OsCUL3a Negatively Regulates Cell Death and Immunity by Degrading OsNPR1 in Rice

CDP-DAG synthases regulate plant growth and broad-spectrum disease resistance

Phosphatidic acid (PA) functions as a cell membrane component and signaling molecule in plants. PA metabolism has multiple routes, in one of which PA is converted into cytidine diphosphate diacylglycerol (CDP-DAG) by CDP-DAG synthases (CDSs). CDS genes are highly conserved in plants. Here, we found that knock-down of the CDS gene enhanced the resistance of Arabidopsis thaliana to multiple pathogens, with a growth penalty. When Arabidopsis leaves were treated with chitin or flg22, reactive oxygen species (ROS) production in cds mutants was significantly higher than that in the wild-type (WT). Similarly, phosphorylation of mitogen-activated protein kinases (MAPKs) in the cds1cds2 double mutant was significantly increased compared to the WT. By integrating lipidomics, transcriptomics, and metabolomics data, PA accumulation was observed in mutants cds1cds2, activating the jasmonic acid (JA) and salicylic acid (SA) signaling pathway, and increasing transcript levels of plant defense-related genes. Significant accumulation of the downstream metabolites including serotonin and 5-methoxyindole was also found, which plays important roles in plant immunity. In conclusion, our study indicated the role of CDSs in broad-spectrum disease resistance in Arabidopsis and that CDSs are involved in plant metabolic regulation.

Plant viruses convergently target NPR1 with various strategies to suppress salicylic acid-mediated antiviral immunity

NONEXPRESSER OF PATHOGENESIS-RELATED GENES 1 (NPR1), the receptor for salicylic acid (SA), plays a central role in the SA-mediated basal antiviral responses. Recent studies have shown that two different plant RNA viruses encode proteins that suppress such antiviral responses by inhibiting its SUMOylation and inducing its degradation, respectively. However, it is unclear whether targeting NPR1 is a general phenomenon in viruses and whether viruses have novel strategies to inhibit NPR1. In the present study, we report that two different positive-sense single-stranded RNA (+ssRNA) viruses, namely, alfalfa mosaic virus (AMV) and potato virus X (PVX); one negative-sense single-stranded RNA (-ssRNA) virus (calla lily chlorotic spot virus, CCSV); and one single-stranded DNA virus (beet severe curly-top virus, BSCTV) that also encode one or more proteins that interact with NPR1. In addition, we found that the AMV-encoded coat protein (CP) can induce NPR1 degradation by recruiting S-phase kinase-associated protein 1 (Skp1), a key component of the Skp1/cullin1/F-box (SCF) E3 ligase. In contrast, the BSCTV-encoded V2 protein inhibits NPR1 function, probably by affecting its nucleocytoplasmic distribution via the nuclear export factor ALY. Taken together, these data suggest that NPR1 is one of the central hubs in the molecular arms race between plants and viruses and that different viruses have independently evolved different strategies to target NPR1 and disrupt its function.

Genetic engineering, including genome editing, for enhancing broad-spectrum disease resistance in crops

Plant diseases, caused by a wide range of pathogens, severely reduce crop yield and quality, posing a significant threat to global food security. Developing broad-spectrum resistance (BSR) in crops is a key strategy for controlling crop diseases and ensuring sustainable crop production. Cloning disease-resistance (R) genes and understanding their underlying molecular mechanisms provide new genetic resources and strategies for crop breeding. Novel genetic engineering and genome editing tools have accelerated the study and engineering of BSR genes in crops, which is the primary focus of this review. We first summarize recent advances in understanding the plant immune system, followed by an examination of the molecular mechanisms underlying BSR in crops. Finally, we highlight diverse strategies employed to achieve BSR, including gene stacking to combine multiple R genes, multiplexed genome editing of susceptibility genes and promoter regions of executor R genes, editing cis-regulatory elements to fine-tune gene expression, RNA interference, saturation mutagenesis, and precise genomic insertions. The genetic studies and engineering of BSR are accelerating the breeding of disease-resistant cultivars, contributing to crop improvement and enhancing global food security.

RBB1 negatively regulates rice disease resistance by modulating protein glycosylation

Glycosylation, a prevalent post-translational modification in eukaryotic secreted and membrane-associated proteins, plays a pivotal role in diverse physiological and pathological processes. Although UDP-N-acetylglucosamine (UDP-GlcNAc) is essential for this modification, the specific glycosylation mechanisms during plant leaf senescence and defense responses remain poorly understood. In our research, we identified a novel rice mutant named rbb1 (resistance to blast and bacterial blight1), exhibiting broad-spectrum disease resistance. This mutant phenotype results from a loss-of-function mutation in the gene encoding glucosamine-6-phosphate acetyltransferase, an important enzyme in D-glucosamine 6-phosphate acetylation. The rbb1 mutant demonstrates enhanced defense responses, evident in increased resistance to rice blast and bacterial blight, along with the upregulation of defense-response genes. Various biochemical markers indicate an activated defense mechanism in the rbb1 mutant, such as elevated levels of reactive oxygen species and malondialdehyde, reduced enzyme activity and UDP-GlcNAc content, and decreased expression of N-glycan and O-glycan modifying proteins. Moreover, proteome analysis of N-glycosylation modifications reveals alterations in the N-glycosylation of several disease-resistance-related proteins, with a significant reduction in Prx4 and Prx13 in rbb1-1. Additionally, the knockout of Prx4 or Prx13 also enhances resistance to Xanthomonas oryzae pv. oryzae (Xoo) and Magnaporthe oryzae (M. oryzae). This study uncovers a novel mechanism of defense response in rice, suggesting potential targets for the development of disease-resistant varieties.

All Roads Lead to Rome: Pathways to Engineering Disease Resistance in Plants

Unlike animals, plants are unable to move and lack specialized immune cells and circulating antibodies. As a result, they are always threatened by a large number of microbial pathogens and harmful pests that can significantly reduce crop yield worldwide. Therefore, the development of new strategies to control them is essential to mitigate the increasing risk of crops lost to plant diseases. Recent developments in genetic engineering, including efficient gene manipulation and transformation methods, gene editing and synthetic biology, coupled with the understanding of microbial pathogenicity and plant immunity, both at molecular and genomic levels, have enhanced the capabilities to develop disease resistance in plants. This review comprehensively explains the fundamental mechanisms underlying the tug-of-war between pathogens and hosts, and provides a detailed overview of different strategies for developing disease resistance in plants. Additionally, it provides a summary of the potential genes that can be employed in resistance breeding for key crops to combat a wide range of potential pathogens and pests, including fungi, oomycetes, bacteria, viruses, nematodes, and insects. Furthermore, this review addresses the limitations associated with these strategies and their possible solutions. Finally, it discusses the future perspectives for producing plants with durable and broad-spectrum disease resistance.

Ubiquitination of OsCSN5 by OsPUB45 activates immunity by modulating the OsCUL3a-OsNPR1 module

The COP9 signalosome (CSN) is a highly conserved protein complex in eukaryotes, with CSN5 serving as its critical catalytic subunit. However, the role of CSN5 in plant immunity is largely unexplored. Here, we found that suppression of OsCSN5 in rice enhances resistance against the fungal pathogen Magnaporthe oryzae and the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) without affecting growth. OsCSN5 is ubiquitinated and degraded by the E3 ligase OsPUB45. Overexpression of OsPUB45 increased resistance against M. oryzae and Xoo, while dysfunction of OsPUB45 decreased resistance. In addition, OsCSN5 stabilized OsCUL3a to promote the degradation of a positive regulator OsNPR1. Overexpression of OsPUB45 compromised accumulation of OsCUL3a, leading to stabilization of OsNPR1, whereas mutations in OsPUB45 destabilized OsNPR1. These findings suggest that OsCSN5 stabilizes OsCUL3a to facilitate the degradation of OsNPR1, preventing its constitutive activation without infection. Conversely, OsPUB45 promotes the degradation of OsCSN5, contributing to immunity activation upon pathogen infection.

