specificity. They are typically produced in transgenic plants to induce silencing at the whole-organism level, although their expression in selected tissues for inactivating genes in distal tissues has not been reported. Here, art-sRNAs designed against the magnesium chelatase subunit CHLI-encoding SULFUR gene (NbSu) were agroinfiltrated in Nicotiana benthamiana leaves, and the induction of local and systemic silencing was analyzed phenotypically by monitoring the appearance of the characteristic bleached phenotype, as well as molecularly by analyzing art-sRNA processing, accumulation and targeting activity and efficacy. We found that the two classes of art-sRNAs, artificial microRNAs (amiRNAs) and synthetic trans-acting small interfering RNAs (syn-tasiRNAs), are able to induce systemic silencing of NbSu, which requires high art-sRNA expression in the vicinity of the leaf petiole but is independent on the production of secondary sRNAs from NbSu mRNAs. Moreover, we revealed that 21-nucleotide amiRNA and syn-tasiRNA duplexes, and not their precursors, are the molecules moving between cells and through the phloem to systemically silence NbSu in upper leaves. In sum, our results indicate that 21-nucleotide art-sRNAs can move throughout the plant to silence plant genes in tissues different from where they are produced. This highlights the biotechnological potential of art-sRNAs, which might be applied locally for triggering whole-plant and highly specific silencing to regulate gene expression or induce resistance against pathogenic RNAs in next-generation crops. The present study demonstrates that artificial small RNAs, such as artificial microRNAs and synthetic trans-acting small interfering RNAs, can move long distances in plants as 21-nucleotide duplexes, specifically silencing endogenous genes in tissues different from where they are applied. This highlights the biotechnological potential of artificial small RNAs, which might be applied locally for triggering whole-plant, highly specific silencing to regulate gene expression or induce resistance against pathogenic RNAs in next-generation crops.

A tau class GST, OsGSTU5, interacts with VirE2 and modulates the Agrobacterium-mediated transformation in rice

OsGSTU5 interacts and glutathionylates the VirE2 protein of Agrobacterium and its (OsGSTU5) overexpression and downregulation showed a low and high AMT efficiency in rice, respectively. During Agrobacterium-mediated transformation (AMT), T-DNA along with several virulence proteins such as VirD2, VirE2, VirE3, VirD5, and VirF enter the plant cytoplasm. VirE2 serves as a single-stranded DNA binding (SSB) protein that assists the cytoplasmic trafficking of T-DNA inside the host cell. Though the regulatory roles of VirE2 have been established, the cellular reaction of their host, especially in monocots, has not been characterized in detail. This study identified a cellular interactor of VirE2 from the cDNA library of rice. The identified plant protein encoded by the gene cloned from rice was designated OsGSTU5, it interacted specifically with VirE2 in the host cytoplasm. OsGSTU5 was upregulated during Agrobacterium infection and involved in the post-translational glutathionylation of VirE2 (gVirE2). Interestingly, the in silico analysis showed that the 'gVirE2 + ssDNA' complex was structurally less stable than the 'VirE2 + ssDNA' complex. The gel shift assay also confirmed the attenuated SSB property of gVirE2 over VirE2. Moreover, knock-down and overexpression of OsGSTU5 in rice showed increased and decreased T-DNA expression, respectively after Agrobacterium infection. The present finding establishes the role of OsGSTU5 as an important target for modulation of AMT efficiency in rice.

The TGN/EE SNARE protein SYP61 and the ubiquitin ligase ATL31 cooperatively regulate plant responses to carbon/nitrogen conditions in Arabidopsis

Ubiquitination is a post-translational modification involving the reversible attachment of the small protein ubiquitin to a target protein. Ubiquitination is involved in numerous cellular processes, including the membrane trafficking of cargo proteins. However, the ubiquitination of the trafficking machinery components and their involvement in environmental responses are not well understood. Here, we report that the Arabidopsis thaliana trans-Golgi network/early endosome localized SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein SYP61 interacts with the transmembrane ubiquitin ligase ATL31, a key regulator of resistance to disrupted carbon (C)/nitrogen/(N)-nutrient conditions. SYP61 is a key component of membrane trafficking in Arabidopsis. The subcellular localization of ATL31 was disrupted in knockdown mutants of SYP61, and the insensitivity of ATL31-overexpressing plants to high C/low N-stress was repressed in these mutants, suggesting that SYP61 and ATL31 cooperatively function in plant responses to nutrient stress. SYP61 is ubiquitinated in plants, and its ubiquitination level is upregulated under low C/high N-nutrient conditions. These findings provide important insights into the ubiquitin signaling and membrane trafficking machinery in plants.

