Phenotypic mechanisms of host resistance against greenbug (Homoptera: Aphididae) revealed by near isogenic lines of wheat

Evaluation of aphid resistance on different rose cultivars and transcriptome analysis in response to aphid infestation

BACKGROUND

The rose is one of the most important ornamental flowers in the world for its aesthetic beauty but can be attacked by many pests such as aphids. Aphid infestation causes tremendous damage on plant tissues leading to harmed petals and leaves. Rose cultivars express different levels of resistance to aphid infestation yet the information remains unclear. Not only that, studies about the transcriptional analysis on defending mechanisms against aphids in rose are limited so far.

RESULTS

In this study, the aphid resistance of 20 rose cultivars was evaluated, and they could be sorted into six levels based on the number ratio of aphids. And then, a transcriptome analysis was conducted after aphid infestation in one high resistance (R, Harmonie) and one highly susceptibility (S, Carefree Wonder) rose cultivar. In open environment the majority of rose cultivars had the highest aphid number at May 6th or May 15th in 2020 and the resistance to infestation could be classified into six levels. Differential expression analysis revealed that there were 1,626 upregulated and 767 downregulated genes in the R cultivar and 481 upregulated and 63 downregulated genes in the S cultivar after aphid infestation. Pathway enrichment analysis of the differentially expressed genes revealed that upregulated genes in R and S cultivars were both enriched in defense response, biosynthesis of secondary metabolites (phenylpropanoid, alkaloid, and flavonoid), carbohydrate metabolism (galactose, starch, and sucrose metabolism) and lipid processing (alpha-linolenic acid and linolenic acid metabolism) pathways. In the jasmonic acid metabolic pathway, linoleate 13S-lipoxygenase was specifically upregulated in the R cultivar, while genes encoding other crucial enzymes, allene oxide synthase, allene oxide cyclase, and 12-oxophytodienoate reductase were upregulated in both cultivars. Transcription factor analysis and transcription factor binding search showed that WRKY transcription factors play a pivotal role during aphid infestation in the R cultivar.

CONCLUSIONS

Our study indicated the potential roles of jasmonic acid metabolism and WRKY transcription factors during aphid resistance in rose, providing clues for future research.

Untapped Sources of Dual Resistance to Hessian Fly and Greenbug in Synthetic Hexaploid Wheats

The Hessian fly (Hf) and greenbugs (Gb) are major pests of wheat, causing severe economic losses globally. Deploying resistant wheat is the most effective strategy for managing these destructive insects. However, the resistance is not effective against all Hf or Gb biotypes and can impose selection pressure on insects, resulting in the development of virulent biotypes. These challenges must be met through the discovery of new and novel sources of resistance to these pests. Synthetic Hexaploid Wheat (SHW)-developed cultivars are a rich source of resistance against a diverse array of pathogens and pests. In this study, 80 SHW lines were evaluated for their resistance to Hf and Gb under controlled environmental conditions. Of these, a total of 36 SHW lines showed resistance independently to Hf biotype L and Gb biotype E, while 27 lines showed combined resistance to both Hf and Gb. Further, a subset of 10 SHW lines showed resistance to additional Hf biotypes, Great Plains and vH13. The identification of SHW lines resistant to multiple insects and biotypes offers an invaluable resource to breeders who are looking to stack resistance traits to develop elite cultivars as a strategy to alleviate economic impacts upon global wheat production.

Genetic Resources of Cereal Crops for Aphid Resistance

The genetic resources of cereal crops in terms of resistance to aphids are reviewed. Phytosanitary destabilization led to a significant increase in the harmfulness of this group of insects. The breeding of resistant plant genotypes is a radical, the cheapest, and environmentally safe way of pest control. The genetic homogeneity of crops hastens the adaptive microevolution of harmful organisms. Both major and minor aphid resistance genes of cereal plants interact with insects differentially. Therefore, rational breeding envisages the expansion of the genetic diversity of cultivated varieties. The possibilities of replenishing the stock of effective resistance genes by studying the collection of cultivated cereals, introgression, and creating mutant forms are considered. The interaction of insects with plants is subject to the gene-for-gene relationship. Plant resistance genes are characterized by close linkage and multiple allelism. The realizing plant genotype depends on the phytophage biotype. Information about the mechanisms of constitutional and induced plant resistance is discussed. Resistance genes differ in terms of stability of expression. The duration of the period when varieties remain resistant is not related either to its phenotypic manifestation or to the number of resistance genes. One explanation for the phenomenon of durable resistance is the association of the virulence mutation with pest viability.

Transcriptomics of induced defense responses to greenbug aphid feeding in near isogenic wheat lines

The greenbug aphid, Schizaphis graminum (Rondani) is an important cereal pest, periodically threatening wheat yields in the United States and around the world. The single dominant gene, Gb3-based resistance is highly durable against prevailing greenbug biotypes under field conditions; however, the molecular mechanisms of Gb3-mediated defense responses remain unknown. We used Affymetrix GeneChip Wheat Genome Arrays to investigate the transcriptomics of host defense responses upon greenbug feeding on resistant and susceptible bulks (RB and SB, respectively) derived from two near-isogenic lines. The study identified 692 differentially expressed transcripts and further functional classification recognized 122 transcripts that are putatively associated to mediate biotic stress responses. In RB, Gb3-mediated resistance resulted in activation of transmembrane receptor kinases and signaling-related transcripts involved in early signal transduction cascades. While in SB, transcripts mediating final steps in jasmonic acid biosynthesis, redox homeostasis, peroxidases, glutathione S-transferases, and notable defense-related secondary metabolites were induced. Also transcripts involved in callose and cell wall decomposition were elevated SB, plausibly to facilitate uninterrupted feeding operations. These results suggest that Gb3-mediated resistance is less vulnerable to cell wall modification and the data provides ample tools for further investigations concerning R gene based model of resistance.

