Role of Transcription Factor HAT1 in Modulating Arabidopsis thaliana Response to Cucumber mosaic virus

Physiological and transcriptomic response reveals new insight into manganese tolerance of Celosia argentea Linn

Celosia argentea is a manganese (Mn) hyperaccumulator with high ornamental value and strong stress resistance. It is important to understand the molecular mechanism of tolerance to heavy metals of hyperaccumulators to improve the efficiency of phytoremediation. In this study, the effects of different Mn concentrations (0, 0.8, 3, and 10 mM) on physiological characteristics and molecular changes were determined. Low concentrations of Mn increased the growth of C. argentea, while high concentrations of Mn suppressed its growth, A concentration up to 3 mM did not affect the growth of C. argentea, and the highest transfer factor (TF) was 6.16. Oxidative damage of different Mn level treatments in C. argentea was verified through relative water content, electrolyte leakage, MDA content, H2O2 content and superoxide contents. With an increase in Mn concentration, the contents of chlorophyll a, chlorophyll b, and carotenoids decreased. Our results indicated that low-concentration manganese treatment can reduce the reactive oxygen burst and MDA, soluble sugar and proline, making C. argentea have strong abiotic stress tolerance. The molecular mechanism of C. argentea after 10 mM Mn treatment was analysed through transcriptome analysis, and differentially expressed genes (DEGs) in these pathways were further verified by qRTPCR. Plantpathogen interactions, plant hormone signal transduction, the MAPK signalling pathway and the phenylpropanoid biosynthesis pathway were important in the response to Mn stress, and the heavy metal-associated isoprenylated plant protein, metal transporter Nramp, and zinc transporter play key roles in the strong ability of C. argentea to tolerate heavy metals. These results suggest that C. argentea exhibits strong manganese tolerance and provide new insight into the molecular mechanisms of plant responses to heavy metal stress.

The Role of Plant Transcription Factors in the Fight against Plant Viruses

Plants are vulnerable to the challenges of unstable environments and pathogen infections due to their immobility. Among various stress conditions, viral infection is a major threat that causes significant crop loss. In response to viral infection, plants undergo complex molecular and physiological changes, which trigger defense and morphogenic pathways. Transcription factors (TFs), and their interactions with cofactors and cis-regulatory genomic elements, are essential for plant defense mechanisms. The transcriptional regulation by TFs is crucial in establishing plant defense and associated activities during viral infections. Therefore, identifying and characterizing the critical genes involved in the responses of plants against virus stress is essential for the development of transgenic plants that exhibit enhanced tolerance or resistance. This article reviews the current understanding of the transcriptional control of plant defenses, with a special focus on NAC, MYB, WRKY, bZIP, and AP2/ERF TFs. The review provides an update on the latest advances in understanding how plant TFs regulate defense genes expression during viral infection.

Arabidopsis SUMO E3 ligase SIZ1 enhances cadmium tolerance via the glutathione-dependent phytochelatin synthesis pathway

Sumoylation is a posttranslational modification (PTM) in which SUMO (small ubiquitin-like modifier) is covalently conjugated to protein substrates via a range of enzymes. SUMO E3 ligase SIZ1 is involved in mediating several essential or nonessential element-responsive SUMO conjugations in Arabidopsis. However, whether SIZ1 is involved in the cadmium (Cd) response remains to be identified. In this study, we found that SIZ1 positively regulates plant Cd tolerance. The loss-of-function siz1-2 mutant exhibited impaired resistance to Cd exposure and accumulated more reactive oxygen species (ROS). Moreover, the transcription of GSH1, GSH2, PCS1, and PCS2 was suppressed while the accumulation of Cd was enhanced in the siz1-2 mutant under Cd exposure. Further analysis revealed that the higher Cd sensitivity of the siz1-2 mutant was partially rescued by the overexpression of GSH1. Consistently, Cd stress stimulated the accumulation of SUMO1 conjugates in wild-type plants but not in the siz1-2 mutant. Together, our results demonstrate that Cd-induced SIZ1 activates GSH- and PC synthesis-related gene expression to increase the synthesis of GSH- and PCs, thereby leading to higher Cd tolerance in plants.