A procedure for Dex-induced gene transactivation in Arabidopsis ovules

BACKGROUND

Elucidating the genetic and molecular control of plant reproduction often requires the deployment of functional approaches based on reverse or forward genetic screens. The loss-of-function of essential genes, however, may lead to plant lethality prior to reproductive development or to the formation of sterile structures before the organ-of-interest can be analyzed. In these cases, inducible approaches that enable a spatial and temporal control of the genetic perturbation are extremely valuable. Genetic induction in reproductive organs, such as the ovule, deeply embedded in the flower, is a delicate procedure that requires both optimization and validation.

RESULTS

Here we report on a streamlined procedure enabling reliable induction of gene expression in Arabidopsis ovule and anther tissues using the popular pOP/LhGR Dex-inducible system. We demonstrate its efficiency and reliability using fluorescent reporter proteins and histochemical detection of the GUS reporter gene.

CONCLUSION

The pOP/LhGR system allows for a rapid, efficient, and reliable induction of transgenes in developing ovules without compromising developmental progression. This approach opens new possibilities for the functional analysis of candidate regulators in sporogenesis and gametogenesis, which is otherwise affected by early lethality in conventional, stable mutants.

CRISPR-Cas-mediated transcriptional control and epi-mutagenesis

Tools for sequence-specific DNA binding have opened the door to new approaches in investigating fundamental questions in biology and crop development. While there are several platforms to choose from, many of the recent advances in sequence-specific targeting tools are focused on developing Clustered Regularly Interspaced Short Palindromic Repeats- CRISPR Associated (CRISPR-Cas)-based systems. Using a catalytically inactive Cas protein (dCas), this system can act as a vector for different modular catalytic domains (effector domains) to control a gene's expression or alter epigenetic marks such as DNA methylation. Recent trends in developing CRISPR-dCas systems include creating versions that can target multiple copies of effector domains to a single site, targeting epigenetic changes that, in some cases, can be inherited to the next generation in the absence of the targeting construct, and combining effector domains and targeting strategies to create synergies that increase the functionality or efficiency of the system. This review summarizes and compares DNA targeting technologies, the effector domains used to target transcriptional control and epi-mutagenesis, and the different CRISPR-dCas systems used in plants.

Rapid Investigation of Functional Roles of Genes in Regulation of Leaf Senescence Using Arabidopsis Protoplasts

Leaf senescence is the final stage of leaf development preceding death, which involves a significant cellular metabolic transition from anabolism to catabolism. Several processes during leaf senescence require coordinated regulation by senescence regulatory genes. In this study, we developed a rapid and systematic cellular approach to dissect the functional roles of genes in senescence regulation through their transient expression in Arabidopsis protoplasts. We established and validated this system by monitoring the differential expression of a luciferase-based reporter that was driven by promoters of SEN4 and SAG12, early and late senescence-responsive genes, depending on effectors of known positive and negative senescence regulators. Overexpression of positive senescence regulators, including ORE1, RPK1, and RAV1, increased the expression of both SEN4- and SAG12-LUC while ORE7, a negative senescence regulator decreased their expression. Consistently with overexpression, knockdown of target genes using amiRNAs resulted in opposite SAG12-LUC expression patterns. The timing and patterns of reporter responses induced by senescence regulators provided molecular evidence for their distinct kinetic involvement in leaf senescence regulation. Remarkably, ORE1 and RPK1 are involved in cell death responses, with more prominent and earlier involvement of ORE1 than RPK1. Consistent with the results in protoplasts, further time series of reactive oxygen species (ROS) and cell death assays using different tobacco transient systems reveal that ORE1 causes acute cell death and RPK1 mediates superoxide-dependent intermediate cell death signaling during leaf senescence. Overall, our results indicated that the luciferase-based reporter system in protoplasts is a reliable experimental system that can be effectively used to examine the regulatory roles of Arabidopsis senescence-associated genes.

