Rapid mini-chromosome divergence among fungal isolates causing wheat blast outbreaks in Bangladesh and Zambia.

The fungal pathogen, Magnaporthe oryzae Triticum pathotype, causing wheat blast disease was first identified in South America and recently spread across continents to South Asia and Africa. Here, we studied the genetic relationship among isolates found on the three continents. Magnaporthe oryzae strains closely related to a South American field isolate B71 were found to have caused the wheat blast outbreaks in South Asia and Africa. Genomic variation among isolates from the three continents was examined using an improved B71 reference genome and whole-genome sequences. We found strong evidence to support that the outbreaks in Bangladesh and Zambia were caused by the introductions of genetically separated isolates, although they were all close to B71 and, therefore, collectively referred to as the B71 branch. In addition, B71 branch strains carried at least one supernumerary mini-chromosome. Genome assembly of a Zambian strain revealed that its mini-chromosome was similar to the B71 mini-chromosome but with a high level of structural variation. Our findings show that while core genomes of the multiple introductions are highly similar, the mini-chromosomes have undergone marked diversification. The maintenance of the mini-chromosome and rapid genomic changes suggest the mini-chromosomes may serve important virulence or niche adaptation roles under diverse environmental conditions.

QTL Pyramiding Provides Marginal Improvement in 2NvS-Based Wheat Blast Resistance.

Wheat blast, caused by the fungus Magnaporthe oryzae Triticum pathotype (MoT), is a devastating disease affecting South America, Bangladesh, and Zambia. Resistance to wheat blast has strongly relied on the 2NvS translocation; however, newer MoT isolates have increased aggressiveness, threatening the 2NvS translocation's effectiveness and durability. To identify genomic regions associated with wheat blast resistance, we performed a quantitative trait loci (QTL) mapping study using 187 double-haploid (DH) lines from a cross between the Brazilian wheat cultivars 'TBIO Alvorada' and 'TBIO Sossego', which are moderately resistant and susceptible to blast, respectively. The DH population was evaluated in a greenhouse in Brazil and Bolivia, and field conditions in Bolivia. Contrasting models best explained the relationship between traits evaluated according to differences in disease levels and the presence of the 2NvS. A large effect-locus, derived from 'TBIO Sossego', was identified on chromosome 2AS, which was confirmed to be 2NvS translocation and explained 33.5 to 82.4% of the phenotypic variance. Additional significant loci were identified on 5AL, 1DS, 4DS, 5DL, and 6DL chromosome arms with phenotypic variance <6%, but they were not consistent across trait-environment combinations. QTL pyramiding analyses showed that some specific loci had an additive effect when combined with the 2NvS, suggesting that stacking multiple loci may be an effective strategy to help manage wheat blast. The markers associated with the 2NvS can be used as dominant diagnostic markers for this alien translocation. Additional characterization of these loci using a broader set of MoT isolates is critical to validate their effectiveness against current MoT populations.

Novel Sources of Wheat Head Blast Resistance in Modern Breeding Lines and Wheat Wild Relatives.

Wheat head blast (WHB), caused by the fungus Magnaporthe oryzae pathotype triticum, is a devastating disease affecting South America and South Asia. Despite 30 years of intensive effort, the 2NVS translocation from Aegilops ventricosa contains the only useful source of resistance to WHB effective against M. oryzae triticum isolates. The objective of this study was to identify non-2NVS sources of resistance to WHB among elite cultivars, breeding lines, landraces, and wild-relative accessions. Over 780 accessions were evaluated under field and greenhouse conditions in Bolivia, greenhouse conditions in Brazil, and at two biosafety level-3 laboratories in the United States. The M. oryzae triticum isolates B-71 (2012), 008 (2015), and 16MoT001 (2016) were used for controlled experiments, while isolate 008 was used for field experiments. Resistant and susceptible checks were included in all experiments. Under field conditions, susceptible spreaders were inoculated at the tillering stage to guarantee sufficient inoculum. Disease incidence and severity were evaluated as the average rating for each 1-m-row plot. Under controlled conditions, heads were inoculated after full emergence and individually rated for percentage of diseased spikelets. The diagnostic marker Ventriup-LN2 was used to test for the presence of the 2NVS translocation. Four non-2NVS spring wheat International Maize and Wheat Improvement Center breeding lines (CM22, CM49, CM52, and CM61) and four wheat wild-relatives (A. tauschii TA10142, TA1624, TA1667, and TA10140) were identified as resistant (<5% of severity) or moderately resistant (5 to <25% severity) to WHB. Experiments conducted at the seedling stage showed little correlation with disease severity at the head stage. M. oryzae triticum isolate 16MoT001 was significantly more aggressive against 2NVS-based varieties. The low frequency of WHB resistance and the increase in aggressiveness of newer M. oryzae triticum isolates highlight the threat that the disease poses to wheat production worldwide and the urgent need to identify and characterize new resistance genes that can be used in breeding for durably resistant varieties.