Phosphorylation of the transcription factor OsNAC29 by OsMAPK3 activates diterpenoid genes to promote rice immunity

Well-conserved mitogen-activated protein kinase (MAPK) cascades are essential for orchestrating of a wide range of cellular processes in plants, including defense responses against pathogen attack. NAC transcription factors (TFs) play important roles in plant immunity, but their targets and how they are regulated remain largely unknown. Here, we identified the TF OsNAC29 as a key component of a MAPK signaling pathway involved in rice (Oryza sativa) disease resistance. OsNAC29 binds directly to CACGTG motifs in the promoters of OsTPS28 and OsCYP71Z2, which are crucial for the biosynthesis of the phytoalexin 5,10-diketo-casbene and consequently rice blast resistance. OsNAC29 positively regulates rice blast resistance by promoting the expression of of OsTPS28 and OsCYP71Z2, and the function of OsNAC29 is genetically dependent on OsCYP71Z2 and OsTPS28. Furthermore, OsNAC29 interacts with OsRACK1A and OsMAPK3/6 to form an immune complex; OsMAPK3 phosphorylates OsNAC29 at Thr304 to prevent its proteasome-mediated degradation and promote its function against rice blast fungus. Phosphorylation of OsNAC29 at Thr304 is induced upon Magnaporthe oryzae infection and chitin treatment. Our data demonstrate the positive role of the OsMAPK3-OsNAC29-OsTPS28/OsCYP71Z2 module in rice blast resistance, providing insights into the molecular regulatory network and fine-tuning of NAC TFs in rice immunity.

Rice E3 ubiquitin ligases: From key modulators of host immunity to potential breeding applications

To combat pathogen attacks, plants have developed a highly advanced immune system, which requires tight regulation to initiate robust defense responses while simultaneously preventing autoimmunity. The ubiquitin-proteasome system (UPS), which is responsible for degrading excess or misfolded proteins, has vital roles in ensuring strong and effective immune responses. E3 ligases, as key UPS components, play extensively documented roles in rice immunity by modulating the ubiquitination and degradation of downstream substrates involved in various immune signaling pathways. Here, we summarize the crucial roles of rice E3 ligases in both pathogen/microbe/damage-associated molecular pattern-triggered immunity and effector-triggered immunity, highlight the molecular mechanisms by which E3 ligases function in rice immune signaling, and emphasize the functions of E3 ligases as targets of pathogen effectors for pathogenesis. We also discuss potential strategies for application of immunity-associated E3 ligases in breeding of disease-resistant rice varieties without growth penalty. This review provides a comprehensive and updated understanding of the sophisticated and interconnected regulatory functions of E3 ligases in rice immunity and in balancing immunity with growth and development.

Trans-crop applications of atypical R genes for multipathogen resistance

Genetic resistance to plant diseases is essential for global food security. Significant progress has been achieved for plant disease-resistance (R) genes comprising nucleotide-binding domain, leucine-rich repeat-containing receptors (NLRs), and membrane-localized receptor-like kinases or proteins (RLKs/RLPs), which we refer to as typical R genes. However, there is a knowledge gap in how non-receptor-type or atypical R genes contribute to plant immunity. Here, we summarize resources and technologies facilitating the study of atypical R genes, examine diverse atypical R proteins for broad-spectrum resistance, and outline potential approaches for trans-crop applications of atypical R genes. Studies of atypical R genes are important for a holistic understanding of plant immunity and the development of novel strategies in disease control and crop improvement.

Arabidopsis class A S-acyl transferases modify the pollen receptors LIP1 and PRK1 to regulate pollen tube guidance

Protein S-acylation catalyzed by protein S-acyl transferases (PATs) is a reversible lipid modification regulating protein targeting, stability, and interaction profiles. PATs are encoded by large gene families in plants, and many proteins including receptor-like cytoplasmic kinases (RLCKs) and receptor-like kinases (RLKs) are subject to S-acylation. However, few PATs have been assigned substrates, and few S-acylated proteins have known upstream enzymes. We report that Arabidopsis (Arabidopsis thaliana) class A PATs redundantly mediate pollen tube guidance and participate in the S-acylation of POLLEN RECEPTOR KINASE1 (PRK1) and LOST IN POLLEN TUBE GUIDANCE1 (LIP1), a critical RLK or RLCK for pollen tube guidance, respectively. PAT1, PAT2, PAT3, PAT4, and PAT8, collectively named PENTAPAT for simplicity, are enriched in pollen and show similar subcellular distribution. Functional loss of PENTAPAT reduces seed set due to male gametophytic defects. Specifically, pentapat pollen tubes are compromised in directional growth. We determine that PRK1 and LIP1 interact with PENTAPAT, and their S-acylation is reduced in pentapat pollen. The plasma membrane (PM) association of LIP1 is reduced in pentapat pollen, whereas point mutations reducing PRK1 S-acylation affect its affinity with its interacting proteins. Our results suggest a key role of S-acylation in pollen tube guidance through modulating PM receptor complexes.

A NAC transcription factor MNAC3-centered regulatory network negatively modulates rice immunity against blast disease

NAC transcription factors (TFs) are pivotal in plant immunity against diverse pathogens. Here, we report the functional and regulatory network of MNAC3, a novel NAC TF, in rice immunity. MNAC3, a transcriptional activator, negatively modulates rice immunity against blast and bacterial leaf blight diseases and pathogen-associated molecular pattern (PAMP)-triggered immune responses. MNAC3 binds to a CACG cis-element and activates the transcription of immune-negative target genes OsINO80, OsJAZ10, and OsJAZ11. The negative function of MNAC3 in rice immunity depends on its transcription of downstream genes such as OsINO80 and OsJAZ10. MNAC3 interacts with immunity-related OsPP2C41 (a protein phosphatase), ONAC066 (a NAC TF), and OsDjA6 (a DnaJ chaperone). ONAC066 and OsPP2C41 attenuate MNAC3 transcriptional activity, while OsDjA6 promotes it. Phosphorylation of MNAC3 at S163 is critical for its negative functions in rice immunity. OsPP2C41, which plays positive roles in rice blast resistance and chitin-triggered immune responses, dephosphorylates MNAC3, suppressing its transcriptional activity on the target genes OsINO80, OsJAZ10, and OsJAZ11 and promoting the translocation of MNAC3 from nucleus to cytoplasm. These results establish a MNAC3-centered regulatory network in which OsPP2C41 dephosphorylates MNAC3, attenuating its transcriptional activity on downstream immune-negative target genes in rice. Together, these findings deepen our understanding of molecular mechanisms in rice immunity and offer a novel strategy for genetic improvement of rice disease resistance.

From trade-off to synergy: microbial insights into enhancing plant growth and immunity

The reduction in crop yield caused by pathogens and pests presents a significant challenge to global food security. Genetic engineering, which aims to bolster plant defence mechanisms, emerges as a cost-effective solution for disease control. However, this approach often incurs a growth penalty, known as the growth-defence trade-off. The precise molecular mechanisms governing this phenomenon are still not completely understood, but they generally fall under two main hypotheses: a "passive" redistribution of metabolic resources, or an "active" regulatory choice to optimize plant fitness. Despite the knowledge gaps, considerable practical endeavours are in the process of disentangling growth from defence. The plant microbiome, encompassing both above- and below-ground components, plays a pivotal role in fostering plant growth and resilience to stresses. There is increasing evidence which indicates that plants maintain intimate associations with diverse, specifically selected microbial communities. Meta-analyses have unveiled well-coordinated, two-way communications between plant shoots and roots, showcasing the capacity of plants to actively manage their microbiota for balancing growth with immunity, especially in response to pathogen incursions. This review centers on successes in making use of specific root-associated microbes to mitigate the growth-defence trade-off, emphasizing pivotal advancements in unravelling the mechanisms behind plant growth and defence. These findings illuminate promising avenues for future research and practical applications.

The RING-finger ubiquitin E3 ligase TaPIR1 targets TaHRP1 for degradation to suppress chloroplast function

Chloroplasts are key players in photosynthesis and immunity against microbial pathogens. However, the precise and timely regulatory mechanisms governing the control of photosynthesis-associated nuclear genes (PhANGs) expression in plant immunity remain largely unknown. Here we report that TaPIR1, a Pst-induced RING-finger E3 ubiquitin ligase, negatively regulates Pst resistance by specifically interacting with TaHRP1, an atypical transcription factor histidine-rich protein. TaPIR1 ubiquitinates the lysine residues K131 and K136 in TaHRP1 to regulate its stability. TaHRP1 directly binds to the TaHRP1-binding site elements within the PhANGs promoter to activate their transcription via the histidine-rich domain of TaHRP1. PhANGs expression induces the production of chloroplast-derived ROS. Although knocking out TaHRP1 reduces Pst resistance, TaHRP1 overexpression contributes to photosynthesis, and chloroplast-derived ROS production, and improves disease resistance. TaPIR1 expression inhibits the downstream activation of TaHRP1 and TaHRP1-induced ROS accumulation in chloroplasts. Overall, we show that the TaPIR1-mediated ubiquitination and degradation of TaHRP1 alters PhANGs expression to disrupt chloroplast function, thereby increasing plant susceptibility to Pst.