Trans-kingdom expression of an insect endogenous microRNA in rice enhances resistance to striped stem borer Chilo suppressalis

BACKGROUND

The striped stem borer (SSB), Chilo suppressalis Walker, is a major pest of rice worldwide. Breeding of transgenic rice expressing Bacillus thuringiensis (Bt) toxins is a powerful strategy to control SSB. However, pests may evolve certain resistance to Bt toxins in transgenic plants. Hence, new controlling strategies must be continuously developed.

RESULTS

We successfully generated SSB-resistant rice (csu-53) expressing the artificial microRNA (amiRNA) of SSB endogenous miRNA (csu-novel-miR53) through the RNAi-based technology. Feeding assays demonstrated that csu-53 rice inhibited larval growth, delayed pupation time, and reduced pupal weight and eclosion rate of SSB larva. In a 10-day feeding experiment, the miRNA mimic of csu-novel-miR53 also suppressed larval growth and more importantly increased larval mortality. Transcriptome analysis identified 28 differentially expressed unigenes (DEGs) in the midgut between SSB larvae fed on csu-53 rice and the wild type. One DEG (DN90065_c0_g12) validated by qRT-PCR had a predicted target site of csu-novel-miR53. In addition, in vitro double-stranded RNA synthesis and further feeding assay proved that DN90065_c0_g12 is most likely the target of csu-novel-miR53.

CONCLUSION

amiRNA-mediated strategy can be applied to the development of insect-resistant crops, and the novel amiRNA csu-novel-miR53 of SSB has important application potential in developing SSB resistant rice. © 2021 Society of Chemical Industry.

In silico identification of sugarcane (Saccharum officinarum L.) genome encoded microRNAs targeting sugarcane bacilliform virus

Sugarcane bacilliform virus (SCBV) is considered one of the most economically damaging pathogens for sugarcane production worldwide. Three open reading frames (ORFs) are characterized in the circular, ds-DNA genome of the SCBV; these encode for a hypothetical protein (ORF1), a DNA binding protein (ORF2), and a polyprotein (ORF3). A comprehensive evaluation of sugarcane (Saccharum officinarum L.) miRNAs for the silencing of the SCBV genome using in silico algorithms were carried out in the present study using mature sugarcane miRNAs. miRNAs of sugarcane are retrieved from the miRBase database and assessed in terms of hybridization with the SCBV genome. A total of 14 potential candidate miRNAs from sugarcane were screened out by all used algorithms used for the silencing of SCBV. The consensus of three algorithms predicted the hybridization site of sof-miR159e at common locus 5534. miRNA-mRNA interactions were estimated by computing the free-energy of the miRNA-mRNA duplex using the RNAcofold algorithm. A regulatory network of predicted candidate miRNAs of sugarcane with SCBV-ORFs, generated using Circos-is used to identify novel targets. The predicted data provide useful information for the development of SCBV-resistant sugarcane plants.

Post-transcriptional Gene Silencing Using Virus-Induced Gene Silencing to Study Plant Gametogenesis in Tomato

Loss-of-function analyses are essential to dissect the complex nature of biological processes, including gametogenesis. Virus-induced gene silencing (VIGS) has been widely used in crop species as an amenable and rapid way to generate gene knockdowns. As a transient assay, VIGS circumvents the generation of stable transgenic lines through laborious and time-consuming tissue culture techniques. VIGS involves inoculating plants during early development with genetically manipulated viral constructs carrying an endogenous gene target sequence. The viral infection triggers the host plant gene silencing machinery to process the viral genomic RNA into small RNAs (sRNAs) including the gene complementary region. The sRNAs with complementary sequences to the endogenous gene mediate posttranscriptional gene silencing of the targeted gene. Here, we provide a simple and reproducible VIGS protocol employing the tobacco rattle virus (TRV) in tomato (Solanum lycopersicum cv. M82). As it is stable at later developmental stages this approach is suitable for many traits in tomato including gametogenesis and it can be adapted to other crop species.

Fine-Tuning Plant Gene Expression with Synthetic Trans-Acting Small Interfering RNAs

RNAi-based tools are widely used in gene function studies and for crop improvement. However, no effective methods for precisely controlling the degree of induced silencing have been reported until recently. Here we report a detailed protocol for designing and generating synthetic trans-acting small interfering RNA (syn-tasiRNA) constructs for fine-tuning gene expression in plants. Recently developed high-throughput AtTAS1c-D2-B/c-based vectors are used to clone and express syn-tasiRNAs that possess different efficacies depending on their precursor location and on their degree of base-pairing with the 5' end of target RNAs.