Discovery of English grain aphid (Hemiptera: Aphididae) biotypes in China

SmCSP4 from aphid saliva stimulates salicylic acid-mediated defence responses in wheat by interacting with transcription factor TaWKRY76

Aphid salivary proteins are critical in modulating plant defence responses. Grain aphid Sitobion miscanthi is an important wheat pest worldwide. However, the molecular basis for the regulation of the plant resistance to cereal aphids remains largely unknown. Here, we show that SmCSP4, a chemosensory protein from S. miscanthi saliva, is secreted into wheat plants during aphid feeding. Delivery of SmCSP4 into wheat leaves activates salicylic acid (SA)-mediated plant defence responses and subsequently reduces aphid performance by deterring aphid feeding behaviour. In contrast, silencing SmCSP4 gene via nanocarrier-mediated RNAi significantly decreases the ability of aphids to activate SA defence pathway. Protein-protein interaction assays showed that SmCSP4 directly interacts with wheat transcriptional factor TaWRKY76 in plant nucleus. Furthermore, TaWRKY76 directly binds to the promoter of SA degradation gene Downy Mildew Resistant 6 (DMR6) and regulates its gene expression as transcriptional activator. SmCSP4 secreted by aphids reduces the transcriptional activation activity of TaWRKY76 on DMR6 gene expression, which is proposed to result in increases of SA accumulation and enhanced plant immunity. This study demonstrated that SmCSP4 acts as salivary elicitor that is involved in activating SA signalling defence pathway of wheat by interacting with TaWRKY76, which provide novel insights into aphid-cereal crops interactions and the molecular mechanism on induced plant immunity.

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.

Responses of Six Wheat Cultivars (Triticum aestivum) to Wheat Aphid (Sitobion avenae) Infestation

Simple Summary

Sitobion avenae Fabricius is an important wheat aphid species in China, causing significant losses to wheat production. Improving host-plant resistance is an effective and environmentally friendly method of aphid control. Sitobion avenae resistance and the total phenolic and flavonoid content accumulation of six wheat cultivars to S. avenae infestation were investigated to elucidate responses of six wheat varieties against S. avenae. Among the six tested wheat cultivars, Yongliang No.15 and Ganchun No.18 demonstrated high resistance to S. avenae. The correlation analysis revealed a positive relationship between total phenol and flavonoid content accumulation and developmental duration (DD), and a negative relationship between accumulation and weight gain (WG) and mean relative growth rate (MRGR). The correlation between flavonoid and biological parameters was statistically stronger than total phenol. Our findings could serve as a theoretical basis for further research into the resistance mechanism of wheat varieties to S. avenae.

Abstract

Resistant variety screening is widely recommended for the management of Sitobion avenae. The purpose of this study was to assess responses of six wheat varieties (lines) to S. avenae. The aphid quantity ratio (AQR) was used to assess S. avenae resistance. Pearson’s correlation coefficient was used to perform a correlation analysis between AQR, biological parameters, and the accumulation of total phenolic and flavonoid content. When compared to the other cultivars, the results showed that two cultivars, Yongliang No.15 and Ganchun No.18, had high resistance against S. avenae. The correlation analysis revealed a positive relationship between total phenol and flavonoid content accumulation and developmental duration (DD), and a negative relationship between accumulation and weight gain (WG) and mean relative growth rate (MRGR). The correlation between flavonoid and biological parameters was statistically stronger than the correlation between total phenol and biological parameters. This research provides critical cues for screening and improving aphid-resistant wheat varieties in the field and will aid in our understanding of the resistance mechanism of wheat varieties against S. avenae.

