The effector repertoire of Puccinia striiformis subdues host-plant immunity
State Key Laboratory of Crop Stress Biology for Arid Areas and College of Agronomy, Northwest A&F University, P. R. China
Rust outbreaks cause severe yield lossespose a serious threat to global food security. As biotrophic pathogens, rust fungi produce effectors to suppress host immunity. In this study, we used a systemic approach to identify and characterize effectors in Puccinia striiformis f. sp. tritici. Among secreted proteins encoded by the Pst genome, we identified 150 putative effectors that are Cys-rich or up-regulated during hostinfection. A systematic screen showed that 14 of them suppressed programmed cell death (PCD) triggered by BAX and INF1 in Nicotiana benthamiana, and all have high intra- and inter-species polymorphisms at the protein level. Although these 14 effectors individually made only minor contributions to Pst virulence, delivery of them into wheat plants via a bacterial type-III secretion system efficiently suppressed PTI and ETI. Interestingly, three of them, Pst_4941, Pst_4884, and Pst_5578, triggered HR-like PCD in the AvSYr1NIL, but only Pst_4941 also caused PCD in the AvSYr7 NIL, suggesting that they may function as avirulence factors. This study suggests that Pst effectors function to suppress PTI and ETI.
Monitoring the South African leaf rust population through a combined phenotyping and SSR genotyping approach
Department of Plant Sciences, University of the Free State, South Africa
View selinga.pdf (1.31 MB)
Leaf rust is a common wheat disease in South Africa. Annual surveys conducted by the Agricultural Research Council - Small Grain Institute (ARC-SGI) during the last 35 years used infection type (IT) data on a defined differential set to identify individual field isolates. Results from these surveys confirmed that the South African Pt population is affected by both local evolution and foreign introductions. A good correlation was found between avirulence/virulence phenotypes and simple sequence repeat (SSR) genotypes in the South African Pt population. We therefore evaluated whether identification of field isolates by SSR analysis would complement the traditional IT analysis using 47 field isolates collected during the 2013 growing season. Of the 39 phenotyped isolates, 35 were correctly genotyped while three were incorrectly genotyped only because the corresponding race was not included as a control. Five isolates that could not be phenotyped due to non-viable spores were successfully genotyped. The dominant race 3SA145 (North American race annotation CCPS) was represented by nine different genotypes sharing 82% genetic similarity. The SSR data further showed that the field isolates formed part of two distinct lineages with little admixture between them. This study confirmed the supporting value of SSR genotyping to traditional race analysis in monitoring the South African Pt population.
Wheat cv. Kingbird is introduced to address a new stem rust threat in Ethiopia
Kulumsa Agricutural Research Center, Ethiopian Institute of Agricultural Research, Ethiopia
Pgt race TKTTF, virulent for the SrTmp gene present in Ethiopian cv. Digalu and first detected in 2012, caused significant yield losses in Digalu during the 2013 and 2014 seasons. No suitable replacement varieties with significant seed volume were available, and alternate solutions were sought. EIAR, with the support of the DRRW project through CIMMYT-Ethiopia, introduced 5 tonnes of adult plant, rust resistant wheat cv. Kingbird from Kenya. Kingbird was evaluated for agronomic performance at seven locations vs. three checks in 2014, and was also evaluated for stem rust reaction in single-race nurseries (TKTTF, TTKSK, TRTTF and JRCQC). With support from USAID/CIMMYT, seed was concurrently multiplied on 37 ha producing 80 tonnes of seed that was distributed to farmers in 2015. Mean grain yield over locations was 2.76 t ha-1. Mean performance of Kingbird was 3.00 t ha-1 compared to 2.79 t ha-1 for Ogolcho, 2.83 t ha-1 for Biqa and 2.42 t ha-1 for Kakaba. Thus Kingbird gave yield advantages of 5 to 22% over the check varieties. Stem rust severities on Kingbird in the single race nurseries ranged from Tr to 15% and reactions ranged from TMR to SMS. The check varieties rated up to 45% severity with S type reactions. Thus Kingbird was superior in terms of yield potential and stem rust resistance as measured in these trials vs. the check varieties. Stem rust resistance of Kingbird is based on Sr2 and Sr57 and is hypothesized to have at least three additional APR loci. Seedling reactions of Kingbird to races TKTTF and Ug99 are characterized as susceptible. Sr57 is pleiotropic and confers partial resistance to all three rusts, powdery mildew, spot blotch, and BYDV. Based on early maturity, yield performance, and stem rust resistance, Kingbird is recommended for low- to mid-altitude wheat-growing areas of Ethiopia.
Characterization of a stripe rust resistance gene in wheat landrace AUS 27969 from the Watkins collection
The University of Sydney, Plant Breeding Institute, Australia
Landraces and wild relatives of wheat are rich repositories of new rust resistance genes. Landraces are preferred over wild relatives for the absence of deleterious effects associated with large alien segments. A common wheat landrace, AUS 27969 (ex Portugal), from the Watkins Collection was resistant under field conditions and produced seedling infection type (IT) 2C against the widely virulent Australian Puccinia striiformis f. sp. tritici (Pst) pathotype 134 E16 A+ Yr17+ Yr27+. AUS 27969 was crossed with the susceptible genotype Avocet S (AvS) and the distribution of F3 lines conformed to monogenic segregation [40 non-segregating resistant (NSR), 93 segregating (Seg), and 37 non-segregating susceptible (NSS); ?2 = 1.61, P2d.f. >0.05] when tested with the same pathotype at the seedling stage. The population is currently being selfed to F6. DNA from NSR and NSS lines will be sent for high throughput analysis to identify the genomic region carrying the resistance gene. Resistance-linked SNPs will be mapped on the F6 RIL population. The resistance gene will be backcrossed into modern Australian wheat backgrounds.
