Constitutive heterologous expression of avrXa27 in rice containing the R gene Xa27 confers enhanced resistance to compatible Xanthomonas oryzae strains

The vascular pathogen Xanthomonas oryzae pv. oryzae ( Xoo ) and nonvascular pathogen Xanthomonas oryzae pv. oryzicola ( Xoc ) cause bacterial blight (BB) and bacterial leaf streak (BLS) diseases of rice, respectively. We have previously identified the avirulence gene avrXa27 from Xoo PXO99^A, which specifically induces the expression of the rice resistance gene Xa27 , ultimately leading to resistance against BB disease in rice. In this study, we have generated a transgenic rice line (L24) that expresses avrXa27 constitutively under the control of the PR1 promoter, and have examined its role in the host–pathogen interaction. L24 is not more susceptible to BB, indicating that avrXa27 does not contribute to virulence. AvrXa27 retains avirulence activity in L24 and, after crossing with a line containing Xa27 , progeny display phenotypic changes including inhibition of tillering, delay in flowering, stiff leaves, early leaf senescence and activation of pathogenesis-related ( PR ) genes. On challenge with a variety of compatible strains of Xoo and Xoc strain L8, lines with both avrXa27 and Xa27 also show enhanced resistance to bacterial infection. The induction of Xa27 and subsequent inhibition of Xoc growth in Xa27 plants are observed on inoculation with Xoc L8 harbouring avrXa27 . Our results indicate that the heterologous expression of avrXa27 in rice containing Xa27 triggers R gene-specific resistance and, at the same time, confers enhanced resistance to compatible strains of Xoo and Xoc . The expression of AvrXa27 and related proteins in plants has the potential to generate broad resistance in plants.     

Over-expression in the nucleotide-binding site-leucine rich repeat gene <Emphasis Type="Italic">DEPG1</Emphasis> increases susceptibility to bacterial leaf streak disease in transgenic rice plants

Bacterial leaf streak of rice (BLS) caused by Xanthomonas oryzae pv. oryzicola (Xoc) is a widely-spread disease in the main rice-producing areas of the world. Investigating the genes that play roles in rice–Xoc interactions helps us to understand the defense signaling pathway in rice. Here we report a differentially expressed protein gene (DEPG1), which regulates susceptibility to BLS. DEPG1 is a nucleotide-binding site (NBS)-leucine rich repeat (LRR) gene, and the deduced protein sequence of DEPG1 has approximately 64% identity with that of the disease resistance gene Pi37. Phylogenetic analysis of DEPG1 and the 18 characterized NBS-LRR genes revealed that DEPG1 is more closely related to Pi37. DEPG1 protein is located to the cytoplasm, which was confirmed by transient expression of DEPG1-GFP (green fluorescent protein) fusion construct in onion epidermal cells. Semi-quantitative PCR assays showed that DEPG1 is widely expressed in rice, and is preferentially expressed in internodes, leaf blades, leaf sheaths and flag leaves. Observation of cross sections of leaves from the transgenic plants with a DEPG1-promoter::glucuronidase (GUS) fusion gene revealed that DEPG1 is also highly expressed in mesophyll tissues where Xoc mainly colonizes. Additionally, Xoc negatively regulates expression of DEPG1 at the early stage of the pathogen infection, and so do the three defense-signal compounds including salicylic acid (SA), methyl jasmonate (MeJA) and 1-aminocyclopropane-1-carboxylic-acid (ACC). Transgenic rice plants overexpressing DEPG1 exhibit enhanced susceptibility to Xoc compared to the wild-type controls. Moreover, enhanced susceptibility to Xoc may be mediated by inhibition of the expression of some SA biosynthesis-related genes and pathogenesis-related genes that may contribute to the disease resistance. Taken together, DEPG1 plays roles in the interactions between rice and BLS pathogen Xoc.     