Screening and Evaluation for Antixenosis Resistance in Wheat Accessions and Varieties to Grain Aphid, Sitobion miscanthi (Takahashi) (Hemiptera: Aphididae)

The grain aphid, Sitobion miscanthi causes serious damage by removing nutritional content from wheat plants and transmitting viral diseases. The use of resistant wheat cultivars is an effective method of aphid management. To identify S. miscanthi resistant cultivars, preliminary antixenosis resistance screening was conducted on 112 Ethiopian and 21 Chinese wheat accessions and varieties along with bioassay to test for further antixenosis resistance, identification of aphid feeding behavior using electrical penetration graph (EPG), and imaging of leaf trichome densities using a 3D microscope. According to antixenosis resistance screening, one highly-resistant, 25 moderately-resistant, and 38 slightly-resistant wheat cultivars to S. miscanthi were identified. Aphid choice tests showed that Luxuan266, 243726, and 213312 were the least preferred after 12, 24, 48, and 72 h of S. miscanthi release. Longer duration of Np, longer time to first probe, and shorter duration of E2 waveforms were recorded in Lunxuan266, 243726, and 213312 than in Beijing 837. The trichome density on adaxial and abaxial leaf surfaces of Lunxuan266, 243726 and 213312 was significantly higher than on those of Beijing 837. We concluded that Lunxuan266, 243726, and 213312 were antixenosis resistant to S. miscanthi based on the choice test, EPG results, and leaf trichome densities.

Temporal analysis of microRNAs associated with wing development in the English grain aphid, Sitobion avenae (F.) (Homoptera: Aphidiae)

Molecular mechanisms underlying wing evolution and development have been a point of scientific inquiry for decades. Phloem-feeding aphids are one of the most devastating global insect pests, where dispersal of winged morphs lead to annual movements, migrations, and range expansions. Aphids show a polyphenic wing dimorphism trait, and offer a model to study the role of environment in determining morphological plasticity of a single genotype. Despite recent progresses in the genetic understanding of wing polyphenism, the influence of environmental cues remains unclear. To investigate the involvement of miRNAs in wing development, we sequenced small RNA libraries of the English grain aphid, Sitobion avenae (F.) across six different developmental stages. As a result, we identified 113 conserved and 193 S. avenae-specific miRNAs. Gene Ontology and KEGG pathway analyses of putative target mRNAs for the six differentially expressed miRNAs are enriched for wing development processes. Dietary uptake of miR-263a, miR-316, and miR-184a agomirs and antagomirs led to significantly higher mortality (>70%) and a lower proportion of winged morphs (<5%). On the other hand, wing malformation was observed in miR-2 and miR-306 agomirs and miR-2 and miR-14 antagomirs, respectively, suggesting their involvement in S. avenae wing morphogenesis. These combined results not only shed light on the regulatory role of miRNAs in wing dimorphism, but also provide potential novel targets for the long-term sustainable management of S. avenae, a devastating global grain pest.

Impacts of Constitutive and Induced Benzoxazinoids Levels on Wheat Resistance to the Grain Aphid (Sitobion avenae)

Benzoxazinoids are important secondary metabolites in gramineae plants and have inhibitory and toxic effects against a wide range of herbivore pests. However, the relationship between benzoxazinoid level and plant resistance to aphids remains controversial. In this study, we investigated the relationship between benzoxazinoids composition and concentration in wheat leaves and the resistance to the grain aphid Sitobion avenae. Overall, six benzoxazinoids were detected and identified by mass spectrometry based metabolites profiling, including three lactams, two hydroxamic acids, and one methyl derivative. The constitutive levels of these benzoxazinoids were significantly different among the wheat varieties/lines. However, none of these benzoxazinoids exhibited considerable correlation with aphid resistance. S. avenae feeding elevated the level of 2-O-β-D-glucopyranosyloxy-4,7-dimethoxy-(2H)-1,4-benzoxazin-3(4H)-one (HDMBOA-Glc) and reduced the level of 2-O-β-D-glucopyranosyloxy-4-hydroxy-7-(2H)-methoxy-1,4-benzoxazin-3(4H)-one (DIMBOA-Glc) in some of the wheat varieties/lines. Moreover, aphid-induced level of DIMBOA-Glc was positively related with callose deposition, which was closely associated with aphid resistance. Wheat leaves infiltrated with DIMBOA-Glc caused a noticeable increase of callose deposition and the effect was in a dose dependent manner. This study suggests that the constitutive level of benzoxazinoids has limited impact on S. avenae. Aphid feeding can affect the balance of benzoxazinoids metabolism and the dynamic level of benzoxazinoids can act as a signal of callose deposition for S. avenae resistance. This study will extend our understanding of aphid–wheat interaction and provides new insights in aphid-resistance wheat breeding.