Improvement of Flowering Stage in Japonica Rice Variety Jiahe212 by Using CRISPR/Cas9 System

The flowering period of rice significantly impacts variety adaptability and yield formation. Properly shortening the reproductive period of rice varieties can expand their ecological range without significant yield reduction. Targeted genome editing, like CRISPR/Cas9, is an ideal tool to fine-tune rice growth stages and boost yield synergistically. In this study, we developed a CRISPR/Cas9-mediated multiplex genome-editing vector containing five genes related to three traits, Hd2, Ghd7, and DTH8 (flowering-stage genes), along with the recessive rice blast resistance gene Pi21 and the aromatic gene BADH2. This vector was introduced into the high-quality japonica rice variety in Zhejiang province, Jiahe212 (JH212), resulting in 34 T0 plants with various effective mutations. Among the 17 mutant T1 lines, several displayed diverse flowering dates, but most exhibited undesirable agronomic traits. Notably, three homozygous mutant lines (JH-C15, JH-C18, and JH-C31) showed slightly earlier flowering dates without significant differences in yield-related traits compared to JH212. Through special Hyg and Cas marker selection of T2 plants, we identified seven, six, and two fragrant glutinous plants devoid of transgenic components. These single plants will serve as sib lines of JH212 and potential resources for breeding applications, including maintenance lines for indica-japonica interspecific three-line hybrid rice. In summary, our research lays the foundation for the creation of short-growth-period CMS (cytoplasmic male sterility, CMS) lines, and also provides materials and a theoretical basis for indica-japonica interspecific hybrid rice breeding with wider adaptability.

OsATL32 ubiquitinates the reactive oxygen species-producing OsRac5-OsRbohB module to suppress rice immunity

Ubiquitination-mediated protein degradation is integral to plant immunity, with E3 ubiquitin ligases acting as key factors in this process. Here, we report the functions of OsATL32, a plasma membrane-localized Arabidopsis Tóxicos En Levadura (ATL)-type E3 ubiquitin ligase, in rice (Oryza sativa) immunity and its associated regulatory network. We found that the expression of OsATL32 is downregulated in both compatible and incompatible interactions between rice and the rice blast fungus Magnaporthe oryzae. The OsATL32 protein level declines in response to infection by a compatible M. oryzae strain or to chitin treatment. OsATL32 negatively regulates rice resistance to blast and bacterial leaf blight diseases, as well as chitin-triggered immunity. Biochemical and genetic studies revealed that OsATL32 suppresses pathogen-induced reactive oxygen species (ROS) accumulation by mediating ubiquitination and degradation of the ROS-producing OsRac5-OsRbohB module, which enhances rice immunity against M. oryzae. The protein phosphatase PHOSPHATASE AND TENSIN HOMOLOG enhances rice blast resistance by dephosphorylating OsATL32 and promoting its degradation, preventing its negative effect on rice immunity. This study provides insights into the molecular mechanism by which the E3 ligase OsATL32 targets a ROS-producing module to undermine rice immunity.

Integration of transcriptomics, metabolomics, and hormone analysis revealed the formation of lesion spots inhibited by GA and CTK was related to cell death and disease resistance in bread wheat (Triticum aestivum L.)

BACKGROUND

Wheat is one of the important grain crops in the world. The formation of lesion spots related to cell death is involved in disease resistance, whereas the regulatory pathway of lesion spot production and resistance mechanism to pathogens in wheat is largely unknown.

RESULTS

In this study, a pair of NILs (NIL-Lm5W and NIL-Lm5M) was constructed from the BC1F4 population by the wheat lesion mimic mutant MC21 and its wild genotype Chuannong 16. The formation of lesion spots in NIL-Lm5M significantly increased its resistance to stripe rust, and NIL-Lm5M showed superiour agronomic traits than NIL-Lm5W under stripe rust infection.Whereafter, the NILs were subjected to transcriptomic (stage N: no spots; stage S, only a few spots; and stage M, numerous spots), metabolomic (stage N and S), and hormone analysis (stage S), with samples taken from normal plants in the field. Transcriptomic analysis showed that the differentially expressed genes were enriched in plant-pathogen interaction, and defense-related genes were significantly upregulated following the formation of lesion spots. Metabolomic analysis showed that the differentially accumulated metabolites were enriched in energy metabolism, including amino acid metabolism, carbohydrate metabolism, and lipid metabolism. Correlation network diagrams of transcriptomic and metabolomic showed that they were both enriched in energy metabolism. Additionally, the contents of gibberellin A7, cis-Zeatin, and abscisic acid were decreased in leaves upon lesion spot formation, whereas the lesion spots in NIL-Lm5M leaves were restrained by spaying GA and cytokinin (CTK, trans-zeatin) in the field.

CONCLUSION

The formation of lesion spots can result in cell death and enhance strip rust resistance by protein degradation pathway and defense-related genes overexpression in wheat. Besides, the formation of lesion spots was significantly affected by GA and CTK. Altogether, these results may contribute to the understanding of lesion spot formation in wheat and laid a foundation for regulating the resistance mechanism to stripe rust.

The potential of genome editing to create novel alleles of resistance genes in rice

Rice, a staple food for a significant portion of the global population, faces persistent threats from various pathogens and pests, necessitating the development of resilient crop varieties. Deployment of resistance genes in rice is the best practice to manage diseases and reduce environmental damage by reducing the application of agro-chemicals. Genome editing technologies, such as CRISPR-Cas, have revolutionized the field of molecular biology, offering precise and efficient tools for targeted modifications within the rice genome. This study delves into the application of these tools to engineer novel alleles of resistance genes in rice, aiming to enhance the plant's innate ability to combat evolving threats. By harnessing the power of genome editing, researchers can introduce tailored genetic modifications that bolster the plant's defense mechanisms without compromising its essential characteristics. In this study, we synthesize recent advancements in genome editing methodologies applicable to rice and discuss the ethical considerations and regulatory frameworks surrounding the creation of genetically modified crops. Additionally, it explores potential challenges and future prospects for deploying edited rice varieties in agricultural landscapes. In summary, this study highlights the promise of genome editing in reshaping the genetic landscape of rice to confront emerging challenges, contributing to global food security and sustainable agriculture practices.

Elevating plant immunity by translational regulation of a rice WRKY transcription factor

Plants have intricate mechanisms that tailor their defence responses to pathogens. WRKY transcription factors play a pivotal role in plant immunity by regulating various defence signalling pathways. Many WRKY genes are transcriptionally activated upon pathogen attack, but how their functions are regulated after transcription remains elusive. Here, we show that OsWRKY7 functions as a crucial positive regulator of rice basal immunity against Xanthomonas oryzae pv. oryzae (Xoo). The activity of OsWRKY7 was regulated at both translational and post-translational levels. Two translational products of OsWRKY7 were generated by alternative initiation. The full-length OsWRKY7 protein is normally degraded by the ubiquitin-proteasome system but was accumulated following elicitor or pathogen treatment, whereas the alternate product initiated from the downstream in-frame start codon was stable. Both the full and alternate OsWRKY7 proteins have transcriptional activities in yeast and rice cells, and overexpression of each form enhanced resistance to Xoo infection. Furthermore, disruption of the main AUG in rice increased the endogenous translation of the alternate stabilized form of OsWRKY7 and enhanced bacterial blight resistance. This study provides insights into the coordination of alternative translation and protein stability in the regulation of plant growth and basal defence mediated by the OsWRKY7 transcription factor, and also suggests a promising strategy to breed disease-resistant rice by translation initiation control.

