Co-overexpression of SWEET sucrose transporters modulates sucrose synthesis and defence responses to enhance immunity against bacterial blight in rice

Enhancing carbohydrate export from source to sink tissues is considered to be a realistic approach for improving photosynthetic efficiency and crop yield. The rice sucrose transporters OsSUT1, OsSWEET11a and OsSWEET14 contribute to sucrose phloem loading and seed filling. Crucially, Xanthomonas oryzae pv. oryzae (Xoo) infection in rice enhances the expression of OsSWEET11a and OsSWEET14 genes, and causes leaf blight. Here we show that co-overexpression of OsSUT1, OsSWEET11a and OsSWEET14 in rice reduced sucrose synthesis and transport leading to lower growth and yield but reduced susceptibility to Xoo relative to controls. The immunity-related hypersensitive response (HR) was enhanced in the transformed lines as indicated by the increased expression of defence genes, higher salicylic acid content and presence of HR lesions on the leaves. The results suggest that the increased expression of OsSWEET11a and OsSWEET14 in rice is perceived as a pathogen (Xoo) attack that triggers HR and results in constitutive activation of plant defences that are related to the signalling pathways of pathogen starvation. These findings provide a mechanistic basis for the trade-off between plant growth and immunity because decreased susceptibility against Xoo compromised plant growth and yield.

Bactericidal bissulfone B7 targets bacterial pyruvate kinase to impair bacterial biology and pathogenicity in plants

The prevention and control of rice bacterial leaf blight (BLB) disease has not yet been achieved due to the lack of effective agrochemicals and available targets. Herein, we develop a series of novel bissulfones and a novel target with a unique mechanism to address this challenge. The developed bissulfones can control Xanthomonas oryzae pv. oryzae (Xoo), and 2-(bis(methylsulfonyl)methylene)-N-(4-chlorophenyl) hydrazine-1-carboxamide (B7) is more effective than the commercial drugs thiodiazole copper (TC) and bismerthiazol (BT). Pyruvate kinase (PYK) in Xoo has been identified for the first time as the target protein of our bissulfone B7. PYK modulates bacterial virulence via a CRP-like protein (Clp)/two-component system regulatory protein (regR) axis. The elucidation of this pathway facilitates the use of B7 to reduce PYK expression at the transcriptional level, block PYK activity at the protein level, and impair the interaction within the PYK-Clp-regR complex via competitive inhibition, thereby attenuating bacterial biology and pathogenicity. This study offers insights into the molecular and mechanistic aspects underlying anti-Xoo strategies that target PYK. We believe that these valuable discoveries will be used for bacterial disease control in the future.

Identification of NADPH Oxidase Genes Crucial for Rice Multiple Disease Resistance and Yield Traits

Reactive oxygen species (ROS) act as a group of signaling molecules in rice functioning in regulation of development and stress responses. Respiratory burst oxidase homologues (Rbohs) are key enzymes in generation of ROS. However, the role of the nine Rboh family members was not fully understood in rice multiple disease resistance and yield traits. In this study, we constructed mutants of each Rboh genes and detected their requirement in rice multiple disease resistance and yield traits. Our results revealed that mutations of five Rboh genes (RbohA, RbohB, RbohE, RbohH, and RbohI) lead to compromised rice blast disease resistance in a disease nursery and lab conditions; mutations of five Rbohs (RbohA, RbohB, RbohC, RbohE, and RbohH) result in suppressed rice sheath blight resistance in a disease nursery and lab conditions; mutations of six Rbohs (RbohA, RbohB, RbohC, RbohE, RbohH and RbohI) lead to decreased rice leaf blight resistance in a paddy yard and ROS production induced by PAMPs and pathogen. Moreover, all Rboh genes participate in the regulation of rice yield traits, for all rboh mutants display one or more compromised yield traits, such as panicle number, grain number per panicle, seed setting rate, and grain weight, resulting in reduced yield per plant except rbohb and rbohf. Our results identified the Rboh family members involved in the regulation of rice resistance against multiple pathogens that caused the most serious diseases worldwide and provide theoretical supporting for breeding application of these Rbohs to coordinate rice disease resistance and yield traits.