Identification and Genetic Differentiation of Sitobion avenae (Hemiptera: Aphididae) Biotypes in China

The development of biotypes of the cereal aphid, Sitobion avenae (Fabricius) (Hemiptera: Aphididae), was initially found only on wheat, but barley can also be critical in the process. To address this issue, S. avenae clones were collected on barley and wheat, genotyped with six microsatellite markers, and tested with 58 wheat/barley varieties. Based on the virulence response profiles on different resistant wheat/barley varieties and three susceptible controls, six biotypes of S. avenae were identified. We developed a new system to distinguish between S. avenae biotypes by using only five barley/wheat varieties (i.e., barley: Dulihuang, Zaoshu No.3, Xiyin No.2; wheat: Zhong 4 wumang, 186-TM12-34). The unique virulence profiles of different S. avenae biotypes were further verified by testing their life-history traits (i.e., 10-d fecundity and total developmental time of nymphs) on the abovementioned five barley/wheat varieties. Among all the identified biotypes, biotype 1 was predominant, occupying over 82% of the total in each province. Biotype 5 was found only in Xinjiang, whereas biotype 6 occurred only in Zhejiang. The principal coordinate analysis with microsatellite data suggested apparently low genetic differentiation between biotypes 1 and 2. In most cases, extents of genetic divergence between different S. avenae biotypes could reflect differences in virulence response profiles of these biotypes, implying a genetic component for evolutionary relationships among these biotypes. Our study provides insights into the development and evolution of aphid biotypes, and a firm basis for clarifying the underlying genetic and evolutionary mechanisms.

Transcriptome Profiling Revealed Potentially Critical Roles for Digestion and Defense-Related Genes in Insects' Use of Resistant Host Plants: A Case Study with Sitobion Avenae

Using host plant resistance (HPR) in management of insect pests is often environmentally friendly and suitable for sustainable development of agricultural industries. However, this strategy can be limited by rapid evolution of insect populations that overcome HPR, for which the underlying molecular factors and mechanisms are not well understood. To address this issue, we analyzed transcriptomes of two distinct biotypes of the grain aphid, Sitobion avenae (Fabricius), on wheat and barley. This analysis revealed a large number of differentially expressed genes (DEGs) between biotypes 1 and 3 on wheat and barley. The majority of them were common DEGs occurring on both wheat and barley. GO and KEGG enrichment analyses for these common DEGs demonstrated significant expression divergence between both biotypes in genes associated with digestion and defense. Top defense-related common DEGs with the most significant expression changes included three peroxidases, two UGTs (UDP-glycosyltransferase), two cuticle proteins, one glutathione S-transferases (GST), one superoxide dismutase, and one esterase, suggesting their potentially critical roles in the divergence of S. avenae biotypes. A relatively high number of specific DEGs on wheat were identified for peroxidases (9) and P450s (8), indicating that phenolic compounds and hydroxamic acids may play key roles in resistance of wheat against S. avenae. Enrichment of specific DEGs on barley for P450s and ABC transporters suggested their key roles in this aphid's detoxification against secondary metabolites (e.g., alkaloids) in barley. Our results can provide insights into the molecular factors and functions that explain biotype adaptation in insects and their use of resistant plants. This study also has significant implications for developing new resistant cultivars, developing strategies that limit rapid development of insect biotypes, and extending resistant crop cultivars' durability and sustainability in integrated management programs.