The E3 ligase TaGW2 mediates transcription factor TaARR12 degradation to promote drought resistance in wheat

Drought stress limits crop yield, but the molecular modulators and their mechanisms underlying the trade-off between drought resistance and crop growth and development remain elusive. Here, a grain width and weight2 (GW2)-like really interesting new gene finger E3 ligase, TaGW2, was identified as a pivotal regulator of both kernel development and drought responses in wheat (Triticum aestivum). TaGW2 overexpression enhances drought resistance but leads to yield drag under full irrigation conditions. In contrast, TaGW2 knockdown or knockout attenuates drought resistance but remarkably increases kernel size and weight. Furthermore, TaGW2 directly interacts with and ubiquitinates the type-B Arabidopsis response regulator TaARR12, promoting its degradation via the 26S proteasome. Analysis of TaARR12 overexpression and knockdown lines indicated that TaARR12 represses the drought response but does not influence grain yield in wheat. Further DNA affinity purification sequencing combined with transcriptome analysis revealed that TaARR12 downregulates stress-responsive genes, especially group-A basic leucine zipper (bZIP) genes, resulting in impaired drought resistance. Notably, TaARR12 knockdown in the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9)-mediated tagw2 knockout mutant leads to significantly higher drought resistance and grain yield compared to wild-type plants. Collectively, these findings show that the TaGW2-TaARR12 regulatory module is essential for drought responses, providing a strategy for improving stress resistance in high-yield wheat varieties.

Association of a Specific OsCULLIN3c Haplotype with Salt Stress Responses in Local Thai Rice

We previously found that OsCUL3c is involved in the salt stress response. However, there are no definitive reports on the diversity of OsCUL3c in local Thai rice. In this study, we showed that the CUL3 group was clearly separated from the other CUL groups; next, we focused on OsCUL3c, the third CUL3 of the CUL3 family in rice, which is absent in Arabidopsis. A total of 111 SNPs and 28 indels over the OsCUL3c region, representing 79 haplotypes (haps), were found. Haplotyping revealed that group I (hap A and hap C) and group II (hap B1 and hap D) were different mutated variants, which showed their association with phenotypes under salt stress. These results were supported by cis-regulatory elements (CREs) and transcription factor binding sites (TFBSs) analyses. We found that LTR, MYC, [AP2; ERF], and NF-YB, which are related to salt stress, drought stress, and the response to abscisic acid (ABA), have distinct positions and numbers in the haplotypes of group I and group II. An RNA Seq analysis of the two predominant haplotypes from each group showed that the OsCUL3c expression of the group I representative was upregulated and that of group II was downregulated, which was confirmed by RT-qPCR. Promoter changes might affect the transcriptional responses to salt stress, leading to different regulatory mechanisms for the expression of different haplotypes. We speculate that OsCUL3c influences the regulation of salt-related responses, and haplotype variations play a role in this regulation.

Characterization and fine mapping of a maize lesion mimic mutant (Les8) with enhanced resistance to Curvularia leaf spot and southern leaf blight

KEY MESSAGE

A novel light-dependent dominant lesion mimic mutant with enhanced multiple disease resistance was physiologically, biochemically, and genetically characterized; the causal gene was fine mapped to a 909 kb interval containing 38 genes. Identification of genes that confer multiple disease resistance (MDR) is crucial for the improvement of maize disease resistance. However, very limited genes are identified as MDR genes in maize. In this study, we characterized a dominant disease lesion mimics 8 (Les8) mutant that had chlorotic lesions on the leaves and showed enhanced resistance to both curvularia leaf spot and southern leaf blight. Major agronomic traits were not obviously altered, while decreased chlorophyll content was observed in the mutant, and the genetic effect of the Les8 mutation was stable in different genetic backgrounds. By BSR-seq analysis and map-based cloning, the LES8 gene was mapped into a 909 kb region containing 38 candidate genes on chromosome 9 wherein no lesion mimic or disease-resistance genes were previously reported. Using transcriptomics analysis, we found that genes involved in defense responses and secondary metabolite biosynthesis were enriched in the significantly up-regulated genes, while genes involved in photosynthesis and carbohydrate-related pathways were enriched in the significantly down-regulated genes in Les8. In addition, there was an overaccumulation of jasmonic acid and lignin but not salicylic acid in Les8. Taken together, this study revealed candidate genes and potential mechanism underlying Les8-conferred MDR in maize.

Editing a rice CDP-DAG synthase confers broad-spectrum resistance

Lesion mimic mutations (LMMs) often confer broad-spectrum resistance (BSR) in plants, but with significant yield penalties. Sha et al. recently demonstrated that genome editing of the rice BSR gene RESISTANCE TO BLAST1 (RBL1), encoding a cytidine diphosphate diacylglycerol (CDP-DAG) synthase involved in phospholipid biosynthesis, confers multipathogen resistance without an obvious trade-off in yield.

PWL1, a G-type lectin receptor-like kinase, positively regulates leaf senescence and heat tolerance but negatively regulates resistance to Xanthomonas oryzae in rice

Plant leaf senescence, caused by multiple internal and environmental factors, has an important impact on agricultural production. The lectin receptor-like kinase (LecRLK) family members participate in plant development and responses to biotic and abiotic stresses, but their roles in regulating leaf senescence remain elusive. Here, we identify and characterize a rice premature withered leaf 1 (pwl1) mutant, which exhibits premature leaf senescence throughout the plant life cycle. The pwl1 mutant displayed withered and whitish leaf tips, decreased chlorophyll content, and accelerated chloroplast degradation. Map-based cloning revealed an amino acid substitution (Gly412Arg) in LOC_Os03g62180 (PWL1) was responsible for the phenotypes of pwl1. The expression of PWL1 was detected in all tissues, but predominantly in tillering and mature leaves. PWL1 encodes a G-type LecRLK with active kinase and autophosphorylation activities. PWL1 is localized to the plasma membrane and can self-associate, mainly mediated by the plasminogen-apple-nematode (PAN) domain. Substitution of the PAN domain significantly diminished the self-interaction of PWL1. Moreover, the pwl1 mutant showed enhanced reactive oxygen species (ROS) accumulation, cell death, and severe DNA fragmentation. RNA sequencing analysis revealed that PWL1 was involved in the regulation of multiple biological processes, like carbon metabolism, ribosome, and peroxisome pathways. Meanwhile, interfering of biological processes induced by the PWL1 mutation also enhanced heat sensitivity and resistance to bacterial blight and bacterial leaf streak with excessive accumulation of ROS and impaired chloroplast development in rice. Natural variation analysis indicated more variations in indica varieties, and the vast majority of japonica varieties harbour the PWL1Hap1 allele. Together, our results suggest that PWL1, a member of LecRLKs, exerts multiple roles in regulating plant growth and development, heat-tolerance, and resistance to bacterial pathogens.

The E3 ubiquitin ligase OsRGLG5 targeted by the Magnaporthe oryzae effector AvrPi9 confers basal resistance against rice blast

Rice blast, caused by Magnaporthe oryzae, is one of the most devastating diseases of rice. During infection, M. oryzae secretes effectors to facilitate blast development. Among these effectors, the avirulence factor AvrPi9 is recognized by Pi9, a broad-spectrum blast resistance protein that triggers Pi9-mediated resistance in rice. However, little is known about the interaction between AvrPi9 and Pi9 and how AvrPi9 exerts virulence to promote infection. In this study, we found that ectopic expression of AvrPi9 in the Pi9-lacking cultivar TP309 suppressed basal resistance against M. oryzae. Furthermore, we identified an AvrPi9-interacting protein in rice, which we named OsRGLG5, encoding a functional RING-type E3 ubiquitin ligase. During infection, AvrPi9 was ubiquitinated and degraded by OsRGLG5. Meanwhile, AvrPi9 affected the stability of OsRGLG5. Infection assays revealed that OsRGLG5 is a positive regulator of basal resistance against M. oryzae, but it is not essential for Pi9-mediated blast resistance in rice. In conclusion, our results revealed that OsRGLG5 is targeted by the M. oryzae effector AvrPi9 and positively regulates basal resistance against rice blast.