Genetic transformation of LoHDZ2 and analysis of its function to enhance stress resistance in Larix olgensis

To study the function of LoHDZ2 in larch, we first constructed a VB191103-LoHDZ2::GUS overexpression vector. Through Agrobacterium-mediated infection, the expression vector was transferred into a larch embryogenic cell line. A stable resistant cell line was subsequently screened, and mature embryos were induced to grow until they developed into seedlings. Antagonistic cell lines were identified at both the DNA and RNA levels. The transgenic cell lines were then subjected to GUS staining, and transgenic cell lines were ultimately identified and obtained. These transgenic cell lines were sequenced to identify differentially expressed genes, and a cluster analysis was performed. The resistant cell lines were cultured under stress conditions involving 20% PEG₆₀₀₀ and 200 mM NaCl proliferation media (1/10-BM). After the stress treatment, the contents of peroxidase (POD), malondialdehyde (MDA) and superoxide dismutase (SOD) in both wild-type and transgenic cell lines were measured. The results are summarized below: (1) When the specific fragment of the target gene in the genome of the resistant cell line was amplified. At the RNA level, the expression of the fragment in four resistant lines increased. In addition, GUS staining showed a blue reaction, indicating that LoHDZ2 was successfully integrated into the larch embryonic cell lines. (2) To verify the accuracy and reliability of the transcriptome data, 10 differentially expressed genes (5 upregulated and 5 down regulated genes) were subjected to qRT-PCR verification. The results showed that the expression trend of the 10 differentially expressed genes was the same as that revealed by RNA-Seq, indicating that the transcriptome data were reliable. (3) The transcriptome sequencing showed that 176 genes were upregulated and that 140 genes were down regulated. Through GO enrichment analysis and KEGG metabolic pathway analysis, the screened differentially expressed genes were related to biological processes such as larch metabolism and response to stimuli, indicating that these genes may be closely involved in the regulation of the larch response to external stimuli, including heat stress, drought stress, metal ion stress and bacterial infection, and may participate in the growth process. (4) After 20% PEG₆₀₀₀ treatment, the POD enzyme activity of the transgenic cell line was greater than that of the wild-type; this activity could effectively remove the amount of peroxide produced. The MDA content of the transgenic cell lines was lower than that of the wild-type cell lines, and the accumulation degree of harmful substances was low, indicating that the degree of oxidative damage of the transgenic cell lines was lower than that of the wild-type cell lines. The SOD content of the transgenic cell lines was lower than that of the wild-type cell lines, indicating that the drought resistance of the transgenic cell lines was enhanced. After 200 mM NaCl treatment, although the increase in SOD content was not obvious, the same trend was detected, indicating that the resistance of the transgenic cell lines was indeed stronger than that of the wild-type cell lines. According to the results of previous experiments, after this gene was overexpressed in tobacco, the transformed plants showed obvious dwarfing, which may indicate that the stress resistance of the plant was enhanced. In conclusion, a transgenic larch cell line was successfully obtained, and transgenic larch seedlings were successfully induced. LoHDZ2 may participate in the response of plants to the external environment, and may participate in the growth and development of Larixolgensis by affecting plant metabolic pathways.

Homeobox 27, a Homeodomain Transcription Factor, Confers Tolerances to CMV by Associating with Cucumber Mosaic Virus 2b Protein

Transcription factors (TFs) play an important role in plant development; however, their role during viral infection largely remains unknown. The present study was designed to uncover the role transcription factors play in Cucumber mosaic virus (CMV) infection. During the screening of an Arabidopsis thaliana (Col-0) transcription factor library, using the CMV 2b protein as bait in the yeast two-hybrid system, the 2b protein interacted with Homeobox protein 27 (HB27). HB27 belongs to the zinc finger homeodomain family and is known to have a regulatory role in flower development, and responses to biotic and abiotic stress. The interaction between CMV 2b and HB27 proteins was further validated using in planta (bimolecular fluorescence complementation assay) and in vitro far-Western blotting (FWB) methods. In the bimolecular fluorescence complementation assay, these proteins reconstituted YFP fluorescence in the nucleus and the cytoplasmic region as small fluorescent dots. In FWB, positive interaction was detected using bait anti-MYC antibody on the target HB27-HA protein. During CMV infection, upregulation (~3-fold) of the HB27 transcript was observed at 14 days post-infection (dpi) in A. thaliana plants, and expression declined to the same as healthy plants at 21 dpi. To understand the role of the HB27 protein during CMV infection, virus accumulation was determined in HB27-overexpressing (HB27 OE) and knockout mutants. In HB27-overexpressing lines, infected plants developed mild symptoms, accumulating a lower virus titer at 21 dpi compared to wild-type plants. Additionally, knockout HB27 mutants had more severe symptoms and a higher viral accumulation than wild-type plants. These results indicate that HB27 plays an important role in the regulation of plant defense against plant virus infection.

