Avirulence proteins from haustoria-forming pathogens

A major insight that has emerged in the study of haustoria-forming plant pathogens over the last few years is that these eukaryotic biotrophs deliver suites of secreted proteins into host cells during infection. This insight has largely derived from successful efforts to identify avirulence (Avr) genes and their products from these pathogens. These Avr genes, identified from a rust and a powdery mildew fungus and three oomycete species, encode small proteins that are recognized by resistance proteins in the host plant cytoplasm, suggesting that they are transported inside plant cells during infection. These Avr proteins probably represent examples of fungal and oomycete effector proteins with important roles in subverting host cell biology during infection. In this respect, they represent a new opportunity to understand the basis of disease caused by these biotrophic pathogens. Elucidating how these pathogen proteins gain entry into plant cells and their biological function will be key questions for future research.     

Outbreak of Rice Blast Disease at Yeoju of Korea in 2020

Rice blast is the most destructive disease threatening stable rice production in rice-growing areas. Cultivation of disease-resistant rice cultivars is the most effective way to control rice blast disease. However, the rice blast resistance is easy to breakdown within years by blast fungus that continually changes to adapt to new cultivars. Therefore, it is important to continuously monitor the incidence of rice blast disease and race differentiation of rice blast fungus in fields. In 2020, a severe rice blast disease occurred nationwide in Korea. We evaluated the incidence of rice blast disease in Yeoju and compared the weather conditions at the periods of rice blast disease in 2019 and 2020. We investigated the races and avirulence genes of rice blast isolates in Yeoju to identify race diversity and genetic characteristics of the isolates. This study will provide empirical support for rice blast control and the breeding of blastresistant rice cultivars.     

AvrE1 and HopR1 from Pseudomonas syringae pv. actinidiae are additively required for full virulence on kiwifruit

Pseudomonas syringae pv. actinidiae ICMP 18884 biovar 3 (Psa3) produces necrotic lesions during infection of its kiwifruit host. Bacterial growth in planta and lesion formation are dependent upon a functional type III secretion system (T3S), which translocates multiple effector proteins into host cells. Associated with the T3S locus is the conserved effector locus (CEL), which has been characterized and shown to be essential for the full virulence in other P. syringae pathovars. Two effectors at the CEL, hopM1 and avrE1, as well as an avrE1-related non-CEL effector, hopR1, have been shown to be redundant in the model pathogen P. syringae pv. tomato DC3000 (Pto), a close relative of Psa. However, it is not known whether CEL-related effectors are required for Psa pathogenicity. The Psa3 allele of hopM1, and its associated chaperone, shcM, have diverged significantly from their orthologs in Pto. Furthermore, the CEL effector hopAA1-1, as well as a related non-CEL effector, hopAA1-2, have both been pseudogenized. We have shown that HopM1 does not contribute to Psa3 virulence due to a truncation in shcM, a truncation conserved in the Psa lineage, probably due to the need to evade HopM1-triggered immunity in kiwifruit. We characterized the virulence contribution of CEL and related effectors in Psa3 and found that only avrE1 and hopR1, additively, are required for in planta growth and lesion production. This is unlike the redundancy described for these effectors in Pto and indicates that these two Psa3 genes are key determinants essential for kiwifruit bacterial canker disease.     

Interspecific somatic hybrids Solanum villosum (+) S. tuberosum, resistant to Phytophthora infestans

The interspecific somatic hybrids 4x S. villosum (+) 2x S. tuberosum clone DG 81-68 (VT hybrids) were obtained and characterized molecularly and cytogenetically.     

江西稻瘟病菌稻巨座壳致病性分化年度动态变化

为明确江西水稻种植区稻巨座壳菌（稻瘟病病菌）的年际变化规律,本研究采用7个中国鉴别寄主与30个单基因鉴别品系两套鉴别寄主分别鉴定分析了2006-2018年间从江西37个水稻主要种植县市分离的1 161个稻瘟病单孢菌的生理小种、致病力、致病类型与无毒基因型等。研究结果表明,江西稻瘟病菌可以分成7群49个生理小种,其中ZA、ZB、ZC群为优势种群,ZB13为优势小种,出现频率为18.00%,以毒性较强的强致病力菌株为主;江西稻瘟病菌在生理小种构成、优势小种、致病力年际变化方面均具有3-5年的周期性;江西历年稻瘟病菌的致病类型较为丰富且存在年度差异,菌株致病类型占各年度总菌株数的82.79%-98.21%,优势致病型菌株占当年总菌株的3.57%-5.77%;历年稻瘟病菌的无毒基因个数为24-29个,其中Avr-Pizt、Avr-Piz5、Avr-Pik、Avr-Pik(C)在历年供试菌株中出现频率较高,说明与之相对应的抗瘟基因在江西抗病育种与抗性品种布局中具有较好的应用价值。     

Isolation of NBS-LRR RGAs from invasive Wedelia trilobata and the calculation of evolutionary rates to understand bioinvasion from a molecular evolution perspective

Supports for the molecular evolution of host–pathogen interactions on enemy release hypothesis are rare. According to the theory of plant immunity and the coevolution of hosts and pathogens, we hypothesized that the evolutionary rate (d_N/d_S) of resistance genes (R-genes) in invasive plants would be greater than in non-invasive plants, assuming that based on the enemy release hypothesis, the former would suffer less selection stress from co-evolutionary specialist pathogens. To test our hypothesis, we isolated and analyzed the conserved nucleotide-binding sites (NBS) of resistance gene analogues (RGAs) of an invasive weed, Wedelia trilobata (WTRGA). We then used the information in GenBank to compare the d_N/d_S of the NBS R-gene/RGAs in invasive and homologous non-invasive plants. Three W. trilobata NBS RGA sequences were obtained, belonging to the Toll/Interleukin-1 receptor (TIR) (WTRGA1 and WTRGA2) and non-TIR subclasses (WTRGA3). Compared with the homologous non-invasive plants, the invasive plants showed a significantly greater d_N/d_S for TIR NBS R-gene/RGAs (p < 0.0001), supporting our hypothesis. Future research should include an examination of R-genes/RGAs from more invasive plants on a population level to understand diversity and R-gene functions in invasive plant species, as well as to explore how disease resistance allows plants to adapt to changing pathogen stresses.     