Brachypodium distachyon as a model to study nonhost resistance to wheat stripe rust
CSIRO Plant Industry, Australia
The model grass Brachypodium distachyon has been used to study nonhost resistance mechanisms to the wheat stripe rust pathogen, Puccinia striiformis f. sp tritici. Numerous B. distachyon accessions were screened with an array of UK and Australian P. striiformis isolates and distinct infection phenotypes identified, ranging from complete resistance to partial susceptibility. Three mapping families were established - BdTR10H x TEK4, BdTR13K x Bd21 and ABR6 x Bd21 - and immunity was dominantly inherited when they were tested with one Australian and three UK isolates. Depending upon the mapping family, between one and three genes for stripe rust resistance were present and designated Yrr1 to Yrr3. Yrr1, which is present in all three families, was effective against all isolates and was fine mapped to a 100 kilobase region containing six candidate genes. Interestingly, no candidate was homologous to a known resistance gene. Yrr2, which is present in the BdTR13K x Bd21 and ABR6 x Bd21 families, is race-specific and was mapped to a 1 megabase region that contains multiple, classic NBS-LRR resistance gene candidates. Yrr3, which is present in the ABR6 x Bd21 family and effective against all isolates, was mapped to a 400 kilobase region also containing NBS-LRR gene candidates. Agrobacterium-mediated transformation of Yrr1 candidates is underway in Brachypodium for complementation, and in common wheat to test for interspecies transfer of characterized resistance.
Characterization of seedling and adult plant resistance to leaf rust in African wheat germplasm
The University of Sydney, Plant Breeding Institute, Australia
Many of the catalogued leaf rust resistance genes in wheat deployed in agriculture have been overcome by variants of Puccinia triticina (Pt), the causal pathogen of leaf rust. Discovery and characterization of new sources of resistance in various germplasms using multipathotype tests and molecular markers could permit future diversification of the genetic base of leaf rust resistance in wheat. In searching for new sources of leaf rust resistance, 140 wheat lines from 14 African countries were tested with 8 Australian Pt pathotypes. Seedling tests revealed that 41% of the lines were susceptible to all pathotypes, 31% were postulated to carry either one of 10 resistance genes (Lr1, Lr2a, Lr3a, Lr13, Lr18, Lr23, Lr24, Lr26, Lr37 or Lr73) or one of five gene combinations (Lr2a+Lr3a, Lr1+Lr13, Lr1+Lr23, Lr1+Lr13+Lr73 and Lr23+Lr73). Twenty-eight percent of the lines were postulated to carry uncharacterized seedling resistance genes. Based on average coefficients of infection (ACI), 101, 25 and 11 lines showed high (ACI 0-19), moderate (ACI 21-38) and low (ACI 41-56) levels adult plant resistance, respectively, whereas three lines were moderately susceptible to susceptible (ACI 63-76). Genotyping of 74-78 lines that were anticipated to carry APR genes, using the molecular markers: csLV34 (linked to Lr34) and KASP SNP markers SNP1G22 and SNPT10 (linked to Lr46 and Lr67), respectively, revealed the presence of Lr34, Lr46 and Lr67 in 11, 22 and 14 wheat lines, respectively. The identities of the APR in the remaining 22 lines are unknown, and potentially represent new resistance sources. Genetic analyses of these uncharacterized APR sources are underway to select single gene lines and allow fine mapping.
Crosstalk between CBL-CIPK and SA signaling pathways in wheat-Puccinia striiformis f. sp. tritici pathosystem
State Key Laboratory of Crop Stress Biology for Arid Areas and College of Plant Protection, Northwest A&F University, PR China
Intracellular calcium changes during plant–pathogen interaction are essential early events leading to both local and systemic acquired resistances. Salicylic acid, a critical messenger, is also required for both responses. However, the relationship between the CBL-CIPK and SA signaling pathways during wheat–Pst interaction is unclear. In this study, we isolated seven wheat CBL and 11 wheat CIPK genes and designated them as TaCBL1, 2, 3, 4, 6, 7, 9 and TaCIPK2, 5, 7, 9, 10, 14, 15, 17, 23, 31, 32. Some wheat CBLs and CIPKs were functionally characterized. Concurrently, wheat TaNPR1 as a master regulator of SA-mediated host response during Pst infection was functionally characterized. Silencing of TaCBL4, TaCIPK10 and TaNPR1 permitted increased rust development in a wheat variety that was resistant to Pst pathotype CYR23. Decreased levels of salicylic acid (SA) were observed in TaCBL4- and TaCIPK10-silenced wheat plants. Yeast two-hybrid and biomolecular fluorescence complementation (BiFC) revealed that TaCIPK10 interacted with both TaCBL4 and TaNPR1. These results suggest that a TaCBL4-TaCIPK10-TaNPR1 complex is involved in innate immunity of wheat to Pst.