The GHKL ATPase MORC1 Modulates Species-Specific Plant Immunity in Solanaceae

Microrchidia OsMORC6 Positively Regulates Cadmium Tolerance and Uptake by Mediating DNA Methylation in Rice

Rice (Oryza sativa) is an extremely important global food crop. However, cadmium (Cd) contamination in paddy fields poses a serious threat to human health worldwide. To generate low-Cd or Cd-free rice germplasms, it is essential to understand the molecular mechanisms involved in Cd tolerance, uptake, and translocation from soil to plant. In this study, we identify three Microrchidia proteins, OsMORC6a, OsMORC6b, and OsMORC6c, that regulate Cd tolerance and accumulation, although they do not alter the translocation of Cd from roots to shoots. Knockout of all three genes results in reducing Cd accumulation and increasing sensitivity to Cd stress. Furthermore, transcriptome analysis reveals 1,127 differentially expressed genes (DEGs) in the morc6abc mutants, which are significantly enriched in 'plant-type cell wall' and 'oxidoreductase activity' pathways. Through an integrating DNA methylome and transcriptome data, we identify 247 hyper-DMR-associated DEGs and 325 hypo-DMR-associated DEGs in morc6abc mutants. Gene Ontology (Go) enrichment analysis reveals that OsMORC6 proteins positively regulate Cd tolerance and uptake by mediating DNA methylation, which regulates the proper expression of genes related to plant cell wall and oxidative stress under Cd stress. Taken together, our findings reveal novel genes that mediate Cd tolerance and accumulation by affecting DNA methylation, offering valuable resource for breeding low-Cd or Cd-free rice germplasms.

Shared and Related Molecular Targets and Actions of Salicylic Acid in Plants and Humans

Salicylic acid (SA) is a phenolic compound produced by all plants that has an important role in diverse processes of plant growth and stress responses. SA is also the principal metabolite of aspirin and is responsible for many of the anti-inflammatory, cardioprotective and antitumor activities of aspirin. As a result, the number of identified SA targets in both plants and humans is large and continues to increase. These SA targets include catalases/peroxidases, metabolic enzymes, protein kinases and phosphatases, nucleosomal and ribosomal proteins and regulatory and signaling proteins, which mediate the diverse actions of SA in plants and humans. While some of these SA targets and actions are unique to plants or humans, many others are conserved or share striking similarities in the two types of organisms, which underlie a host of common biological processes that are regulated or impacted by SA. In this review, we compare shared and related SA targets and activities to highlight the common nature of actions by SA as a hormone in plants versus a therapeutic agent in humans. The cross examination of SA targets and activities can help identify new actions of SA and better explain their underlying mechanisms in plants and humans.

Progresses of CRISPR/Cas9 genome editing in forage crops

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) mediated-genome editing has evolved into a powerful tool that is widely used in plant species to induce editing in the genome for analyzing gene function and crop improvement. CRISPR/Cas9 is an RNA-guided genome editing tool consisting of a Cas9 nuclease and a single-guide RNA (sgRNA). The CRISPR/Cas9 system enables more accurate and efficient genome editing in crops. In this review, we summarized the advances of the CRISPR/Cas9 technology in plant genome editing and its applications in forage crops. We described briefly about the development of CRISPR/Cas9 technology in plant genome editing. We assessed the progress of CRISPR/Cas9-mediated targeted-mutagenesis in various forage crops, including alfalfa, Medicago truncatula, Hordeum vulgare, Sorghum bicolor, Setaria italica and Panicum virgatum. The potentials and challenges of CRISPR/Cas9 in forage breeding were discussed.

Epigenetics: a catalyst of plant immunity against pathogens

The plant immune system protects against pests and diseases. The recognition of stress-related molecular patterns triggers localised immune responses, which are often followed by longer-lasting systemic priming and/or up-regulation of defences. In some cases, this induced resistance (IR) can be transmitted to following generations. Such transgenerational IR is gradually reversed in the absence of stress at a rate that is proportional to the severity of disease experienced in previous generations. This review outlines the mechanisms by which epigenetic responses to pathogen infection shape the plant immune system across expanding time scales. We review the cis- and trans-acting mechanisms by which stress-inducible epigenetic changes at transposable elements (TEs) regulate genome-wide defence gene expression and draw particular attention to one regulatory model that is supported by recent evidence about the function of AGO1 and H2A.Z in transcriptional control of defence genes. Additionally, we explore how stress-induced mobilisation of epigenetically controlled TEs acts as a catalyst of Darwinian evolution by generating (epi)genetic diversity at environmentally responsive genes. This raises questions about the long-term evolutionary consequences of stress-induced diversification of the plant immune system in relation to the long-held dichotomy between Darwinian and Lamarckian evolution.