CRISPR/Cas9-mediated simultaneous mutation of three salicylic acid 5-hydroxylase (OsS5H) genes confers broad-spectrum disease resistance in rice

Salicylic acid (SA) is an essential plant hormone that plays critical roles in basal defence and amplification of local immune responses and establishes resistance against various pathogens. However, the comprehensive knowledge of the salicylic acid 5-hydroxylase (S5H) in rice-pathogen interaction is still elusive. Here, we reported that three OsS5H homologues displayed salicylic acid 5-hydroxylase activity, converting SA into 2,5-dihydroxybenzoic acid (2,5-DHBA). OsS5H1, OsS5H2, and OsS5H3 were preferentially expressed in rice leaves at heading stage and responded quickly to exogenous SA treatment. We found that bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) strongly induced the expression of OsS5H1, OsS5H2, and OsS5H3. Rice plants overexpressing OsS5H1, OsS5H2, and OsS5H3 showed significantly decreased SA contents and increased 2,5-DHBA levels, and were more susceptible to bacterial blight and rice blast. A simple single guide RNA (sgRNA) was designed to create oss5h1oss5h2oss5h3 triple mutants through CRISPR/Cas9-mediated gene mutagenesis. The oss5h1oss5h2oss5h3 exhibited stronger resistance to Xoo than single oss5h mutants. And oss5h1oss5h2oss5h3 plants displayed enhanced rice blast resistance. The conferred pathogen resistance in oss5h1oss5h2oss5h3 was attributed to the significantly upregulation of OsWRKY45 and pathogenesis-related (PR) genes. Besides, flg22-induced reactive oxygen species (ROS) burst was enhanced in oss5h1oss5h2oss5h3. Collectively, our study provides a fast and effective approach to generate rice varieties with broad-spectrum disease resistance through OsS5H gene editing.

RHO OF PLANT proteins are essential for pollen germination in Arabidopsis

Pollen germination is a process of polarity establishment, through which a single and unique growth axis is established. Although most of the intracellular activities associated with pollen germination are controlled by RHO OF PLANTs (ROPs) and increased ROP activation accompanies pollen germination, a critical role of ROPs in this process has not yet been demonstrated. Here, by genomic editing of all 4 Arabidopsis (Arabidopsis thaliana) ROPs that are preferentially expressed in pollen, we showed that ROPs are essential for polarity establishment during pollen germination. We further identified and characterized 2 ROP effectors in pollen germination (REGs) through genome-wide interactor screening, boundary of ROP domain (BDR) members BDR8 and BDR9, whose functional loss also resulted in no pollen germination. BDR8 and BDR9 were distributed in the cytosol and the vegetative nucleus of mature pollen grains but redistributed to the plasma membrane (PM) of the germination site and to the apical PM of growing pollen tubes. We demonstrated that the PM redistribution of BDR8 and BDR9 during pollen germination relies on ROPs but not vice versa. Furthermore, enhanced expression of BDR8 partially restored germination of rop1 pollen but had no effects on that of the quadruple rop pollen, supporting their genetic epistasis. Results presented here demonstrate an ROP signaling route essential for pollen germination, which supports evolutionarily conserved roles of Rho GTPases in polarity establishment.

OsMKK6 Regulates Disease Resistance in Rice

Mitogen-activated protein kinase cascades play important roles in various biological programs in plants, including immune responses, but the underlying mechanisms remain elusive. Here, we identified the lesion mimic mutant rsr25 (rust spots rice 25) and determined that the mutant harbored a loss-of-function allele for OsMKK6 (MITOGEN-ACTIVATED KINASE KINASE 6). rsr25 developed reddish-brown spots on its leaves at the heading stage, as well as on husks. Compared to the wild type, the rsr25 mutant exhibited enhanced resistance to the fungal pathogen Magnaporthe oryzae (M. oryzae) and to the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). OsMKK6 interacted with OsMPK4 (MITOGEN-ACTIVATED KINASE 4) in vivo, and OsMKK6 phosphorylated OsMPK4 in vitro. The Osmpk4 mutant is also a lesion mimic mutant, with reddish-brown spots on its leaves and husks. Pathogen-related genes were significantly upregulated in Osmpk4, and this mutant exhibited enhanced resistance to M. oryzae compared to the wild type. Our results indicate that OsMKK6 and OsMPK4 form a cascade that regulates immune responses in rice.

Genome editing of a rice CDP-DAG synthase confers multipathogen resistance

The discovery and application of genome editing introduced a new era of plant breeding by giving researchers efficient tools for the precise engineering of crop genomes1. Here we demonstrate the power of genome editing for engineering broad-spectrum disease resistance in rice (Oryza sativa). We first isolated a lesion mimic mutant (LMM) from a mutagenized rice population. We then demonstrated that a 29-base-pair deletion in a gene we named RESISTANCE TO BLAST1 (RBL1) caused broad-spectrum disease resistance and showed that this mutation caused an approximately 20-fold reduction in yield. RBL1 encodes a cytidine diphosphate diacylglycerol synthase that is required for phospholipid biosynthesis2. Mutation of RBL1 results in reduced levels of phosphatidylinositol and its derivative phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). In rice, PtdIns(4,5)P2 is enriched in cellular structures that are specifically associated with effector secretion and fungal infection, suggesting that it has a role as a disease-susceptibility factor3. By using targeted genome editing, we obtained an allele of RBL1, named RBL1Δ12, which confers broad-spectrum disease resistance but does not decrease yield in a model rice variety, as assessed in small-scale field trials. Our study has demonstrated the benefits of editing an LMM gene, a strategy relevant to diverse LMM genes and crops.

Different viral effectors suppress hormone-mediated antiviral immunity of rice coordinated by OsNPR1

Salicylic acid (SA) and jasmonic acid (JA) are plant hormones that typically act antagonistically in dicotyledonous plants and SA and JA signaling is often manipulated by pathogens. However, in monocotyledonous plants, the detailed SA-JA interplay in response to pathogen invasion remains elusive. Here, we show that different types of viral pathogen can disrupt synergistic antiviral immunity mediated by SA and JA via OsNPR1 in the monocot rice. The P2 protein of rice stripe virus, a negative-stranded RNA virus in the genus Tenuivirus, promotes OsNPR1 degradation by enhancing the association of OsNPR1 and OsCUL3a. OsNPR1 activates JA signaling by disrupting the OsJAZ-OsMYC complex and boosting the transcriptional activation activity of OsMYC2 to cooperatively modulate rice antiviral immunity. Unrelated viral proteins from different rice viruses also interfere with the OsNPR1-mediated SA-JA interplay to facilitate viral pathogenicity, suggesting that this may be a more general strategy in monocot plants. Overall, our findings highlight that distinct viral proteins convergently obstruct JA-SA crosstalk to facilitate viral infection in monocot rice.

Rice ubiquitin-conjugating enzyme OsUbc13 negatively regulates immunity against pathogens by enhancing the activity of OsSnRK1a

Ubc13 is required for Lys63-linked polyubiquitination and innate immune responses in mammals, but its functions in plant immunity still remain largely unknown. Here, we used molecular biological, pathological, biochemical, and genetic approaches to evaluate the roles of rice OsUbc13 in response to pathogens. The OsUbc13-RNA interference (RNAi) lines with lesion mimic phenotypes displayed a significant increase in the accumulation of flg22- and chitin-induced reactive oxygen species, and in defence-related genes expression or hormones as well as resistance to Magnaporthe oryzae and Xanthomonas oryzae pv oryzae. Strikingly, OsUbc13 directly interacts with OsSnRK1a, which is the α catalytic subunit of SnRK1 (sucrose non-fermenting-1-related protein kinase-1) and acts as a positive regulator of broad-spectrum disease resistance in rice. In the OsUbc13-RNAi plants, although the protein level of OsSnRK1a did not change, its activity and ABA sensitivity were obviously enhanced, and the K63-linked polyubiquitination was weaker than that of wild-type Dongjin (DJ). Overexpression of the deubiquitinase-encoding gene OsOTUB1.1 produced similar effects with inhibition of OsUbc13 in affecting immunity responses, M. oryzae resistance, OsSnRK1a ubiquitination, and OsSnRK1a activity. Furthermore, re-interfering with OsSnRK1a in one OsUbc13-RNAi line (Ri-3) partially restored its M. oryzae resistance to a level between those of Ri-3 and DJ. Our data demonstrate OsUbc13 negatively regulates immunity against pathogens by enhancing the activity of OsSnRK1a.