Quantitative differences in aphid virulence and foliar symptom development on tomato plants carrying the Mi resistance gene

The Mi resistance gene in tomato reduces the feeding, fecundity, and survival of certain isolates of the potato aphid (Macrosiphum euphorbiae Thomas). This study compared the performance of two potato aphid isolates, WU11 and WU12, on nearly isogenic susceptible (Mi-) and resistant (Mi+) tomato cultivars. Although Mi significantly reduced the population growth of both aphids, WU12 numbers decreased by only 15% compared with 95% for isolate WU11. These results show that there are quantitative differences in virulence among potato aphid isolates. Compared with WU11 aphids, isolate WU12 caused more necrosis on both resistant and susceptible plants, and this increased damage may play a role in the partial virulence of isolate WU12. However, infestation with aphid isolate WU12 did not compromise plant defenses against isolate WU11 in resistant plants. Prior inoculation with either aphid isolate caused a modest reduction in the survival of WU12 adults, but this form of induced resistance was observed on both resistant and susceptible cultivars. Thus, Mi did not play a role in acquired resistance or mediate any indirect interactions between the two aphid isolates. Notably, the mode of action of Mi-mediated resistance seemed to differ depending on the aphid isolate tested. Mi dramatically deterred feeding by WU11 aphids, whereas the effects of resistance on isolate WU12 seemed to be caused primarily by antibiosis. Tolerance did not seem to be a major component of Mi-mediated responses, although resistant plants showed a modest reduction in the amount of foliar necrosis induced per aphid compared with susceptible plants.

Characterization of resistance to multiple aphid species (Hemiptera: Aphididae) in Medicago truncatula

Aphids are phloem-feeding insects that damage many important crops throughout the world yet, compared to plant-pathogen interactions, little is known about the mechanisms by which plants become resistant to aphids. Medicago truncatula (barrel medic) is widely considered as the pre-eminent model legume for genetic and biological research and in Australia is an important pasture species. Six cultivars of M. truncatula with varying levels of resistance to two pests of pasture and forage legumes, the bluegreen aphid Acyrthosiphon kondoi Shinji and the spotted alfalfa aphid Therioaphis trifolii f. maculata. (Buckton) are investigated. Two resistance phenotypes against T. trifolii f. maculata are described, one of which is particularly effective, killing most aphids within 24 h of infestation. Each resistance phenotype provided a similar but somewhat less effective degree of resistance to the closely-related spotted clover aphid Therioaphis trifolii (Monell). In the case of A. kondoi only one resistance phenotype was observed, which did not vary among different genetic backgrounds. None of the observed resistance against A. kondoi or T. trifolii f. maculata significantly affected the performance of green peach aphid Myzus persicae (Sulzer) or cowpea aphid Aphis craccivora Koch. The existence of multiple aphid resistance mechanisms in similar genetic backgrounds of this model plant provides a unique opportunity to characterize the fundamental basis of plant defence to these serious agricultural pests.

Microsatellite markers associated with two Aegilops tauschii-derived greenbug resistance loci in wheat

A new source of greenbug (Schizaphis graminum Rondani) resistance derived from Aegilops tauschii (Coss.) Schmal was identified in W7984, a synthetic hexaploid wheat line and one parent of the International Triticeae Mapping Initiative (ITMI) mapping population. Segregation analysis of responses to greenbug feeding in a set of recombinant inbred lines (RILs) identified a single, dominant gene governing the greenbug resistance in W7984, which was placed in chromosome arm 7DL by linkage analysis with molecular markers in the ITMI population. Allelism tests based on the segregation of responses to greenbug feeding in F2 and testcross plants revealed that the greenbug resistance in W7984 and Largo, another synthetic line carrying the greenbug resistance gene Gb3, was controlled by different but linked loci. Using the ITMI reference map and a target mapping strategy, we have constructed a microsatellite map of Gb3 in a mapping population of 130 F7 RILs from Largo x TAM 107 and identified one marker (Xwmc634) co-segregating with Gb3 and four markers (Xbarc76, Xgwm037, Xgwm428 and Xwmc824) closely linked with Gb3. Deletion mapping of selected microsatellite markers flanking the Gb3 locus placed this resistance gene into the distal 18% region of 7DL. Comparative mapping in the ITMI and Largo x TAM 107 populations using the same set of microsatellite markers provided further evidence that greenbug resistance in W7984 and Largo is conditioned by two different loci. We suggest that the greenbug resistance gene in W7984 be designated Gb7. The microsatellite map of Gb3 constructed from this study should be a valuable tool for marker-assisted selection of Gb3-conferred greenbug resistance in wheat breeding.