CRISPR/SpCas9-mediated double knockout of barley Microrchidia MORC1 and MORC6a reveals their strong involvement in plant immunity, transcriptional gene silencing and plant growth

The Microrchidia (MORC) family proteins are important nuclear regulators in both animals and plants with critical roles in epigenetic gene silencing and genome stabilization. In the crop plant barley (Hordeum vulgare), seven MORC gene family members have been described. While barley HvMORC1 has been functionally characterized, very little information is available about other HvMORC paralogs. In this study, we elucidate the role of HvMORC6a and its potential interactors in regulating plant immunity via analysis of CRISPR/SpCas9-mediated single and double knockout (dKO) mutants, hvmorc1 (previously generated and characterized by our group), hvmorc6a, and hvmorc1/6a. For generation of hvmorc1/6a, we utilized two different strategies: (i) successive Agrobacterium-mediated transformation of homozygous single mutants, hvmorc1 and hvmorc6a, with the respective second construct, and (ii) simultaneous transformation with both hvmorc1 and hvmorc6a CRISPR/SpCas9 constructs. Total mutation efficiency in transformed homozygous single mutants ranged from 80 to 90%, while upon simultaneous transformation, SpCas9-induced mutation in both HvMORC1 and HvMORC6a genes was observed in 58% of T0 plants. Subsequent infection assays showed that HvMORC6a covers a key role in resistance to biotrophic (Blumeria graminis) and necrotrophic (Fusarium graminearum) plant pathogenic fungi, where the dKO hvmorc1/6a showed the strongest resistant phenotype. Consistent with this, the dKO showed highest levels of basal PR gene expression and derepression of TEs. Finally, we demonstrate that HvMORC1 and HvMORC6a form distinct nucleocytoplasmic homo-/heteromers with other HvMORCs and interact with components of the RNA-directed DNA methylation (RdDM) pathway, further substantiating that MORC proteins are involved in the regulation of TEs in barley.

Role of Chromatin Architecture in Plant Stress Responses: An Update

Sessile plants possess an assembly of signaling pathways that perceive and transmit environmental signals, ultimately resulting in transcriptional reprogramming. Histone is a key feature of chromatin structure. Numerous histone-modifying proteins act under different environmental stress conditions to help modulate gene expression. DNA methylation and histone modification are crucial for genome reprogramming for tissue-specific gene expression and global gene silencing. Different classes of chromatin remodelers including SWI/SNF, ISWI, INO80, and CHD are reported to act upon chromatin in different organisms, under diverse stresses, to convert chromatin from a transcriptionally inactive to a transcriptionally active state. The architecture of chromatin at a given promoter is crucial for determining the transcriptional readout. Further, the connection between somatic memory and chromatin modifications may suggest a mechanistic basis for a stress memory. Studies have suggested that there is a functional connection between changes in nuclear organization and stress conditions. In this review, we discuss the role of chromatin architecture in different stress responses and the current evidence on somatic, intergenerational, and transgenerational stress memory.

MORC Domain Definition and Evolutionary Analysis of the MORC Gene Family in Green Plants

Microrchidia (MORC) proteins have been described as epigenetic regulators and plant immune mediators in Arabidopsis. Typically, plant and animal MORC proteins contain a hallmark GHKL-type (Gyrase, Hsp90, Histidine kinase, MutL) ATPase domain in their N-terminus. Here, 356 and 83 MORC orthologues were identified in 60 plant and 27 animal genomes. Large-scale MORC sequence analyses revealed the presence of a highly conserved motif composition that defined as the MORC domain. The MORC domain was present in both plants and animals, indicating that it originated in the common ancestor before the divergence of plants and animals. Phylogenetic analyses showed that MORC genes in both plant and animal lineages were clearly classified into two major groups, named Plants-Group I, Plants-Group II and Animals-Group I, Animals-Group II, respectively. Further analyses of MORC genes in green plants uncovered that Group I can be subdivided into Group I-1 and Group I-2. Group I-1 only contains seed plant genes, suggesting that Group I-1 and I-2 divergence occurred at least before the emergence of spermatophytes. Group I-2 and Group II have undergone several gene duplications, resulting in the expansion of MORC gene family in angiosperms. Additionally, MORC gene expression analyses in Arabidopsis, soybean, and rice revealed a higher expression level in reproductive tissues compared with other organs, and showed divergent expression patterns for several paralogous gene pairs. Our studies offered new insights into the origins, phylogenetic relationships, and expressional patterns of MORC family members in green plants, which would help to further reveal their functions as plant epigenetic regulators.