Genetic Structure and TALome Analysis Highlight a High Level of Diversity in Burkinabe Xanthomonas Oryzae pv. oryzae Populations

Bacterial Leaf Blight of rice (BLB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a major threat for food security in many rice growing countries including Burkina Faso, where the disease was first reported in the 1980's. In line with the intensification of rice cultivation in West-Africa, BLB incidence has been rising for the last 15 years. West-African strains of Xoo differ from their Asian counterparts as they (i) are genetically distant, (ii) belong to new races and, (iii) contain reduced repertoires of Transcription Activator Like (TAL) effector genes. In order to investigate the evolutionary dynamics of Xoo populations in Burkina Faso, 177 strains were collected from 2003 to 2018 in three regions where BLB is occurring. Multilocus VNTR Analysis (MLVA-14) targeting 10 polymorphic loci discriminated 24 haplotypes and showed that Xoo populations were structured according to their geographical localization and year of collection. Considering their major role in Xoo pathogenicity, we assessed the TAL effector repertoires of the 177 strains upon RFLP-based profiling. Surprisingly, an important diversity was revealed with up to eight different RFLP patterns. Finally, comparing neutral vs. tal effector gene diversity allowed to suggest scenarios underlying the evolutionary dynamics of Xoo populations in Burkina Faso, which is key to rationally guide the deployment of durably resistant rice varieties against BLB in the country.

Molecular Cloning and Characterization of a Serotonin N-Acetyltransferase Gene, xoSNAT3, from Xanthomonas oryzae pv. oryzae

Rice bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the top ten bacterial plant diseases worldwide. Serotonin N-acetyltransferase (SNAT) is one of the key rate-limiting enzymes in melatonin (MT) biosynthesis. However, its function in pathogenic bacteria remains unclear. In this study, a Xoo SNAT protein (xoSNAT3) that showed 27.39% homology with sheep SNAT was identified from a collection of 24 members of GCN5-related N-acetyltransferase (GNAT) superfamily in Xoo. This xoSNAT3 could be induced by MT. In tobacco-based transient expression system, xoSNAT3 was found localized on mitochondria. In vitro studies indicated that xoSNAT3 showed the optima enzymatic activity at 50 °C. The recombinant enzyme showed K_m and V_max values of 709.98 μM and 2.21 nmol/min/mg protein, respectively. Mutant △xoSNAT3 showed greater impaired MT biosynthesis than the wild-type strain. Additionally, △xoSNAT3 showed 14.06% less virulence and 26.07% less biofilm formation. Collectively, our results indicated that xoSNAT3 services as a SNAT involved in MT biosynthesis and pathogenicity in Xoo.

Development of Designer Transcription Activator-Like Effector-Based Plant Growth Regulator for Higher Yield in Rice

Recent studies have shown that reprogramming of gene expression in a genome can induce the production of proteins enabling yield increase. The transcription activator-like effectors (TALEs) from several species of bacterial Xanthomonas have been extensively studied, and a series of research tools, such as genome editing tool TALENs and gene expression activators, have been developed based on the specific protein-nucleic acid recognition and binding mechanisms of TALEs. In this proof-of-principle study, we designed and constructed a designer TALE (dTALE), designated as dTALE-NOG1, to specifically target the promoter of OsNOG1 gene in rice, and demonstrated that this dTALE can be used as a new type of plant growth regulator for better crop growth and harvest. In doing so, the dTALE-NOG1 was transferred into the non-pathogenic Xanthomonas oryzae pv. oryzae (Xoo) strain PH to generate a genetically engineered bacteria (GEB) strain called PH-dtNOG1. Functional verification showed that dTALE-NOG1 could significantly induce the expression of OsNOG1. By spraying cell suspension of PH-dtNOG1 on the rice plants during the tillering stage, the transcription level of OsNOG1 was highly enhanced, the grain number of rice plants was increased by more than 11.40%, and the grain yield per plant increased by more than 11.08%, demonstrating that the dTALE-NOG1 was highly effective in enhancing rice yield. This work provided a new strategy for manipulating agronomical traits by reprogramming gene expression in a crop genome.

