Key message

The diploid wheat stem rust resistance gene Sr21 confers temperature-sensitive resistance to isolates of the Ug99 group and maps to the middle of the long arm of chromosome 2A (m). A race of Puccinia graminis f. sp. tritici, the causal pathogen of stem rust of wheat, known as Ug99, and its variants, are virulent to plants carrying stem rust resistance genes currently deployed in most wheat cultivars worldwide. Therefore, identification, mapping and deployment of effective resistance genes are critical to reduce this threat. Resistance gene Sr21 identified in diploid wheat T. monococcum can be effective against races from the Ug99 race group, but both susceptible and partial resistant reactions have been reported in previous studies. To clarify this conflicting information we screened four monogenic lines with Sr21 and four susceptible controls with 16 Pgt isolates including five isolates of the Ug99 race group under three different temperatures and three different photoperiods. We observed that, temperature influences the interaction between monogenic lines with Sr21 and Ug99 race group isolates, and may be one source of previous inconsistencies. This result indicates that, although Sr21 confers partial resistance against Ug99, its effectiveness can be modulated by environmental conditions and should not be deployed alone. Using two large diploid wheat-mapping populations (total 3,788 F2 plants) we mapped Sr21 approximately 50 cM from the centromere on the long arm of chromosome 2A(m) within a 0.20 cM interval flanked by sequence-based markers FD527726 and EX594406. The closely linked markers identified in this study will be useful to reduce the T. monococcum segments introgressed into common wheat, accelerate Sr21 deployment in wheat breeding programs, and facilitate the map-based cloning of this gene.

Instantaneous control over the orientation of anisotropically shaped plasmonic nanostructures allows for selective excitation of plasmon modes and enables dynamic tuning of the plasmonic properties. Herein we report the synthesis of rod-shaped magnetic/plasmonic core-shell nanocomposite particles and demonstrate the active tuning of their optical property by manipulating their orientation using an external magnetic field. We further design and construct an IR-photoelectric coupling system, which generates an output voltage depending on the extinction property of the measured nanocomposite sample. We employ the device to demonstrate that the nanocomposite particles can serve as units for information encryption when immobilized in a polymer film and additionally when dispersed in solution can be employed as a new type of magnetic-field-direction sensor.

Wheat stem rust caused by Puccinia graminis f. sp. tritici, can cause significant yield losses. To combat the disease, breeders have deployed resistance genes both individually and in combinations to increase resistance durability. A new race, TTKSK (Ug99), identified in Uganda in 1999 is virulent on most of the resistance genes currently deployed, and is rapidly spreading to other regions of the world. It is therefore important to identify, map, and deploy resistance genes that are still effective against TTKSK. One of these resistance genes, Sr13, was previously assigned to the long arm of chromosome 6A, but its precise map location was not known. In this study, the genome location of Sr13 was determined in four tetraploid wheat (T. turgidum ssp. durum) mapping populations involving the TTKSK resistant varieties Kronos, Kofa, Medora and Sceptre. Our results showed that resistance was linked to common molecular markers in all four populations, suggesting that these durum lines carry the same resistance gene. Based on its chromosome location and infection types against different races of stem rust, this gene is postulated to be Sr13. Sr13 was mapped within a 1.2-2.8 cM interval (depending on the mapping population) between EST markers CD926040 and BE471213, which corresponds to a 285-kb region in rice chromosome 2, and a 3.1-Mb region in Brachypodium chromosome 3. These maps will be the foundation for developing high-density maps, identifying diagnostic markers, and positional cloning of Sr13.

The stem rust resistance genes Sr24, Sr26, Sr36, and Sr1RSᴬᵐⁱᵍᵒ confer resistance to race TTKSK (= Ug99) of Puccinia graminis f. sp. tritici Pers. (Pgt). A collection of 776 cultivars and breeding lines of wheat (Triticum aestivum L.) from all growing regions of the United States were screened with simple sequence repeat and sequence tagged site markers linked to Sr24, Sr26, Sr36, and Sr1RSᴬᵐⁱᵍᵒ to determine frequencies of these genes in U.S. wheat germplasm. Marker efficacy in predicting the presence of these genes was evaluated via comparison with assayed seedling infection type. Among the lines evaluated, the most predominant gene is Sr24, present in hard winter, hard spring, and soft winter wheat lines. Resistance in soft winter wheat is primarily due to Sr36 The 1RS·1AL rye translocation carrying Sr1RSᴬᵐⁱᵍᵒ is present at equal frequencies in hard winter and soft winter wheat. Utilization of marker-assisted selection for stem rust resistance genes can hasten the development of wheat cultivars resistant to TTKSK and its variants and allow for the development of resistance gene pyramids for more durable stem rust resistance.

