Untargeted metabolomics profiling of oat (Avena sativa L.) and wheat (Triticum aestivum L.) infested with wheat stem sawfly (Cephus cinctus Norton) reveals differences associated with plant defense and insect nutrition.

Introduction: Wheat stem sawfly (WSS), Cephus cinctus Norton, is a major pest of common bread wheat (Triticum aestivum L.) and other cultivated cereals in North America. Planting of cultivars with solid stems has been the primary management strategy to prevent yield loss due to WSS infestation, however expression of this phenotype can vary depending on environmental conditions and solid stems hinder biological control of WSS via braconid parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck. In the hollow stems of oat (Avena sativa L.), WSS larvae experience 100% mortality before they reach late instars, but the mechanisms for this observed resistance have not been characterized. Objective: The objective of this study was to explore additional sources of resistance outside of the historic solid stem phenotype. Methods: Here, we use an untargeted metabolomics approach to examine the response of the metabolome of two cultivars of oat and four cultivars of spring wheat to infestation by WSS. Using liquid chromatography-mass spectrometry (LC-MS), differentially expressed metabolites were identified between oat and wheat which were associated with the phenylpropanoid pathway, phospholipid biosynthesis and signaling, the salicylic acid signaling pathway, indole-3-acetic acid (IAA) degradation, and biosynthesis of 1,4-benzoxazin-3-ones (Bxs). Several phospho- and galacto- lipids were found in higher abundance in oat, and with the exception of early stem solidness cultivar Conan, both species experienced a decrease in abundance once infested. In all wheat cultivars except Conan, an increase in abundance was observed for Bxs HMDBOA-glc and DIBOA-ß-D-glucoside after infestation, indicating that this pathway is involved in wheat response to infestation in both solid and hollow stemmed cultivars. Differences between species in compounds involved in IAA biosynthesis, degradation and inactivation suggest that wheat may respond to infestation by inactivating IAA or altering the IAA pool in stem tissue. Conclusion: We propose that the species differences found here likely affect the survival of WSS larvae and may also be associated with differences in stem architecture at the molecular level. Our findings suggest pathways to focus on for future studies in elucidating plant response to WSS infestation.

Analysis of recombinant inbred line populations derived from wheat landraces to identify new genes for wheat stem sawfly resistance.

Wheat landrace accessions were chosen from areas of the world with historical European wheat stem sawfly (Cephus pygmaeus L.) selection pressure to develop six recombinant inbred line (RIL) populations. Molecular maps were constructed, and resistance due to antibiosis and antixenosis was assessed at sites in Montana naturally infested by Cephus cinctus Norton, the wheat stem sawfly (WSS). Novel QTLs were identified along with QTL previously identified in elite germplasm. A newly identified QTL on chromosome 1B provided a new source for pith-filled solid stems. An allele for resistance on chromosome 4A unrelated to solid stems was identified in four of the six RIL populations. A landrace from Turkey, PI 166471, contained alleles at three QTLs causing high levels of larval mortality. None of the QTLs were related to stem solidness, but their combined effect provided resistance similar to that observed in a solid-stemmed check cultivar. These results show the utility of genetic populations derived from geographically targeted landrace accessions to identify new alleles for insect resistance. New PCR-based molecular markers were developed for introgression of novel alleles for WSS resistance into elite lines. Comparison of results with previous analysis of elite cultivars addresses changes in allele frequencies during the wheat breeding process.

A Novel QTL in Durum Wheat for Resistance to the Wheat Stem Sawfly Associated with Early Expression of Stem Solidness.

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.

Correction to: A large-scale multiomics analysis of wheat stem solidness and the wheat stem sawfly feeding response, and syntenic associations in barley, Brachypodium, and rice.

The original version of this article contained a mistake. The word "RefSeq v.1" was incorrectly inserted on page 7. The correct sentence should be: To identify the differentially regulated transcripts, clean RNA-Seq reads were mapped onto the T. aestivum Chinese Spring chromosome 3B pseudomolecule.

A large-scale multiomics analysis of wheat stem solidness and the wheat stem sawfly feeding response, and syntenic associations in barley, Brachypodium, and rice.

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.

Characterization of Resistance to Cephus cinctus (Hymenoptera: Cephidae) in Barley Germplasm.