Clonal Diversity and Genetic Differentiation of Sitobion avenae (Hemiptera: Aphididae) From Wheat and Barley in China

The English grain aphid, Sitobion avenae (Fabricius), is a cosmopolitan insect pest on cereals. Many studies on life-history traits indicate that S. avenae clones from different areas have diverged on various host plants. However, direct genetic evidence for this phenomenon is rare. Thus, S. avenae clones were collected from barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) in four provinces (i.e., Hubei, Henan, Jiangsu, and Zhejiang) of China, and characterized using six microsatellite markers. In total, 92 multilocus genotypes were found from 302 individuals of S. avenae. Population Jiangsu was found to have relatively high levels of genotypic diversity among the four geographical populations. Substantial long-distance migration of S. avenae was found from Zhejiang to the other three provinces. Thus, relatively low genetic differentiation was found between these geographic populations. Barley clones of S. avenae showed higher gene diversity compared with wheat clones. The gene flow from barley to wheat clones appeared to be more likely than that in the reverse direction. Diversity indices and structure for S. avenae clones suggested highest level of genetic divergence between barley and wheat clones in Jiangsu among all sampling locations. Besides Jiangsu, pairwise FST values indicated moderate levels of genetic divergence between barley and wheat clones in Zhejiang. Thus, compared with geographical factors, plant factors could be relatively more important in promoting genetic differentiation in S. avenae. Our results provide insights into genetic differentiation of S. avenae on different plants, as well as a basis for exploring the molecular mechanism for its differentiation on plants and biotype development.

Maternal effects of the English grain aphids feeding on the wheat varieties with different resistance traits

The maternal effects of the English grain aphid, Sitobion avenae on offspring phenotypes and performance on wheat varieties with different resistance traits were examined. We found that both conditioning wheat varieties(the host plant for over 3 months) and transition wheat varieties affected the biological parameters of aphid offspring after they were transferred between wheat varieties with different resistance traits. The conditioning varieties affected weight gain, development time (DT), and the intrinsic rate of natural increase (rm), whereas transition varieties affected the fecundity, rm, net reproductive rate, and fitness index. The conditioning and transition wheat varieties had significant interaction effects on the aphid offspring's DT, mean relative growth rate, and fecundity. Our results showed that there was obvious maternal effects on offspring when S. avenae transferred bwteen wheat varieties with different resistance level, and the resistance traits of wheat varieties could induce an interaction between the conditioning and transition wheat varieties to influence the growth, development, reproduction, and even population dynamics of S. avenae. The conditioning varieties affected life-history traits related to individual growth and development to a greater extent, whereas transition varieties affected fecundity and population parameters more.

Geographical Variations in the Life Histories of Rhopalosiphum padi (Hemiptera: Aphididae) in China

Population dynamics of an insect can be affected by its life-history traits. Understanding of the life-history variations among geographical populations of an insect pest is important for developing the corresponding control strategies in different regions. Rhopalosiphum padi is a cosmopolitan cereal pest, and life-history traits identification of R. padi from different geographical regions is rare. Here, we experimentally evaluated variations in the life histories and life-table parameters of R. padi populations from six widely ranging geographical regions, including Baoding (HB), Xianyang (SX), Nanyang (HN), Wuhan (HW), Beibei (CB), and Guiyang (GG), in China. Total longevity was similar for HB, CB, and GG populations and generally shorter than that of the SX, HN, and HW populations. The age at first reproduction of HB and HW populations was significantly different from that of the other four populations. Mean fecundity of the SX population was significantly higher than that of the other five populations, and the mean fecundity of the GG population was the lowest. Principal component analysis (PCA) also showed that the life-history and life-table parameters of R. padi differed among the six geographical populations. According to PCA results, the HW population was in one cluster; the GG population was in another cluster; the SX, HN, and CB populations were in one cluster; and the HB population differed from the above three clusters. The complexity of local genetic adaptation in aphids was discussed. The current results will likely be of importance to management practices and population dynamics forecasting of R. padi.