An OsPRMT5-OsAGO2/miR1875-OsHXK1 module regulates rice immunity to blast disease

Rice ARGONAUTE2 (OsAGO2) is a core component of the rice RNA-induced silencing complex (RISC), which is repressed by Magnaporthe oryzae (M. oryzae) infection. Whether and how OsAGO2-mediated gene silencing plays a role in rice blast resistance and which sRNAs participate in this process are unknown. Our results indicate that OsAGO2 is a key immune player that manipulates rice defense responses against blast disease. OsAGO2 associates with the 24-nt miR1875 and binds to the promoter region of HEXOKINASE1 (OsHXK1), which causes DNA methylation and leads to gene silencing. Our multiple genetic evidence showed that, without M. oryzae infection, OsAGO2/miR1875 RISC promoted OsHXK1 promoter DNA methylation and OsHXK1 silencing; after M. oryzae infection, the reduced OsAGO2/miR1875 led to a relatively activated OsHXK1 expression. OsHXK1 acts as a positive regulator of blast disease resistance that OsHXK1-OE rice exhibited enhanced resistance, whereas Cas9-Oshxk1 rice showed reduced resistance against M. oryzae infection. OsHXK1 may function through its sugar sensor activity as glucose induced defense-related gene expression and reactive oxygen species (ROS) accumulation in Nipponbare and OsHXK1-OE but not in Cas9-Oshxk1 rice. OsAGO2 itself is delicately regulated by OsPRMT5, which senses M. oryzae infection and attenuates OsAGO2-mediated gene silencing through OsAGO2 arginine methylation. Our study reveals an OsPRMT5-OsAGO2/miR1875-OsHXK1 regulatory module that fine tunes the rice defense response to blast disease.

A ubiquitin-specific protease functions in regulating cell death and immune responses in rice

Ubiquitin-specific proteases (UBPs) process deubiquitination in eukaryotic organisms and are widely involved in plant development and responses to environmental stress. However, their role in cell death and plant immunity remains largely unknown. Here, we identified a rice lesion mimic mutant (LMM) and cloned its causative gene, LMM22. Both dysfunction and overexpression of LMM22 gave rise to the hypersensitive response-like cell death, reactive oxygen species bursts, and activated defence responses. LMM22 encodes an active UBP that is localised to the endoplasmic reticulum (ER) and displays a constitutive expression pattern in rice. LMM22 interacts with SPOTTED LEAF 35 (SPL35), a coupling of ubiquitin conjugation to ER degradation domain-containing protein that is known to participate in ubiquitination and the regulation of cell death and disease response in rice. Additional analyses suggest that LMM22 can positively regulate and stabilise the abundance of SPL35 protein likely through its deubiquitination activity. These data therefore improve our understanding of the function of UBP in rice innate immune responses by demonstrating that LMM22 functions as a critical regulator of SPL35 in cell death and disease resistance.

Emerging Roles of Salicylic Acid in Plant Saline Stress Tolerance

One of the most important phytohormones is salicylic acid (SA), which is essential for the regulation of plant growth, development, ripening, and defense responses. The role of SA in plant-pathogen interactions has attracted a lot of attention. Aside from defense responses, SA is also important in responding to abiotic stimuli. It has been proposed to have great potential for improving the stress resistance of major agricultural crops. On the other hand, SA utilization is dependent on the dosage of the applied SA, the technique of application, and the status of the plants (e.g., developmental stage and acclimation). Here, we reviewed the impact of SA on saline stress responses and the associated molecular pathways, as well as recent studies toward understanding the hubs and crosstalk between SA-induced tolerances to biotic and saline stress. We propose that elucidating the mechanism of the SA-specific response to various stresses, as well as SA-induced rhizosphere-specific microbiome modeling, may provide more insights and support in coping with plant saline stress.

Jasmonic Acid-Induced β-Cyclocitral Confers Resistance to Bacterial Blight and Negatively Affects Abscisic Acid Biosynthesis in Rice

Jasmonic acid (JA) regulates the production of several plant volatiles that are involved in plant defense mechanisms. In this study, we report that the JA-responsive volatile apocarotenoid, β-cyclocitral (β-cyc), negatively affects abscisic acid (ABA) biosynthesis and induces a defense response against Xanthomonas oryzae pv. oryzae (Xoo), which causes bacterial blight in rice (Oryza sativa L.). JA-induced accumulation of β-cyc was regulated by OsJAZ8, a repressor of JA signaling in rice. Treatment with β-cyc induced resistance against Xoo and upregulated the expression of defense-related genes in rice. Conversely, the expression of ABA-responsive genes, including ABA-biosynthesis genes, was downregulated by JA and β-cyc treatment, resulting in a decrease in ABA levels in rice. β-cyc did not inhibit the ABA-dependent interactions between OsPYL/RCAR5 and OsPP2C49 in yeast cells. Furthermore, we revealed that JA-responsive rice carotenoid cleavage dioxygenase 4b (OsCCD4b) was localized in the chloroplast and produced β-cyc both in vitro and in planta. These results suggest that β-cyc plays an important role in the JA-mediated resistance against Xoo in rice.

Engineering plant immune circuit: walking to the bright future with a novel toolbox

Plant pathogens destroy crops and cause severe yield losses, leading to an insufficient food supply to sustain the human population. Apart from relying on natural plant immune systems to combat biological agents or waiting for the appropriate evolutionary steps to occur over time, researchers are currently seeking new breakthrough methods to boost disease resistance in plants through genetic engineering. Here, we summarize the past two decades of research in disease resistance engineering against an assortment of pathogens through modifying the plant immune components (internal and external) with several biotechnological techniques. We also discuss potential strategies and provide perspectives on engineering plant immune systems for enhanced pathogen resistance and plant fitness.

The Gain-of-Function Mutation, OsSpl26, Positively Regulates Plant Immunity in Rice

Rice spotted-leaf mutants are ideal materials to study the molecular mechanism underlying programmed cell death and disease resistance in plants. LOC_Os07g04820 has previously been identified as the candidate gene responsible for the spotted-leaf phenotype in rice Spotted-leaf 26 (Spl26) mutant. Here, we cloned and validated that LOC_Os07g04820 is the locus controlling the spotted-leaf phenotype of Spl26 by reverse functional complementation and CRISPR/Cas9-mediated knockout of the mutant allele. The recessive wild-type spl26 allele (Oryza sativa spotted-leaf 26, Osspl26) is highly conservative in grass species and encodes a putative G-type lectin S-receptor-like serine/threonine protein kinase with 444 amino acid residuals. OsSPL26 localizes to the plasma membrane and can be detected constitutively in roots, stems, leaves, sheaths and panicles. The single base substitution from T to A at position 293 leads to phenylalanine/tyrosine replacement at position 98 in the encoded protein in the mutant and induces excessive accumulation of H₂O₂, leading to oxidative damage to cells, and finally, formation of the spotted-leaf phenotype in Spl26. The formation of lesions not only affects the growth and development of the plants but also activates the defense response and enhances the resistance to the bacterial blight pathogen, Xanthomonas oryzae pv. oryzae. Our results indicate that the gain-of-function by the mutant allele OsSpl26 positively regulates cell death and immunity in rice.

Knockdown of NtCPS2 promotes plant growth and reduces drought tolerance in Nicotiana tabacum

Drought stress is one of the primary environmental stress factors that gravely threaten crop growth, development, and yields. After drought stress, plants can regulate the content and proportion of various hormones to adjust their growth and development, and in some cases to minimize the adverse effects of drought stress. In our previous study, the tobacco cis-abienol synthesis gene (NtCPS2) was found to affect hormone synthesis in tobacco plants. Unfortunately, the role of NtCPS2 genes in the response to abiotic stress has not yet been investigated. Here, we present data supporting the role of NtCPS2 genes in drought stress and the possible underlying molecular mechanisms. NtCPS2 gene expression was induced by polyethylene glycol, high-temperature, and virus treatments. The results of subcellular localization showed that NtCPS2 was localized in the cell membrane. The NtCPS2-knockdown plants exhibited higher levels of gibberellin (GA) content and synthesis pathway genes expression but lower abscisic acid (ABA) content and synthesis pathway genes expression in response to drought stress. In addition, the transgenic tobacco lines showed higher leaf water loss and electrolyte loss, lower soluble protein and reactive oxygen species content (ROS), and lower antioxidant enzyme activity after drought treatment compared to wild type plants (WT). In summary, NtCPS2 positively regulates drought stress tolerance possibly by modulating the ratio of GA to ABA, which was confirmed by evidence of related phenotypic and physiological indicators. This study may provide evidence for the feedback regulation of hormone to abiotic and biotic stresses.