Puccinia graminis f. sp. tritici is the causal agent of stem rust of wheat. A new race designated TTKSK (also known as Ug99) and its variants (TTKST and TTTSK) are virulent to most of the stem rust resistance genes currently deployed in wheat cultivars worldwide. Therefore, identification, mapping, and deployment of effective resistance genes are critical components of global efforts to mitigate this threat. Multipathotype seedling tests demonstrated that resistance gene Sr35 is effective against the three TTKS variants and another broadly virulent race from Yemen, TRTTF. Two genetic maps of Sr35 are presented in diploid (Triticum monococcum) and two in hexaploid wheat (T. aestivum). The Sr35 resistance to TRTTF and RKQQC races was mapped in diploid wheat within a 2.2 to 3.1 cM interval on the long arm of chromosome 3Aᵐ between markers XBF483299 and XCJ656351 This interval corresponds to a 174-kb region in Brachypodium that includes 16 annotated genes. The Sr35 map location was confirmed in two backcross-derived hexaploid populations segregating for Sr35 Recombination between diploid and hexaploid chromosomes was 10-fold lower than between homologous chromosomes, but was sufficient to reduce the introgressed diploid segment. These maps provide markers closely linked to Sr35 that will be useful to accelerate its deployment and pyramiding with other stem rust resistance genes.

Legumes form symbiotic associations with either nitrogen-fixing bacteria or arbuscular mycorrhizal fungi. Formation of these two symbioses is regulated by a common set of signalling components that act downstream of recognition of rhizobia or mycorrhizae by host plants. Central to these pathways is the calcium and calmodulin-dependent protein kinase (CCaMK)-IPD3 complex which initiates nodule organogenesis following calcium oscillations in the host nucleus. However, downstream signalling events are not fully understood. Here we show that Medicago truncatula DELLA proteins, which are the central regulators of gibberellic acid signalling, positively regulate rhizobial symbiosis. Rhizobia colonization is impaired in della mutants and we provide evidence that DELLAs can promote CCaMK-IPD3 complex formation and increase the phosphorylation state of IPD3. DELLAs can also interact with NSP2-NSP1 and enhance the expression of Nod-factor-inducible genes in protoplasts. We show that DELLA is able to bridge a protein complex containing IPD3 and NSP2. Our results suggest a transcriptional framework for regulation of root nodule symbiosis.

Stem rust is one of the most destructive diseases of wheat worldwide. The recent emergence of wheat stem rust race Ug99 (TTKS based on the North American stem rust race nomenclature system) and related strains threaten global wheat production because they overcome widely used genes that had been effective for many years. Host resistance is likely to be more durable when several stem rust resistance genes are pyramided in a single wheat variety; however, little is known about the resistance genotypes of widely used wheat germplasm. In this study, a diverse collection of wheat germplasm was haplotyped for stem rust resistance genes Sr2, Sr22, Sr24, Sr25, Sr26, Sr36, Sr40, and 1A.1R using linked microsatellite or simple sequence repeat (SSR) and sequence tagged site (STS) markers. Haplotype analysis indicated that 83 out of 115 current wheat breeding lines from the International Maize and Wheat Improvement Center (CIMMYT) likely carry Sr2. Among those, five out of 94 CIMMYT spring lines tested had both Sr2 and Sr25 haplotypes. Five out of 22 Agriculture Research Service (ARS) lines likely have Sr2 and a few have Sr24, Sr36, and 1A.1R. Two out of 43 Chinese accessions have Sr2. No line was found to have the Sr26 and Sr40 haplotypes in this panel of accessions. DArT genotyping was used to identify new markers associated with the major stem resistance genes. Four DArT markers were significantly associated with Sr2 and one with Sr25. Principal component analysis grouped wheat lines from similar origins. Almost all CIMMYT spring wheats were clustered together as a large group and separated from the winter wheats. The results provide useful information for stem rust resistance breeding and pyramiding.