New Axially Chiral Molecular Scaffolds with Antibacterial Activities against Xanthomonas oryzae pv. oryzae for Protection of Rice

A new class of axially chiral thiazine molecules were constructed and showed promising antibacterial activities against the plant pathogen Xanthomonas oryzae pv. oryzae (Xoo). The axial chiralities of these compounds (R- or S-atropisomer) showed clear impacts on the in vitro inhibitory activities against Xoo. An optimal molecule of this class with the (S)-axially chiral configuration was identified to exhibit inhibitory activity against Xoo with an EC₅₀ value of 4.18 μg/mL. This inhibition efficiency is superior to that of two commercial antibacterial agrochemicals, thiodiazole-copper and bismerthiazol, as the positive controls. This hit compound also performed better than the controls in our in vivo studies. Preliminary mechanistic studies via scanning electron microscopy images showed that our hit compound at a concentration of 10 μg/mL destroyed the bacterial integrity of Xoo. Label-free quantitative proteomics analysis indicated that a total of 366 differentially expressed proteins of the rice plants were significantly influenced in the presence of our hit molecule.

OsTGAL1 suppresses the resistance of rice to bacterial blight disease by regulating the expression of salicylic acid glucosyltransferase OsSGT1

Many TGA transcription factors participate in immune responses in the SA-mediated signaling pathway in Arabidopsis. This study identified a transcription factor OsTGAL1, which is induced upon infection by Xoo. Overexpression of OsTGAL1 increased the susceptibility of rice to Xoo. Plants overexpressing OsTGAL1 could affect the expression of many SA signaling-related genes. OsTGAL1 was able to interact with the promoter of OsSGT1, which encodes a key enzyme for SA metabolism. The transcript of OsSGT1 was induced by Xoo and this responsive expression was further increased in plants overexpressing OsTGAL1. OsSGT1 knockout lines had enhanced resistance to Xoo, and knocking out OsSGT1 in plants overexpressing OsTGAL1 blocked the susceptibility caused by OsTGAL1. Altered expression levels of several OsPRs in all the transgenic plants demonstrated that SA-mediated signaling had been affected. Furthermore, we identified an oxidoreductase of CC-type glutaredoxin, OsGRX17, which interacted with OsTGAL1. OsGRX17 reduced the regulation of OsTGAL1 on OsSGT1, and this may be due to its redox modulation. Thus, our results demonstrate that OsTGAL1 negatively regulates resistance to Xoo by its effects on SA metabolism via the activation of OsSGT1, which provides valuable targets for plant breeders in developing new cultivars that are resistant to Xoo.

The OsWRKY6 transcriptional cascade functions in basal defense and Xa1-mediated defense of rice against Xanthomonas oryzae pv. oryzae

The rice protein OsWRKY6 directly activates OsWRKY45 and OsWRKY47 expression, and also activates OsPR1a and OsPR1b through the two OsWRKYs, and this transcriptional module participates in Xa1-mediated defense against the pathogen Xanthomonas oryzae pv. oryzae. Biotic stress, the pathogen Xanthomonas oryzae pv. oryzae (Xoo) in particular, negatively impacts worldwide productivity and yield in the staple crop rice (Oryza sativa). OsWRKY transcription factors are involved in various biotic stress responses in rice, and OsWRKY6 specifically acts as an important defense regulator against Xoo. However, the relationship between OsWRKY6 and other OsWRKYs, as well as its role in resistance (R) gene-mediated defense, have yet to be studied in depth. Here, we characterized a transcriptional cascade triggered by OsWRKY6 that regulated defense against Xoo infection mediated by the NBS-LRR protein Xa1. OsWRKY45 and OsWRKY47 were identified as direct transcriptional targets of OsWRKY6, and their two gene products reciprocally activated their two genes. Furthermore, OsWRKY6 activated OsPR1a and OsPR1b via the OsWRKY45 and OsWRKY47. Two OsWRKY6 RNAi knockdown lines showed significantly reduced defense even against an incompatible Xoo infection, and the expression of OsWRKY6 was not regulated by OsWRKY51 and OsWRKY88. This study reveals that a novel downstream transcriptional pathway activated by OsWRKY6 is involved in Xa1-mediated defense against Xoo.