Synergetic modulation of plant cadmium tolerance via MYB75-mediated ROS homeostasis and transcriptional regulation

KEY MESSAGE

MYB75 enhances plant cadmium tolerance by mediating ROS homeostasis and cadmium tolerance-related genes expression. Cadmium (Cd) is a heavy metal with biological toxicity, which can be detoxified through chelation and compartmentation in plants. Transcriptional regulation mediates plant Cd tolerance by modulating these processes. However, the mechanism remains to be studied. Our results showed a previously unknown function of MYB75 transcription factor in the regulation of Cd tolerance. Cd exposure stimulates anthocyanin accumulation by raising MYB75 expression. Enhanced Cd tolerance was observed in the MYB75-overexpressing plants, whereas increased Cd sensitivity was found in the MYB75 loss-of-function mutants. Under Cd stress conditions, lower reactive oxygen species (ROS) levels were detected in MYB75-overexpressing plants than in wild type plants. In contrast, higher ROS levels were found in MYB75 loss-of-function mutants. Overexpression of MYB75 was associated with increased glutathione (GSH) and phytochelatin (PC) content under Cd exposure. Furthermore, the expression of Cd stress-related gene including ACBP2 and ABCC2 was elevated in MYB75-overexpressing plants, and this upregulation was mediated through the mechanism by which MYB75 directly bind to the promoter of ACBP2 and ABCC2. Our findings reveal an important role for MYB75 in the regulation of plant Cd tolerance via anthocyanin-mediated ROS homeostasis, and through upregulation of Cd stress-related gene at the transcriptional level.

Transcriptome Profiling Reveals a Petunia Transcription Factor, PhCOL4, Contributing to Antiviral RNA Silencing

RNA silencing is a common antiviral mechanism in eukaryotic organisms. However, the transcriptional regulatory mechanism that controls the RNA silencing process remains elusive. Here, we performed high-depth transcriptome analysis on petunia (Petunia hybrida) leaves infected with tobacco rattle virus (TRV) strain PPK20. A total of 7,402 differentially expressed genes (DEGs) were identified. Of them, some RNA silencing-related transcripts, such as RNA-dependent RNA polymerases (RDRs), Dicer-like RNase III enzymes (DCLs), and Argonautes (AGOs), were induced by viral attack. Furthermore, we performed TRV-based virus-induced gene silencing (VIGS) assay on 39 DEGs encoding putative transcription factors (TFs), using green fluorescent protein (GFP) and phytoene desaturase (PhPDS) as reporters. Results showed that the down-regulation of PhbHLH41, PhbHLH93, PhZPT4-3, PhCOL4, PhHSF-B3A, PhNAC90, and PhWRKY75 led to enhanced TRV accumulation and inhibited PhPDS-silenced photobleaching phenotype. In contrast, silencing of PhERF22 repressed virus accumulation and promoted photobleaching development. Thus, these TFs were identified as potential positive and negative regulators of antiviral RNA silencing, respectively. One positive regulator PhCOL4, belonging to the B-box zinc finger family, was selected for further functional characterization. Silencing and transient overexpression of PhCOL4 resulted in decreased and increased expression of several RNA silencing-related genes. DNA affinity purification sequencing analysis revealed that PhCOL4 targeted PhRDR6 and PhAGO4. Dual luciferase and yeast one-hybrid assays determined the binding of PhCOL4 to the PhRDR6 and PhAGO4 promoters. Our findings suggest that TRV-GFP-PhPDS-based VIGS could be helpful to identify transcriptional regulators of antiviral RNA silencing.