Gain of virulence by Soybean mosaic virus on Rsv4‐genotype soybeans is associated with a relative fitness loss in a susceptible host

‘Gene‐for‐gene’ theory predicts that gain of virulence by an avirulent pathogen on plants expressing resistance (R) genes is associated with fitness loss in susceptible hosts. However, the validity of this prediction has been studied in only a few plant viral pathosystems. In this study, the Soybean mosaic virus (SMV)–Rsv4 pathosystem was exploited to test this prediction. In Rsv4‐genotype soybeans, P3 of avirulent SMV strains provokes an as yet uncharacterized resistance mechanism that restricts the invading virus to the inoculated leaves. A single amino acid substitution in P3 functionally converts an avirulent to a virulent strain, suggesting that the genetic composition of P3 plays a crucial role in virulence on Rsv4‐genotype soybeans. In this study, we examined the impact of gain of virulence mutation(s) on the fitness of virulent variants derived from three avirulent SMV strains in a soybean genotype lacking the Rsv4 gene. Our data demonstrate that gain of virulence mutation(s) by all avirulent viruses on Rsv4‐genotype soybean is associated with a relative fitness loss in a susceptible host. The implications of this finding on the durable deployment of the Rsv4 gene in soybean are discussed.     

Molecular communication between host plant and the fungal tomato pathogenCladosporium fulvum

Host genotype specificity in interactions between biotrophic fungal pathogens and plants in most cases complies with the gene-for-gene model. Success or failure of infection is determined by absence or presence of complementary genes, avirulence and resistance genes, in the pathogen and the host plant, respectively. Resistance, expressed by the induction of a hypersensitive response followed by other defence responses in the host, is envisaged to be based on recognition of the pathogen, mediated through direct interaction between products of avirulence genes of the pathogen (the so-called race-specific elicitors) and receptors in the host plant, the putative products of resistance genes. The interaction between the biothrophic fungusCladosporium fulvum and its only host tomato is a model system to study fungus-plant gene-for-gene relationships. Here we report on isolation, characterization and biological function of putative pathogenicity factors ECP1 and ECP2 and the race-specific elicitors AVR4 and AVR9 ofC. fulvum and cloning and regulation of their encoding genes. Disruption ofecp1 andecp2 genes has no clear effect on pathogenicity ofC. fulvum. Disruption of theavr9 gene, which codes for the race-specific 28 amino acid AVR9 elicitor, in wild type avirulent races, leads to virulence on tomato genotypes carrying the complementary resistance geneCf9. The avirulence geneavr4 encodes a 105 amino acid race-specific elicitor. A single basepair change in the avirulence geneavr4 leads to virulence on tomato genotypes carrying theCf4 resistance gene.     

Molecular and biochemical basis of the interaction between tomato and its fungal pathogen Cladosporium fulvum

The interaction between the biotrophic fungal pathogen Cladosporium fulvum and tomato complies with the genefor-gene model. Resistance, expressed as a hypersensitive response (HR) followed by other defence responses, is based on recognition of products of avirulence genes from C. fulvum (race-specific elicitors) by receptors (putative products of resistance genes) in the host plant tomato. The AVR9 elicitor is a 28 amino acid (aa) peptide and the AVR4 elicitor a 106 aa peptide which both induce HR in tomato plants carrying the complementary resistance genes Cf9 and Cf4, respectively. The 3-D structure of the AVR9 peptide, as determined by 1H NMR, revealed that AVR9 belongs to a family of peptides with a cystine knot motif. This motif occurs in channel blockers, peptidase inhibitors and growth factors. The Cf9 resistance gene encodes a membrane-anchored extracellular glycoprotein which contains leucine-rich repeats (LRRs). 125I labeled AVR9 peptide shows the same affinity for plasma membranes of Cf9+ and Cf9- tomato leaves. Membranes of solanaceous plants tested so far all contain homologs of the Cf9 gene and show similar affinities for AVR9. It is assumed that for induction of HR, at least two plant proteins (presumably CF9 and one of his homologs) interact directly or indirectly with the AVR9 peptide which possibly initiates modulation and dimerisation of the receptor, and activation of various other proteins involved in downstream events eventually leading to HR. We have created several mutants of the Avr9 gene, expressed them in the potato virus X (PVX) expression system and tested their biological activity on Cf9 genotypes of tomato. A positive correlation was observed between the biological activity of the mutant AVR9 peptides and their affinity for tomato plasma membranes. Recent results on structure and biological activity of AVR4 peptides encoded by avirulent and virulent alleles of the Avr4 gene (based on expression studies in PVX) are also discussed as well as early defence responses induced by elicitors in tomato leaves and tomato cell suspensions.