Identification of a novel NPR1 homolog gene, OsNH5N16, which contributes to broad-spectrum resistance in rice

Over half of the earth's population consumes rice as the primary food crop for dietary calories. However, severe loss of rice yield occurs due to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) and bakanae disease caused by Fusarium fujikuroi (F. fujikuroi). Therefore, broad-spectrum resistance (BSR) to these pathogens is essential for rice cultivation. The Nonexpressor of Pathogenesis-Related Genes1 (NPR1), which is related to the signal molecule salicylic acid (SA) and the expression of pathogenesis-related (PR) genes, is a key regulator of systemic acquired resistance (SAR). Although five NPR1 homologs (NHs) have been identified in rice thus far, their cellular and biological functions remain largely unexplored. In this study, we identified a novel rice NH gene from Oryza sativa L. cv. Dongjin. The genetic variation of single nucleotide polymorphisms in OsNH5 caused a single amino acid substitution of asparagine for serine at residue 16. OsNH5N16 was mainly located in the nucleus, and its transcription was induced by Xoo. We generated transgenic rice lines constitutively expressing OsNH5N16 to investigate its function. Plants that overexpressed OsNH5N16 displayed enhanced BSR to Xoo and F. fujikuroi compared with wild varieties, and the transcription of PR genes such as OsPR1, GLUC, and CHIT2 was considerably upregulated. Moreover, we revealed that SA increases the transcription of OsNH5N16 and the promoter activity of OsPR1 regulated by OsNH5N16. These results showed that OsNH5N16 enhances BSR by regulating the expression of PR genes related to SAR and it is controlled by SA at the transcriptional and post-translational levels. This is the first report on the innate immune response conferring BSR associated with NH5.

Overexpression of OsPUB41, a Rice E3 ubiquitin ligase induced by cell wall degrading enzymes, enhances immune responses in Rice and Arabidopsis

BACKGROUND

Cell wall degrading enzymes (CWDEs) induce plant immune responses and E3 ubiquitin ligases are known to play important roles in regulating plant defenses. Expression of the rice E3 ubiquitin ligase, OsPUB41, is enhanced upon treatment of leaves with Xanthomonas oryzae pv. oryzae (Xoo) secreted CWDEs such as Cellulase and Lipase/Esterase. However, it is not reported to have a role in elicitation of immune responses.

RESULTS

Expression of the rice E3 ubiquitin ligase, OsPUB41, is induced when rice leaves are treated with either CWDEs, pathogen associated molecular patterns (PAMPs), damage associated molecular patterns (DAMPs) or pathogens. Overexpression of OsPUB41 leads to induction of callose deposition, enhanced tolerance to Xoo and Rhizoctonia solani infection in rice and Arabidopsis respectively. In rice, transient overexpression of OsPUB41 leads to enhanced expression of PR genes and SA as well as JA biosynthetic and response genes. However, in Arabidopsis, ectopic expression of OsPUB41 results in upregulation of only JA biosynthetic and response genes. Transient overexpression of either of the two biochemically inactive mutants (OsPUB41C40A and OsPUB41V51R) of OsPUB41 in rice and stable transgenics in Arabidopsis ectopically expressing OsPUB41C40A failed to elicit immune responses. This indicates that the E3 ligase activity of OsPUB41 protein is essential for induction of plant defense responses.

CONCLUSION

The results presented here suggest that OsPUB41 is possibly involved in elicitation of CWDE triggered immune responses in rice.