OsDWD1 E3 ligase-mediated OsNPR1 degradation suppresses basal defense in rice

Many ubiquitin E3 ligases function in plant immunity. Here, we show that Oryza sativa (rice) DDB1 binding WD (OsDWD1) suppresses immune responses by targeting O. sativa non-expresser of pathogenesis-related gene 1 (OsNPR1) for degradation. Knock-down and overexpression experiments in rice plants showed that OsDWD1 is a negative regulator of the immune response and that OsNPR1 is a substrate of OsDWD1 and a substrate receptor of OsCRL4. After constructing the loss-of-function mutant OsDWD1_R239A , we showed that the downregulation of OsNPR1 seen in rice lines overexpressing wild-type (WT) OsDWD1 (OsDWD1_WT -ox) was compromised in OsDWD1_R239A -ox lines, and that OsNPR1 upregulation enhanced resistance to pathogen infection, confirming that OsCRL4^OsDWD1 regulates OsNPR1 protein levels. The enhanced disease resistance seen in OsDWD1 knock-down (OsDWD1-kd) lines contrasted with the reduced disease resistance in double knock-down (OsDWD1/OsNPR1-kd) lines, indicating that the enhanced disease resistance of OsDWD1-kd resulted from the accumulation of OsNPR1. Moreover, an in vivo heterologous protein degradation assay in Arabidopsis thaliana ddb1 mutants confirmed that the CUL4-based E3 ligase system can also influence OsNPR1 protein levels in Arabidopsis. Although OsNPR1 was degraded by the OsCRL4^OsDWD1 -mediated ubiquitination system, the phosphodegron-motif-mutated NPR1 was partially degraded in the DWD1-ox protoplasts. This suggests that there might be another degradation process for OsNPR1. Taken together, these results indicate that OsDWD1 regulates OsNPR1 protein levels in rice to suppress the untimely activation of immune responses.

Mining of the CULLIN E3 ubiquitin ligase genes in the whole genome of Salvia miltiorrhiza

CULLIN (CUL) proteins are E3 ubiquitin ligases that are involved in a wide variety of biological processes as well as in response to stress in plants. In Salvia miltiorrhiza, CUL genes have not been characterized and its role in plant development, stress response and secondary metabolite synthesis have not been studied. In this study, genome-wide analyses were performed to identify and to predict the structure and function of CUL of S. miltiorrhiza. Eight CUL genes were identified from the genome of S. miltiorrhiza. The CUL genes were clustered into four subgroups according to phylogenetic relationships. The CUL domain was highly conserved across the family of CUL genes. Analysis of cis-acting elements suggested that CUL genes might play important roles in a variety of biological processes, including abscission reaction acid (ABA) processing. To investigate this hypothesis, we treated hairy roots of S. miltiorrhiza with ABA. The expression of CUL genes varied obviously after ABA treatment. Co-expression network results indicated that three CUL genes might be involved in the biosynthesis of phenolic acid or tanshinone. In summary, the mining of the CUL genes in the whole genome of S. miltiorrhiza contribute novel information to the understanding of the CUL genes and its functional roles in plant secondary metabolites, growth and development.

ATP-citrate lyase B (ACLB) negatively affects cell death and resistance to Verticillium wilt

BACKGROUND

ATP-citrate lyase (ACL) plays a pivotal role in histone acetylation and aerobic glycolysis. In plant, ACL is a heteromeric enzyme composed of ACLA (45 kD) and ACLB (65 kD). So far, the function of ACL genes in cotton still remains unknown.

RESULTS

Here, we identified three ACLA homologous sequences and two ACLB homologous in each genome/sub-genome of cotton species. Silencing ACLB in cotton led to cell death at newly-grown leaves and stem apexes. Simultaneously, in ACLB-silenced plants, transcription factors related to senescence including SGR, WRKY23 and Osl57 were observed to be activated. Further investigation showed that excessive H₂O₂ was accumulated, salicylic acid-dependent defense response and pathogenesis-related gene expressions were evidently enhanced in ACLB-silenced plants, implying that knockdown of ACLB genes leads to hypersensitive response-like cell death in cotton seedlings. However, as noted, serious cell death happened in newly-grown leaves and stem apexes in ACLB-silenced plants, which led to the failure of subsequent fungal pathogenicity assays. To confirm the role of ACLB gene in regulating plant immune response, the dicotyledonous model plant Arabidopsis was selected for functional verification of ACLB gene. Our results indicate the resistance to Verticillium dahliae infection in the Arabidopsis mutant aclb-2 were enhanced without causing strong cell death. Ectopic expression of GausACLB-2 in Arabidopsis weakened its resistance to V. dahliae either in Col-0 or in aclb-2 background, in which the expression level of ACLB is negatively correlated with the resistance to V. dahliae.

CONCLUSIONS

These results indicate that ACLB has a new function in negatively affecting the induction of plant defense response and cell death in cotton, which provides theoretical guidance for developing cotton varieties with resistance against Verticillium wilt.

Advances in the Genetic Basis and Molecular Mechanism of Lesion Mimic Formation in Rice

Plant lesion mutation usually refers to the phenomenon of cell death in green tissues before senescence in the absence of external stress, and such mutants also show enhanced resistance to some plant pathogens. The occurrence of lesion mimic mutants in rice is affected by gene mutation, reactive oxygen species accumulation, an uncontrolled programmed cell death system, and abiotic stress. At present, many lesion mimic mutants have been identified in rice, and some genes have been functionally analyzed. This study reviews the occurrence mechanism of lesion mimic mutants in rice. It analyzes the function of rice lesion mimic mutant genes to elucidate the molecular regulation pathways of rice lesion mimic mutants in regulating plant disease resistance.

A VQ-motif-containing protein fine-tunes rice immunity and growth by a hierarchical regulatory mechanism

Rice blast and bacterial blight, caused by the fungus Magnaporthe oryzae and the bacterium Xanthomonas oryzae pv. oryzae (Xoo), respectively, are devastating diseases affecting rice. Here, we report that a rice valine-glutamine (VQ) motif-containing protein, OsVQ25, balances broad-spectrum disease resistance and plant growth by interacting with a U-Box E3 ligase, OsPUB73, and a transcription factor, OsWRKY53. We show that OsPUB73 positively regulates rice resistance against M. oryzae and Xoo by interacting with and promoting OsVQ25 degradation via the 26S proteasome pathway. Knockout mutants of OsVQ25 exhibit enhanced resistance to both pathogens without a growth penalty. Furthermore, OsVQ25 interacts with and suppresses the transcriptional activity of OsWRKY53, a positive regulator of plant immunity. OsWRKY53 downstream defense-related genes and brassinosteroid signaling genes are upregulated in osvq25 mutants. Our findings reveal a ubiquitin E3 ligase-VQ protein-transcription factor module that fine-tunes plant immunity and growth at the transcriptional and posttranslational levels.

A Single Amino Acid Substitution in MIL1 Leads to Activation of Programmed Cell Death and Defense Responses in Rice

Lesion mimic mutants are an ideal model system for elucidating the molecular mechanisms of programmed cell death and defense responses in rice. In this study, we identified a lesion mimic mutant termed miner infection like 1-1 (mil1-1). The mil1-1 exhibited lesions on the leaves during development, and the chloroplasts of mil1-1 leaves were disrupted. Reactive oxygen species were found to accumulate in mil1-1 leaves. Cell death and DNA fragmentation were observed in mil1-1 leaves, indicating that the cells in the spots of mil1-1 leaves experienced programmed cell death. Most agronomic traits decreased in mil1-1, suggesting that the growth retardation in mil1-1 caused reduced per-plant grain yield. However, the mutation of MIL1 activated the expression of pathogen response genes and enhanced resistance to bacterial blight. The MIL1 gene was cloned using the positional cloning approach. A missense mutation 751 bp downstream of ATG was found in mil1-1. The defects of mil1-1 were able to be rescued by delivering a wild-type MIL1 gene into mil1-1. MIL1 encoded hydroperoxide lyase 3 (OsHPL3), and the expression of OsHPL3 was induced via hormone and abiotic stresses. Our findings provide insights into the roles of MIL1 in regulating programmed cell death, development, yield, and defense responses in rice.