水稻对白叶枯病和细菌性条斑病的抗性遗传研究进展

水稻白叶枯病和水稻细菌性条斑病是由稻黄单胞细菌(Xanthomonas oryzae)不同致病变种引起的两种最重要的水稻细菌性病害。发掘和利用抗性基因,培育抗病品种是防治这两种病害的最有效手段之一。本文分别综述了这两种高度相关的病害的抗性遗传研究进展,包括已发掘和利用的主效抗性基因特点及目前国内外对这两种病害的抗性QTL定位研究进展,为水稻抗白叶枯病和细菌性条斑病育种研究提供有用信息。     

Novel broad-spectrum bacteriophages against Xanthomonas oryzae and their biocontrol potential in rice bacterial diseases

Bacterial leaf blight (BLB) and bacterial leaf streak (BLS)—caused by Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xoc), respectively—are two major bacterial diseases that threaten the safe production of rice, one of the most important food crops. Bacteriophages are considered potential biocontrol agents against rice bacterial pathogens, due to their host specificity and environmental safety. It is common for BLB and BLS to occur together in fields, which highlights the need for broad-spectrum phages capable of infecting both Xoo and Xoc. In this study, two lytic broad-spectrum phages (pXoo2106 and pXoo2107) that can infect various strains of Xoo and Xoc were assessed. Both phages belong to the class Caudoviricetes and one of them to the family Autographiviridae, while the other belongs to an unclassified family. Two phages alone or combined in a phage cocktail could effectively inhibit Xoo and Xoc growth in vitro. In an in vivo biocontrol experiment, the phage cocktail reduced the total CFU and significantly eased the symptoms caused by Xoo or Xoc. Our results suggest that pXoo2106 and pXoo2107 have a broad-spectrum host range targeting different X. oryzae strains, and have strong biocontrol potential in field applications against both BLB and BLS.     

Bacterial leaf streak 1 encoding a mitogen-activated protein kinase confers resistance to bacterial leaf streak in rice

Bacterial leaf streak (BLS) is a major bacterial disease of rice. Utilization of host genetic resistance has become one of the most important strategies for controlling BLS. However, only a few resistance genes have been characterized. Previously, a recessive BLS resistance gene bls1 was roughly mapped on chromosome 6. Here, we further delineated bls1 to a 21 kb region spanning four genes. Genetic analysis confirmed that the gene encoding a mitogen-activated protein kinase (OsMAPK6) is the target of the allelic genes BLS1 and bls1. Overexpression of BLS1 weakened resistance to the specific Xanthomonas oryzae pv. oryzicola (Xoc) strain JZ-8, while low expression of bls1 increased resistance. However, both overexpression of BLS1 and low expression of bls1 could increase no-race-specific broad-spectrum resistance. These results indicate that BLS1 and bls1 negatively regulate race-specific resistance to Xoc strain JZ-8 but positively and negatively control broad-spectrum resistance, respectively. Subcellular localization demonstrated that OsMAPK6 was localized in the nucleus. RGA4, which is known to mediate resistance to Xoc, is the potential target of OsMAPK6. Overexpression of BLS1 and low expression of bls1 showed increase in salicylic acid and induced expression of defense-related genes, simultaneously increasing broad-spectrum resistance. Moreover, low expression of bls1 showed increase an in jasmonic acid and abscisic acid, in company with an increase in resistance to Xoc strain JZ-8. Collectively, our study provides new insights into the understanding of BLS resistance and facilitates the development of rice host-resistant cultivars.     

Domain Dissection of AvrRxo1 for Suppressor,Avirulence and Cytotoxicity Functions

AvrRxo1, a type III effector from Xanthomonas oryzae pv. oryzicola (Xoc) which causes bacterial leaf streak (BLS) in rice, can be recognised by non-host resistance protein Rxo1. It triggers a hypersensitive response (HR) in maize. Little is known regarding the virulence function of AvrRxo1. In this study, we determined that AvrRxo1 is able to suppress the HR caused by the non-host resistance recognition of Xanthomonas oryzae pv. oryzae (Xoo) by Nicotiana benthamiana. It is toxic, inducing cell death from transient expression in N. benthamiana, as well as in yeast. Among the four AvrRxo1 alleles from different Xoc strains, we concluded that the toxicity is abolished by a single amino acid substitution at residue 344 in two AvrRxo1 alleles. A series of truncations from the carboxyl terminus (C-terminus) indicate that the complete C-terminus of AvrRxo1 plays an essential role as a suppressor or cytotoxic protein. The C-terminus was also required for the avirulence function, but the last two residues were not necessary. The first 52 amino acids of N-terminus are unessential for toxicity. Point mutagenesis experiments indicate that the ATP/GTP binding site motif A is required for all three functions of AvrRxo1, and NLS is required for both the avirulence and the suppression of non-host resistance. The putative thiol protease site is only required for the cytotoxicity function. These results determine that AvrRxo1 plays a role in the complex interaction with host proteins after delivery into plant cells.