The OsMPK15 Negatively Regulates Magnaporthe oryza and Xoo Disease Resistance via SA and JA Signaling Pathway in Rice

Mitogen-activated protein kinase (MAPK) cascades play central roles in response to biotic and abiotic stresses. However, the mechanisms by which various MAPK members regulate the plant immune response in rice remain elusive. In this article, to characterize the mechanisms, the knock-out and overexpression mutants of OsMPK15 were constructed and the disease resistance was investigated under the various fungal and bacterial inoculations. The knock-out mutant of OsMPK15 resulted in the constitutive expression of pathogenesis-related (PR) genes, increased accumulation of reactive oxygen species (ROS) triggered by the pathogen-associated molecular pattern (PAMP) elicitor chitin, and significantly enhanced the disease resistance to different races of Magnaporthe oryzae and Xanthomonas oryzae pv. oryzae (Xoo), which cause the rice blast and bacterial blight diseases, respectively. On contrary, the expression of PR genes and ROS were down-regulated in the OsMPK15-overexpressing (OsMPK15-OE) lines. Meanwhile, phytohormones such as salicylic acid (SA) and jasmonic acid (JA) were accumulated in the mpk15 mutant lines but decreased in the OsMPK15-OE lines. The expression of SA- and JA-pathway associated genes were significantly upregulated in the mpk15 mutant, whereas it was down regulated in the OsMPK15-OE lines. We conclude that OsMPK15 may negatively regulate the disease resistance through modulating SA- and JA-mediated signaling pathway.

Variation and inheritance of the Xanthomonas raxX-raxSTAB gene cluster required for activation of XA21-mediated immunity

The rice XA21-mediated immune response is activated on recognition of the RaxX peptide produced by the bacterium Xanthomonas oryzae pv. oryzae (Xoo). The 60-residue RaxX precursor is post-translationally modified to form a sulfated tyrosine peptide that shares sequence and functional similarity with the plant sulfated tyrosine (PSY) peptide hormones. The 5-kb raxX-raxSTAB gene cluster of Xoo encodes RaxX, the RaxST tyrosylprotein sulfotransferase, and the RaxA and RaxB components of a predicted type I secretion system. To assess raxX-raxSTAB gene cluster evolution and to determine its phylogenetic distribution, we first identified rax gene homologues in other genomes. We detected the complete raxX-raxSTAB gene cluster only in Xanthomonas spp., in five distinct lineages in addition to X. oryzae. The phylogenetic distribution of the raxX-raxSTAB gene cluster is consistent with the occurrence of multiple lateral (horizontal) gene transfer events during Xanthomonas speciation. RaxX natural variants contain a restricted set of missense substitutions, as expected if selection acts to maintain peptide hormone-like function. Indeed, eight RaxX variants tested all failed to activate the XA21-mediated immune response, yet retained peptide hormone activity. Together, these observations support the hypothesis that the XA21 receptor evolved specifically to recognize Xoo RaxX.

Pest categorisation of Xanthomonas oryzae pathovars oryzae and oryzicola

The EFSA Panel on Plant Health performed a pest categorisation for Xanthomonas oryzae pathovars oryzae (Xoo) and oryzicola (Xoc), the causal agents of the bacterial blight and the bacterial leaf streak of rice, respectively. These pathovars are widely distributed in Asia, Africa and Australia. Xoo is also reported in some states of the USA and in some other countries of America. The identity of both pathovars is well established and efficient identification methods are available. The major host is cultivated rice (Oryza sativa), but different Oryza spp. as well as Poaceae weeds are reported as alternative hosts, with some uncertainty concerning the actual host range. Both pathovars are seed associated, despite the fact that seed transmission is still controversial for Xoo. Both pathovars are already regulated in Directives 2000/29/EC, on harmful organisms for plants, and 66/402/EEC, on the marketing of cereal seeds. The main pathway for entry is seed. Should these pathovars enter into EU, they may establish and spread, and they may have an impact on the rice crops, with uncertainties. The knowledge gaps identified are (1) the quantity of EU importation of rice seeds, (2) the risk of introduction through unprocessed rice for consumption, (3) the suitability of the EU growing climate conditions for the bacteria to establish and spread, (4) role of seed transmission (Xoo), (5) the role of weeds in the epidemiology and especially in seed transmission and dispersal, (6) host range of weeds. As none of the pathovars is known to occur in the EU, they do not meet one of the criteria for being considered as Union regulated non-quarantine pests. Nevertheless, both pathovars meet the criteria assessed by EFSA for consideration as Union quarantine pest.