OsSPL88 Encodes a Cullin Protein that Regulates Rice Growth and Development

Plant lesion mimics refer to necrotic spots spontaneously produced by the plant without mechanical damage, pathogen invasion, and adversity stress. Here, we isolated and characterized two rice (Oryza sativa L) mutants, namely, spl88-1 (spotted leaf88-1) and spl88-2 (spotted leaf88-2), which were identified from an ethyl methanesulfonate-mutagenized japonica cultivar Xiushui 11 population. Physiological and biochemical experiments indicated that more ROS accumulated in spl88-1 and spl88-2 than in wild type. spl88-1 and spl88-2 displayed spontaneous cell death and enhanced their resistance to bacterial blight by affecting the expression of defense-related genes. We isolated SPL88 by map-based cloning, which encoded a highly conserved Cullin protein. A single base deletion was detected in spl88-1 and spl88-2, in which the 132nd base C of SPL88-1 and the 381th base T of SPL88-2 were deleted, causing premature termination of protein translation. SPL88 was expressed in root, stem, leaf, leaf sheath, and panicle. The Cullin protein was localized in the cytoplasm and nucleus. The aforementioned results indicate that SPL88 regulates the growth and development of rice by affecting the expression of defense-related genes.

Wheat E3 ubiquitin ligase TaGW2-6A degrades TaAGPS to affect seed size

TaGW2 has been identified as a key determinant of the grain weight in wheat (Triticum aestivum L.). In our previous study, we found that the grain size differs in Chinese Spring (CS) and its TaGW2-6A allelic variant (NIL31). In addition, the expression of the key starch biosynthesis enzyme gene TaAGPS differs significantly in the two materials. However, the underlying molecular mechanism associated with the action of TaGW2-6A has not been reported. In the present study, we found that TaGW2-6A-CS interacted with TaAGPS, whereas TaGW2-6A-NIL31 did not interact with it in vitro and in vivo. Furthermore, we found that the C-terminal LXLX domain (376-424 aa) of TaGW2-6A recognized TaAGPS. However, the TaGW2-6A allelic variant lacked this key interaction region due to premature translation termination. We also found that TaGW2-6A-CS can ubiquitinate TaAGPS and degrade it via the 26 S proteasome pathway. In addition, our analysis of the activity of ADP-glucose pyrophosphorylase (AGPase) indicated that the AGPase level in the endosperm cells was lower in CS than NIL31. Cytological observations demonstrated that the average number of starch granules and the average area of starch granules in endosperm cells were lower in CS than NIL31. The overexpression of TaAGPS positively regulated the seed size in transgenic Arabidopsis. Our findings provide novel insights into the molecular mechanism that allows TaGW2-6A-TaAGPS to regulate seed size via the starch synthesis pathway.

Underlying mechanism of accelerated cell death and multiple disease resistance in a maize lethal leaf spot 1 allele

Multiple disease resistance (MDR) in maize has attracted increasing attention. However, the interplay between cell death and metabolite changes and their contributions to MDR remains elusive in maize. In this study, we identified a mutant named as lesion mimic 30 (les30) that showed 'suicidal' lesion formation in the absence of disease and had enhanced resistance to the fungal pathogen Curvularia lunata. Using map-based cloning, we identified the causal gene encoding pheophorbide a oxidase (PAO), which is known to be involved in chlorophyll degradation and MDR, and is encoded by LETHAL LEAF SPOT1 (LLS1). LLS1 was found to be induced by both biotic and abiotic stresses. Transcriptomics analysis showed that genes involved in defense responses and secondary metabolite biosynthesis were mildly activated in leaves of the les30 mutant without lesions, whilst they were strongly activated in leaves with lesions. In addition, in les30 leaves with lesions, there was overaccumulation of defense-associated phytohormones including jasmonic acid and salicylic acid, and of phytoalexins including phenylpropanoids, lignin, and flavonoids, suggesting that their biosynthesis was activated in a lesion-dependent manner. Taken together, our study implies the existence of an interactive amplification loop of interrupted chlorophyll degradation, cell death, expression of defense-related genes, and metabolite changes that results in suicidal lesion formation and MDR, and this has the potential to be exploited by genetic manipulation to improve maize disease resistance.

Identification of a unique allele in the quantitative trait locus for crown root number in japonica rice from Japan using genome-wide association studies

To explore the genetic resources that could be utilized to help improve root system architecture phenotypes in rice (Oryza sativa), we have conducted genome-wide association studies to investigate maximum root length and crown root number in 135 10-day-old Japanese rice accessions grown hydroponically. We identified a quantitative trait locus for crown root number at approximately 32.7 Mbp on chromosome 4 and designated it qNCR1 (quantitative trait locus for Number of Crown Root 1). A linkage disequilibrium map around qNCR1 suggested that three candidate genes are involved in crown root number: a cullin (LOC_Os04g55030), a gibberellin 20 oxidase 8 (LOC_Os04g55070), and a cyclic nucleotide-gated ion channel (LOC_Os04g55080). The combination of haplotypes for each gene was designated as a haploblock, and haploblocks 1, 2, and 3 were defined. Compared to haploblock 1, the accessions with haploblocks 2 and 3 had fewer crown roots; approximately 5% and 10% reductions in 10-day-old plants and 15% and 25% reductions in 42-day-old plants, respectively. A Japanese leading variety Koshihikari and its progenies harbored haploblock 3. Their crown root number could potentially be improved using haploblocks 1 and 2.

Rice Lesion Mimic Gene Cloning and Association Analysis for Disease Resistance

Lesion mimic mutants refer to a class of mutants that naturally form necrotic lesions similar to allergic reactions on leaves in the absence of significant stress or damage and without being harmed by pathogens. Mutations in most lesion mimic genes, such as OsACL-A2 and OsSCYL2, can enhance mutants’ resistance to pathogens. Lesion mimic mutants are ideal materials for studying programmed cell death (PCD) and plant defense mechanisms. Studying the genes responsible for the rice disease-like phenotype is of great significance for understanding the disease resistance mechanism of rice. In this paper, the nomenclature, occurrence mechanism, genetic characteristics, regulatory pathways, and the research progress on the cloning and disease resistance of rice lesion mimic mutant genes were reviewed, in order to further analyze the various lesion mimic mutants of rice. The mechanism lays a theoretical foundation and provides a reference for rice breeding.

The Rice Malectin Regulates Plant Cell Death and Disease Resistance by Participating in Glycoprotein Quality Control

In animals, malectin is well known to play an essential role in endoplasmic reticulum quality control (ERQC) by interacting with ribophorin I, one unit of the oligosaccharyltransferase (OST) complex. However, the functions of malectin in plants remain largely unknown. Here, we demonstrate the rice OsMLD1 is an ER- and Golgi-associated malectin protein and physically interacts with rice homolog of ribophorin I (OsRpn1), and its disruption leads to spontaneous lesion mimic lesions, enhanced disease resistance, and prolonged ER stress. In addition, there are many more N-glycosites and N-glycoproteins identified from the mld1 mutant than wildtype. Furthermore, OsSERK1 and OsSERK2, which have more N-glycosites in mld1, were demonstrated to interact with OsMLD1. OsMLD1 can suppress OsSERK1- or OsSERK2-induced cell death. Thus, OsMLD1 may play a similar role to its mammalian homologs in glycoprotein quality control, thereby regulating cell death and immunity of rice, which uncovers the function of malectin in plants.

An evolutionarily conserved C4HC3-type E3 ligase regulates plant broad-spectrum resistance against pathogens

Deployment of broad-spectrum disease resistance against multiple pathogen species is an efficient way to control plant diseases. Here, we identify a Microtubule-associated C4HC3-type E3 Ligase (MEL) in both Nicotiana benthamiana and Oryza sativa, and show that it is able to integrate and initiate a series of host immune signaling, conferring broad-spectrum resistance to viral, fungal, and bacterial pathogens. We demonstrate that MEL forms homodimer through intermolecular disulfide bonds between its cysteine residues in the SWIM domain, and interacts with its substrate serine hydroxymethyltrasferase 1 (SHMT1) through the YφNL motif. Ubiquitin ligase activity, homodimerization and YφNL motif are indispensable for MEL to regulate plant immunity by mediating SHMT1 degradation through the 26S proteasome pathway. Our findings provide a fundamental basis for utilizing the MEL-SHMT1 module to generate broad-spectrum-resistant rice to global destructive pathogens including rice stripe virus, Magnaporthe oryzae, and Xanthomonas oryzae pv. oryzae.