Resistance induction based on the understanding of molecular interactions between plant viruses and host plants

BACKGROUND

Viral diseases cause significant damage to crop yield and quality. While fungi- and bacteria-induced diseases can be controlled by pesticides, no effective approaches are available to control viruses with chemicals as they use the cellular functions of their host for their infection cycle. The conventional method of viral disease control is to use the inherent resistance of plants through breeding. However, the genetic sources of viral resistance are often limited. Recently, genome editing technology enabled the publication of multiple attempts to artificially induce new resistance types by manipulating host factors necessary for viral infection.

MAIN BODY

In this review, we first outline the two major (R gene-mediated and RNA silencing) viral resistance mechanisms in plants. We also explain the phenomenon of mutations of host factors to function as recessive resistance genes, taking the eIF4E genes as examples. We then focus on a new type of virus resistance that has been repeatedly reported recently due to the widespread use of genome editing technology in plants, facilitating the specific knockdown of host factors. Here, we show that (1) an in-frame mutation of host factors necessary to confer viral resistance, sometimes resulting in resistance to different viruses and that (2) certain host factors exhibit antiviral resistance and viral-supporting (proviral) properties.

CONCLUSION

A detailed understanding of the host factor functions would enable the development of strategies for the induction of a new type of viral resistance, taking into account the provision of a broad resistance spectrum and the suppression of the appearance of resistance-breaking strains.

A HAT1-DELLA signaling module regulates trichome initiation and leaf growth by achieving gibberellin homeostasis

Trichome initiation and leaf growth are two critical developmental processes in the plant life cycle, which need to be optimized in accordance with developmental stage and immediate surroundings. To a large extent, this optimization is achieved by fine-tuning of hormonal pathways, including the gibberellin (GA) pathway. However, the mechanism by which plants control GA homeostasis to optimize these two developmental processes is unknown. Here, we report that HAT1, a HD-ZIP II transcription factor, negatively regulates GA-mediated trichome initiation and cotyledon expansion. Both protein and transcript levels indicated that HAT1 was induced by GA, while an increased abundance of HAT1, in turn, was found to suppress GA biosynthesis and signaling, thus forming a regulatory negative feedback loop that controls GA homeostasis to fine-tune trichome development and cotyledon expansion. We also found that HAT1 interacts with DELLAs, including GAI and RGA. GAI inhibits both protein stability and the binding activity of HAT1 to its target genes. Overexpression of HAT1 in della5 can completely suppress the enhanced trichome initiation and enlarged cotyledon of della5. Our findings demonstrate that HAT1 functions as a critical repressor to regulate GA-mediated trichome initiation and cotyledon growth; in addition, we describe a novel mechanism by which the plant regulates trichome initiation and cotyledon expansion through a HAT1-DELLA regulatory module under various GA concentrations.

Next-Generation Sequencing and the CRISPR-Cas Nexus: A Molecular Plant Virology Perspective

In recent years, next-generation sequencing (NGS) and contemporary Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-CRISPR-associated (Cas) technologies have revolutionized the life sciences and the field of plant virology. Both these technologies offer an unparalleled platform for sequencing and deciphering viral metagenomes promptly. Over the past two decades, NGS technologies have improved enormously and have impacted plant virology. NGS has enabled the detection of plant viruses that were previously undetectable by conventional approaches, such as quarantine and archeological plant samples, and has helped to track the evolutionary footprints of viral pathogens. The CRISPR-Cas-based genome editing (GE) and detection techniques have enabled the development of effective approaches to virus resistance. Different versions of CRISPR-Cas have been employed to successfully confer resistance against diverse plant viruses by directly targeting the virus genome or indirectly editing certain host susceptibility factors. Applications of CRISPR-Cas systems include targeted insertion and/or deletion, site-directed mutagenesis, induction/expression/repression of the gene(s), epigenome re-modeling, and SNPs detection. The CRISPR-Cas toolbox has been equipped with precision GE tools to engineer the target genome with and without double-stranded (ds) breaks or donor templates. This technique has also enabled the generation of transgene-free genetically engineered plants, DNA repair, base substitution, prime editing, detection of small molecules, and biosensing in plant virology. This review discusses the utilities, advantages, applications, bottlenecks of NGS, and CRISPR-Cas in plant virology.