Further analysis of barley MORC1 using a highly efficient RNA-guided Cas9 gene-editing system

Microrchidia (MORC) proteins comprise a family of proteins that have been identified in prokaryotes and eukaryotes. They are defined by two hallmark domains: a GHKL-type ATPase and an S5-fold. In plants, MORC proteins were first discovered in a genetic screen for Arabidopsis thaliana mutants compromised for resistance to a viral pathogen. Subsequent studies expanded their role in plant immunity and revealed their involvement in gene silencing and genome stabilization. Little is known about the role of MORC proteins of cereals, especially because knockout (KO) mutants were not available and assessment of loss of function relied only on RNAi strategies, which were arguable, given that MORC proteins in itself are influencing gene silencing. Here, we used a Streptococcus pyogenes Cas9 (SpCas9)-mediated KO strategy to functionally study HvMORC1, one of the current seven MORC members of barley. Using a novel barley RNA Pol III-dependent U3 small nuclear RNA (snRNA) promoter to drive expression of the synthetic single guide RNA (sgRNA), we achieved a very high mutation frequency in HvMORC1. High frequencies of mutations were detectable by target sequencing in the callus, the T0 generation (77%) and T1 generation (70%-100%), which constitutes an important improvement of the gene-editing technology in cereals. Corroborating and extending earlier findings, SpCas9-edited hvmorc1-KO barley, in clear contrast to Arabidopsis atmorc1 mutants, had a distinct phenotype of increased disease resistance to fungal pathogens, while morc1 mutants of either plant showed de-repressed expression of transposable elements (TEs), substantiating that plant MORC proteins contribute to genome stabilization in monocotyledonous and dicotyledonous plants.

MORC Proteins: Novel Players in Plant and Animal Health

Microrchidia (MORC) proteins comprise a family of proteins that have been identified in prokaryotes and eukaryotes. They are defined by two hallmark domains: a GHKL-type ATPase and an S5 fold. MORC proteins in plants were first discovered via a genetic screen for Arabidopsis mutants compromised for resistance to a viral pathogen. Subsequent studies expanded their role in plant immunity and revealed their involvement in gene silencing and transposable element repression. Emerging data suggest that MORC proteins also participate in pathogen-induced chromatin remodeling and epigenetic gene regulation. In addition, biochemical analyses recently demonstrated that plant MORCs have topoisomerase II (topo II)-like DNA modifying activities that may be important for their function. Interestingly, animal MORC proteins exhibit many parallels with their plant counterparts, as they have been implicated in disease development and gene silencing. In addition, human MORCs, like plant MORCs, bind salicylic acid and this inhibits some of their topo II-like activities. In this review, we will focus primarily on plant MORCs, although relevant comparisons with animal MORCs will be provided.

Plant Resistance Inducers against Pathogens in Solanaceae Species-From Molecular Mechanisms to Field Application

This review provides a current summary of plant resistance inducers (PRIs) that have been successfully used in the Solanaceae plant family to protect against pathogens by activating the plant's own defence. Solanaceous species include many important crops such as potato and tomato. We also present findings regarding the molecular processes after application of PRIs, even if the number of such studies still remains limited in this plant family. In general, there is a lack of patterns regarding the efficiency of induced resistance (IR) both between and within solanaceous species. In many cases, a hypersensitivity-like reaction needs to form in order for the PRI to be efficient. "-Omics" studies have already given insight in the complexity of responses, and can explain some of the differences seen in efficacy of PRIs between and within species as well as towards different pathogens. Finally, examples of field applications of PRIs for solanaceous crops are presented and discussed. We predict that PRIs will play a role in future plant protection strategies in Solanaceae crops if they are combined with other means of disease control in different spatial and temporal combinations.