Most barley cultivars have some degree of resistance to the wheat stem sawfly (WSS), Cephus cinctus Norton (Hymenoptera: Cephidae). Damage caused by WSS is currently observed in fields of barley grown in the Northern Great Plains, but the impact of WSS damage among cultivars due to genetic differences within the barley germplasm is not known. Specifically, little is known about the mechanisms underlying WSS resistance in barley. We characterized WSS resistance in a subset of the spring barley CAP (Coordinated Agricultural Project) germplasm panel containing 193 current and historically important breeding lines from six North American breeding programs. Panel lines were grown in WSS infested fields for two consecutive years. Lines were characterized for stem solidness, stem cutting, WSS infestation (antixenosis), larval mortality (antibiosis), and parasitism (indirect plant defense). Variation in resistance to WSS in barley was compared to observations made for solid-stemmed resistant and hollow-stemmed susceptible wheat lines. Results indicate that both antibiosis and antixenosis are involved in the resistance of barley to the WSS, but antibiosis seems to be more prevalent. Almost all of the barley lines had greater larval mortality than the hollow-stemmed wheat lines, and only a few barley lines had mortality as low as that observed in the solid-stemmed wheat line. Since barley lines lack solid stems, it is apparent that barley has a different form of antibiosis. Our results provide information for use of barley in rotation to control the WSS and may provide a basis for identification of new approaches for improving WSS resistance in wheat.

Tolerance: the forgotten child of plant resistance.

Plant resistance against insect herbivory has greatly focused on antibiosis, whereby the plant has a deleterious effect on the herbivore, and antixenosis, whereby the plant is able to direct the herbivore away from it. Although these two types of resistance may reduce injury and yield loss, they can produce selection pressures on insect herbivores that lead to pest resistance. Tolerance, on the other hand, is a more sustainable pest management strategy because it involves only a plant response and therefore does not cause evolution of resistance in target pest populations. Despite its attractive attributes, tolerance has been poorly studied and understood. In this critical, interpretive review, we discuss tolerance to insect herbivory and the biological and socioeconomic factors that have limited its use in plant resistance and integrated pest management. First, tolerance is difficult to identify, and the mechanisms conferring it are poorly understood. Second, the genetics of tolerance are mostly unknown. Third, several obstacles hinder the establishment of high-throughput phenotyping methods for large-scale screening of tolerance. Fourth, tolerance has received little attention from entomologists because, for most, their primary interest, research training, and funding opportunities are in mechanisms which affect pest biology, not plant biology. Fifth, the efforts of plant resistance are directed at controlling pest populations rather than managing plant stress. We conclude this paper by discussing future research and development activities.

Host plant quantitative trait loci affect specific behavioral sequences in oviposition by a stem-mining insect.

KEY MESSAGE: Genetic diversity in quantitative loci associated with plant traits used by insects as cues for host selection can influence oviposition behavior and maternal choice. Host plant selection for oviposition is an important determinant of progeny performance and survival for phytophagous insects. Specific cues from the plant influence insect oviposition behavior; but, to date, no set of host plant quantitative trait loci (QTLs) have been shown to have an effect on behavioral sequences leading to oviposition. Three QTLs in wheat (Triticum aestivum L.) have been identified as influencing resistance to the wheat stem sawfly (WSS) (Cephus cinctus Norton). Wheat near-isogenic lines (NILs) for each of the three QTLs were used to test whether foraging WSS were able to discriminate variation in plant cues resulting from allelic changes. A QTL on chromosome 3B (Qss-msub-3BL) previously associated with stem solidness and larval antibiosis was shown to affect WSS oviposition behavior, host preference, and field infestation. Decreased preference for oviposition was also related to a QTL allele on chromosome 2D (Qwss.msub-2D). A QTL on chromosome 4A (Qwss.msub-4A.1) affected host plant attractiveness to foraging females, but did not change oviposition preference after females landed on the stem. These findings show that oviposition decisions regarding potential plant hosts require WSS females to discriminate signals from the plant associated with allelic variation at host plant quantitative loci. Allele types in a host plant QTL associated with differential survival of immature progeny can affect maternal choices for oviposition. The multidisciplinary approach used here may lead to the identification of plant genes with important community consequences, and may complement the use of antibiosis due to solid stems to control the wheat stem sawfly in agroecosystems.