Control of Plant Viruses by CRISPR/Cas System-Mediated Adaptive Immunity

Plant diseases caused by invading plant viruses pose serious threats to agricultural production in the world, and the antiviral engineering initiated by molecular biotechnology has been an effective strategy to prevent and control plant viruses. Recent advances in clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system-mediated DNA or RNA editing/interference in plants make them very attractive tools applicable to the plant protection field. Here, we review the development of CRISPR/Cas systems and summarize their applications in controlling different plant viruses by targeting viral sequences or host susceptibility genes. We list some potential recessive resistance genes that can be utilized in antiviral breeding and emphasize the importance and promise of recessive resistance gene-based antiviral breeding to generate transgene-free plants without developmental defects. Finally, we discuss the challenges and opportunities for the application of CRISPR/Cas techniques in the prevention and control of plant viruses in the field.

The C-terminal region of the 2a protein and 2b protein of cucumber mosaic virus are involved in the induction of shoestring-like leaf blade in tomato

Cucumber mosaic virus (CMV) has numerous strains with distinct pathological properties in nature. In this study, we focused on the distinct host-specificity of two isolates of CMV regarding induction of the shoestring-like leaf blade (SLB) in tomato (Solanum lycopersicum cv. Sekaiichi). During the initial infection stage, plants inoculated with CMV-D8 and CMV-Y developed green/yellow systemic mosaic and stunting. Late in infection, CMV-D8 caused severe systemic symptoms with SLB on the newly emerged leaves, whereas CMY-Y caused severe yellow mosaic with stunting. Accumulation of viral RNA of CMV-D8 during initial infection was higher than for CMV-Y, but their levels did not differ significantly at 5 weeks post inoculation. Pseudorecombination and recombination analyses between CMV-D8 and CMV-Y genomic RNAs showed that recombinant that contained the C-terminal region of 2a and the entire 2b protein of CMV-D8 (D2a-C/D2b) induced SLB. Changing isoleucine to valine at position 830 in the 2a ORF played an important role in formation of chronic SLB. We further elucidated that infection with CMV-D8 or the recombinant Y1Y2(D2a-C/D2b)D3, but not with CMV-Y, upregulated miRNAs and transcript levels of AGO1, which is involved in RNA silencing, and of HD-ZIP, TCP4, and PHAN, which are essential for leaf morphogenesis. The present results first demonstrated that the cooperative function of D2a-C/D2b is involved indispensably in SLB formation. In addition, we suggest that D2a-C/D2b region interferes with the miRNA pathway that is associated with RNA silencing and leaf morphogenesis, leading to the enhanced virulence of CMV-D8.

Emerging Molecular Links Between Plant Photomorphogenesis and Virus Resistance

Photomorphogenesis refers to photoreceptor-mediated morphological changes in plant development that are triggered by light. Multiple photoreceptors and transcription factors (TFs) are involved in the molecular regulation of photomorphogenesis. Likewise, light can also modulate the outcome of plant-virus interactions since both photosynthesis and many viral infection events occur in the chloroplast. Despite the apparent association between photosynthesis and virus infection, little is known about whether there are also interplays between photomorphogenesis and plant virus resistance. Recent research suggests that plant-virus interactions are potentially regulated by several photoreceptors and photomorphogenesis regulators, including phytochromes A and B (PHYA and PHYB), cryptochromes 2 (CRY2), phototropin 2 (PHOT2), the photomorphogenesis repressor constitutive photomorphogenesis 1 (COP1), the NAM, ATAF, and CUC (NAC)-family TF ATAF2, the Aux/IAA protein phytochrome-associated protein 1 (PAP1), the homeodomain-leucine zipper (HD-Zip) TF HAT1, and the core circadian clock component circadian clock associated 1 (CCA1). Particularly, the plant growth promoting brassinosteroid (BR) hormones play critical roles in integrating the regulatory pathways of plant photomorphogenesis and viral defense. Here, we summarize the current understanding of molecular mechanisms linking plant photomorphogenesis and defense against viruses, which represents an emerging interdisciplinary research topic in both molecular plant biology and virology.