The broad bacterial blight resistance of rice line CBB23 is triggered by a novel transcription activator-like (TAL) effector of Xanthomonas oryzae pv. oryzae

Bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is not only a disease devastating rice production worldwide, but also an ideal model system for the study of the interaction between plants and their bacterial pathogens. The rice near-isogenic line (NIL) CBB23, derived from a cross between a wild rice Oryza rufipogon accession (RBB16) and a susceptible indica rice variety (Jingang 30), is highly resistant to all field Xoo strains tested so far. Although the BB resistance of CBB23 has been widely used in rice breeding programmes, the mechanism of its extremely broad-spectrum resistance remains unknown. Here, we report the molecular cloning of an avirulence gene, designated as avrXa23, from Xoo strain PXO99(A) . We validate that AvrXa23, a novel transcription activator-like effector, specifically triggers the broad-spectrum BB resistance in CBB23. The prevalence of avrXa23 in all 38 Xoo strains surveyed may explain the broad-spectrum feature of BB resistance in CBB23. The results will significantly facilitate the molecular cloning of the corresponding resistance (R) gene in the host, and provide new insights into our understanding of the molecular mechanism for broad-spectrum disease resistance in plants.

Analysis of transcript and metabolite levels in Italian rice (Oryza sativa L.) cultivars subjected to osmotic stress or benzothiadiazole treatment

One of the major objectives of rice (Oryza sativa L.) breeding programs is the development of new varieties with higher tolerance/resistance to both abiotic and biotic stresses. In this study, Italian rice cultivars were subjected to osmotic stress or benzothiadiazole (BTH) treatments. An analysis of the expression of selected genes known to be involved in the stress response and (1)H nuclear magnetic resonance ((1)H NMR) metabolic profiling were combined with multivariate statistical analyses to elucidate potential correlations between gene expression or metabolite content and observed tolerant/resistant phenotypes. We observed that the expression of three chosen genes (two WRKY genes and one peroxidase encoding gene) differed between susceptible and resistant cultivars in response to BTH treatments. Moreover, the analysis of metabolite content, in particular in the osmotic stress experiment, enabled discrimination between selected cultivars based on differences in the accumulation of some primary metabolites, primarily sugars. This research highlights the potential usefulness of this approach to characterise rice varieties based on transcriptional or metabolic changes due to adverse environmental conditions.

The endoplasmic reticulum-quality control component SDF2 is essential for XA21-mediated immunity in rice

Plant genomes contain large number of plasma membrane (PM)-localized immune receptors, also called pattern recognition receptors (PRRs). PRRs are synthesized in the endoplasmic reticulum (ER) and then translocated to the PM, where they recognize conserved pathogen-associated molecular patterns (PAMPs) and activate innate immune response. The rice XA21 immune receptor confers resistance to the Gram-negative bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). To identify components that mediate XA21-mediated signaling, we performed co-purification experiments using C-terminal GFP tagged XA21 protein. Several endoplasmic reticulum-quality control (ER-QC) proteins including stromal-derived factor 2 (SDF2) co-purified with XA21. Silencing of the SDF2 genes in the XA21 rice genetic background compromises resistance to Xoo but does not affect plant growth and development.

Expression analysis of rice VQ genes in response to biotic and abiotic stresses

WRKY transcription factors are encoded by a large gene superfamily with a broad range of roles in plants. Proteins containing a short VQ (FxxxVQxLTG) motif have been recently shown to interact with WRKY transcription factors, implying that AtVQ proteins are important in the plant defense responses in Arabidopsis, either as positive or negative cofactors of WRKY transcription factors. Thirty-nine Oryza sativa genes containing the VQ motif (OsVQs) were identified and the genome structures of OsVQ proteins were characterized through genome-wide analysis in rice. Also, phylogenetic tree analysis was performed with the VQ domain of Arabidopsis and rice. The expression patterns of these OsVQ genes in plants under several stress treatments were assessed, specifically, following infection with the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo), treatment with abscisic acid (ABA), or exposure to drought. The cellular localization of a few OsVQ proteins was examined using rice protoplast system. Based on our results, we suggest that OsVQ proteins function as important co-regulators during the plant defense response to biotic and abiotic stresses.