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The wheat stem sawfly (WSS), Cephus cinctus Norton (Hymenoptera: Cephidae), is an important pest of wheat and other cereals, threatening the quality and quantity of grain production. WSS larvae feed and develop inside the stem where they are protected from the external environment; therefore, pest management strategies primarily rely on host plant resistance. A major locus on the long arm of wheat chromosome 3B underlies most of the variation in stem solidness; however, the impact of stem solidness on WSS feeding has not been completely characterized. Here, we used a multiomics approach to examine the response to WSS in both solid- and semi-solid-stemmed wheat varieties. The combined transcriptomic, proteomic, and metabolomic data revealed that two important molecular pathways, phenylpropanoid and phosphate pentose, are involved in plant defense against WSS. We also detected a general downregulation of several key defense transcripts, including those encoding secondary metabolites such as DIMBOA, tricetin, and lignin, which suggested that the WSS larva might interfere with plant defense. We comparatively analyzed the stem solidness genomic region known to be associated with WSS tolerance in wild emmer, durum, and bread wheats, and described syntenic regions in the close relatives barley, Brachypodium, and rice. Additionally, microRNAs identified from the same genomic region revealed potential regulatory pathways associated with the WSS response. We propose a model outlining the molecular responses of the WSS-wheat interactions. These findings provide insight into the link between stem solidness and WSS feeding at the molecular level. [ABSTRACT FROM AUTHOR]

Several species of wheat stem sawflies ( WSS) are pests of wheat, including Cephus cinctus Norton in North America. Larvae feed inside the stems and cut the stem near plant maturity. The primary means of control is resistance due to solid stems, largely controlled by a locus on chromosome 3B ( Qss.msub-3 BL). Cultivars that differed for WSS resistance, but with similar stem solidness, were crossed to determine the genetic basis for the differences. The cultivar 'Scholar' is susceptible, while 'Conan' shows resistance. Scholar and Conan possessed different alleles at Qss.msub-3 BL. Both alleles conferred solidness, yet the Conan allele conferred higher WSS resistance. An allele from Conan on chromosome 4A also decreased infestation and stem cutting. The 3B and 4A alleles from Conan acted in an additive fashion to provide increased WSS resistance without increasing stem solidness. Stem solidness has long been used by breeders as a proxy for WSS resistance because of its simplicity. Our results suggest that other resistance mechanisms may complement solid stems. [ABSTRACT FROM AUTHOR]

The wheat stem sawfly (WSS) ( Cephus cinctus Norton) is a major yield-reducing pest of wheat ( Triticum aestivum L.). Varieties with pith-filled, or solid, stems provide a measure of resistance by inhibiting larval survival inside the stem. Durum wheat ( Triticum turgidum L.) has resistance to the wheat stem sawfly even in the absence of known genes for stem solidness. To determine the genetic basis of resistance in durum wheat, a susceptible durum wheat, PI 41353, was identified from among 1,211 landrace accessions from around the world screened in WSS-infested sites. A recombinant inbred line (RIL) population of 105 individuals was developed from a cross of PI 41353 with a typically resistant variety, Pierce. The RIL were screened in a total of three WSS-infested locations in Montana over a two year period. A genetic map was constructed with 2,867 SNP-based markers. A quantitative trait locus (QTL) analysis identified six QTL associated with resistance. An allele from resistant cultivar Pierce at a QTL on chromosome 3A, Qss.msub-3AL , caused a 25% reduction in stem cutting. Assessment of near-isogenic lines that varied for alleles at Qss.msub-3AL showed that the Pierce allele was also associated with higher stem solidness as measured early in stem development, which is a critical stage for WSS oviposition and larval development. Stem solidness of Pierce and other resistant durum wheat lines largely disappeared later in plant development. Results suggest a genetic mechanism for WSS resistance observed in durum wheat, and provide an additional source of WSS resistance for hexaploid bread wheat., (Copyright © 2019 Varella et al.)