Transcriptome analysis of two cultivars of tobacco in response to Cucumber mosaic virus infection

Cucumber mosaic virus (CMV) is among the most important plant virus infections, inducing a variety of disease symptoms. However, the molecular mechanisms underlying plant responses to CMV infection remain poorly understood. In this study, we performed RNA sequencing analysis of tolerant (Taiyan8) and susceptible (NC82) tobacco cultivars on CMV-infected plants, using mock-inoculated plants as a control. The propagation of CMV in inoculated leaves did not show obvious difference between two cultivars, whereas virus accumulation in systemic leaves of Taiyan8 was smaller than those of NC82 at the same time point. We observed 765 and 1,011 differentially expressed genes (DEGs) in Taiyan8 and NC82, respectively, in CMV-inoculated leaves. DEGs related to reactive oxygen species, salicylic acid signal transduction, and plant-pathogen interaction were upregulated or downregulated in Taiyan8, which indicates that defense response pathways to CMV were activated in the tolerant cultivar. In addition, we identified several DEGs related to disease defense and stress resistance showing opposing expression patterns in the two cultivars. Our comparative transcriptome analysis will improve our understanding of the mechanisms of CMV tolerance in plants, and will be of great importance in the molecular breeding of CMV- tolerant genotypes.

Multiple Links between HD-Zip Proteins and Hormone Networks

HD-Zip proteins are unique to plants, and contain a homeodomain closely linked to a leucine zipper motif, which are involved in dimerization and DNA binding. Based on homology in the HD-Zip domain, gene structure and the presence of additional motifs, HD-Zips are divided into four families, HD-Zip I–IV. Phylogenetic analysis of HD-Zip genes using transcriptomic and genomic datasets from a wide range of plant species indicate that the HD-Zip protein class was already present in green algae. Later, HD-Zips experienced multiple duplication events that promoted neo- and sub-functionalizations. HD-Zip proteins are known to control key developmental and environmental responses, and a growing body of evidence indicates a strict link between members of the HD-Zip II and III families and the auxin machineries. Interactions of HD-Zip proteins with other hormones such as brassinolide and cytokinin have also been described. More recent data indicate that members of different HD-Zip families are directly involved in the regulation of abscisic acid (ABA) homeostasis and signaling. Considering the fundamental role of specific HD-Zip proteins in the control of key developmental pathways and in the cross-talk between auxin and cytokinin, a relevant role of these factors in adjusting plant growth and development to changing environment is emerging.

α-Expansin EXPA4 Positively Regulates Abiotic Stress Tolerance but Negatively Regulates Pathogen Resistance in Nicotiana tabacum

Since they function as cell wall-loosening proteins, expansins can affect plant growth, developmental processes and environmental stress responses. Our previous study demonstrated that changes in Nicotiana tabacum α-expansin 4 (EXPA4) expression affect the sensitivity of tobacco to Tobacco mosaic virus [recombinant TMV encoding green fluorescent protein (TMV-GFP)] infection by Agrobacterium-mediated transient expression. In this study, to characterize the function of tobacco EXPA4 further, EXPA4 RNA interfernce (RNAi) mutants and overexpression lines were generated and assayed for their tolerance to abiotic stress and resistance to pathogens. First, the differential phenotypes and histomorphology of transgenic plants with altered EXPA4 expression indicated that EXPA4 is essential for normal tobacco growth and development. By utilizing tobacco EXPA4 mutants with abiotic stress, it was demonstrated that RNAi mutants have increased hypersensitivity to salt and drought stress. In contrast, the overexpression of EXPA4 in tobacco conferred greater tolerance to salt and drought stress, as indicated by less cell damage, higher fresh weight, higher soluble sugar and proline accumulation, and higher expression levels of several stress-responsive genes. In addition, the overexpression lines were more susceptible to the viral pathogen TMV-GFP when compared with the wild type or RNAi mutants. The induction of the antioxidant system, several defense-associated phytohormones and gene expression was down-regulated in overexpression lines but up-regulated in RNAi mutants when compared with the wild type following TMV-GFP infection. In addition, EXPA4 overexpression also accelerated the disease development of Pseudomonas syringae DC3000 on tobacco. Taken together, these results suggested that EXPA4 appears to be important in tobacco growth and responses to abiotic and biotic stress.

Transcription factor HAT1 is a substrate of SnRK2.3 kinase and negatively regulates ABA synthesis and signaling in Arabidopsis responding to drought

Drought is a major threat to plant growth and crop productivity. The phytohormone abscisic acid (ABA) plays a critical role in plant response to drought stress. Although ABA signaling-mediated drought tolerance has been widely investigated in Arabidopsis thaliana, the feedback mechanism and components negatively regulating this pathway are less well understood. Here we identified a member of Arabidopsis HD-ZIP transcription factors HAT1 which can interacts with and be phosphorylated by SnRK2s. hat1hat3, loss-of-function mutant of HAT1 and its homolog HAT3, was hypersensitive to ABA in primary root inhibition, ABA-responsive genes expression, and displayed enhanced drought tolerance, whereas HAT1 overexpressing lines were hyposensitive to ABA and less tolerant to drought stress, suggesting that HAT1 functions as a negative regulator in ABA signaling-mediated drought response. Furthermore, expression levels of ABA biosynthesis genes ABA3 and NCED3 were repressed by HAT1 directly binding to their promoters, resulting in the ABA level was increased in hat1hat3 and reduced in HAT1OX lines. Further evidence showed that both protein stability and binding activity of HAT1 was repressed by SnRK2.3 phosphorylation. Overexpressing SnRK2.3 in HAT1OX transgenic plant made a reduced HAT1 protein level and suppressed the HAT1OX phenotypes in ABA and drought response. Our results thus establish a new negative regulation mechanism of HAT1 which helps plants fine-tune their drought responses.

Alpha-momorcharin enhances Tobacco mosaic virus resistance in tobaccoNN by manipulating jasmonic acid-salicylic acid crosstalk

Alpha-momorcharin (α-MMC) is a type-I ribosome inactivating protein (RIP) with a molecular weight of 29 kDa found in plants. This protein has been shown to be effective against a broad range of human viruses and also has anti-tumor activities. However, the mechanism by which α-MMC induces plant defense responses and regulates the N gene to promote resistance to the Tobacco mosaic virus (TMV) is still not clear. By using pharmacological and infection experiments, we found that α-MMC enhances TMV resistance of tobacco plants containing the N gene (tobaccoNN). Our results showed that plants pretreated with 0.5 mg/ml α-MMC could relieve TMV-induced oxidative damage, had enhanced the expression of the N gene and increased biosynthesis of jasmonic acid (JA) and salicylic acid (SA). Moreover, transcription of JA and SA signaling pathway genes were increased, and their expression persisted for a longer period of time in plants pretreated with α-MMC compared with those pretreated with water. Importantly, exogenous application of 1-Aminobenzotriazole (ABT, SA inhibitor) and ibuprofen (JA inhibitor) reduced α-MMC induced plant resistance under viral infection. Thus, our results revealed that α-MMC enhances TMV resistance of tobaccoNN plants by manipulating JA-SA crosstalk.

Recessive Resistance to Plant Viruses: Potential Resistance Genes Beyond Translation Initiation Factors

The ability of plant viruses to propagate their genomes in host cells depends on many host factors. In the absence of an agrochemical that specifically targets plant viral infection cycles, one of the most effective methods for controlling viral diseases in plants is taking advantage of the host plant’s resistance machinery. Recessive resistance is conferred by a recessive gene mutation that encodes a host factor critical for viral infection. It is a branch of the resistance machinery and, as an inherited characteristic, is very durable. Moreover, recessive resistance may be acquired by a deficiency in a negative regulator of plant defense responses, possibly due to the autoactivation of defense signaling. Eukaryotic translation initiation factor (eIF) 4E and eIF4G and their isoforms are the most widely exploited recessive resistance genes in several crop species, and they are effective against a subset of viral species. However, the establishment of efficient, recessive resistance-type antiviral control strategies against a wider range of plant viral diseases requires genetic resources other than eIF4Es. In this review, we focus on recent advances related to antiviral recessive resistance genes evaluated in model plants and several crop species. We also address the roles of next-generation sequencing and genome editing technologies in improving plant genetic resources for recessive resistance-based antiviral breeding in various crop species.