Resistance of Wheat Accessions to the English Grain Aphid Sitobion avenae

The English grain aphid, Sitobion avenae, is a major pest species of wheat crops; however, certain varieties may have stronger resistance to infestation than others. Here, we investigated 3 classical resistance mechanisms (antixenosis, antibiosis, and tolerance) by 14 wheat varieties/lines to S. avenae under laboratory and field conditions. Under laboratory conditions, alatae given the choice between 2 wheat varieties, strongly discriminated against certain varieties. Specifically, the 'Amigo' variety had the lowest palatability to S. avenae alatae of all varieties. 'Tm' (Triticum monococcum), 'Astron,' 'Xanthus,' 'Ww2730,' and 'Batis' varieties also had lower palatability than other varieties. Thus, these accessions may use antibiosis as the resistant mechanism. In contrast, under field conditions, there were no significant differences in the number of alatae detected on the 14 wheat varieties. One synthetic line (98-10-30, a cross between of Triticum aestivum (var. Chris) and Triticum turgidum (var. durum) hybridization) had low aphid numbers but high yield loss, indicating that it has high antibiosis, but poor tolerance. In comparison, 'Amigo,' 'Xiaoyan22,' and some '186Tm' samples had high aphid numbers but low yield loss rates, indicating they have low antibiosis, but good tolerance. Aphid population size and wheat yield loss rates greatly varied in different fields and years for '98-10-35,' 'Xiaoyan22,' 'Tp,' 'Tam200,' 'PI high,' and other '186Tm' samples, which were hybrid offspring of T. aestivum and wheat related species. Thus, these germplasm should be considered for use in future studies. Overall, S. avenae is best adapted to 'Xinong1376,' because it was the most palatable variety, with the greatest yield loss rates of all 14 wheat varieties. However, individual varieties/lines influenced aphid populations differently in different years. Therefore, we strongly recommend a combination of laboratory and long-term field experiments in targeted planting regions to identify varieties/lines that consistently show high resistance to S. avenae infestation.

Soybean aphid intrabiotype variability based on colonization of specific soybean genotypes

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is one of the most destructive insect pests on soybeans in the United States. One method for managing this pest is through host plant resistance. Since its arrival in 2000, 4 aphid biotypes have been identified that are able to overcome soybean aphid resistance (Rag) genes. A soybean aphid isolate collected from Moline, Illinois readily colonized soybean plants with the soybean aphid resistance gene Rag2, unlike biotypes 1 and 2, but similar to soybean aphid biotype 3. Two no-choice experiments compared the virulence of the Moline isolate with biotype 3. In both experiments, differences in aphid population counts were not significant (P > 0.05) on soybean genotypes LD08-12957a (Rag2) and LD11-5413a (Rag2), but the aphid counts for the Moline isolate were significantly (P < 0.05) lower than the aphid counts for the biotype 3 isolate on the soybean genotypes Dowling (Rag1), LD05-16611 (Rag1), LD11-4576a (Rag1), and PI 567598B (rag1b and rag3). The Moline isolate was a variant of aphid biotype 3, which is the first report showing that soybean aphid isolates classified as the same biotype, based on virulence against specific Rag genes, can differ in aggressiveness or ability to colonize specific host genotypes.

Genetic diversity of Sitobion avenae (Homoptera: Aphididae) populations from different geographic regions in China

Sitobion avenae is a major agricultural pest of wheat in China. Using microsatellite markers, we studied the potential gene flow, genetic diversity, genetic differentiation, and genetic structure of seven S. avenae populations from different regions of China (Beijing, Hebei, Henan, Hubei, Jiangsu, Shandong, and Shanxi provinces). The populations from Henan, Shandong, and Jiangsu showed high levels of genic and genotypic diversity. By contrast, the genic diversity in the Beijing and Hebei populations was much lower. Despite this low genic diversity, the genotypic diversity of the Beijing population was higher than that of all of the other populations, except those from Jiangsu and Shandong. Overall, the genetic divergence among the seven S. avenae populations tested was high, though there was almost no differentiation between the Shandong and Henan populations. We observed significant negative correlation between the strength of gene flow and the geographic distances among populations. Based on genetic analysis, the seven S. avenae populations studied can be divided into four distinct clusters; (i) Hubei, (ii) Shanxi, (iii) Beijing and Hebei, and (iv) Shandong, Henan, and Jiangsu. The present results provide a basis for potentially optimizing integrated pest management (IPM) programs in China, through adapting control methods that target biological traits shared by various populations of the same genotype.

Genetic basis and selection for life-history trait plasticity on alternative host plants for the cereal aphid Sitobion avenae

Sitobion avenae (F.) can survive on various plants in the Poaceae, which may select for highly plastic genotypes. But phenotypic plasticity was often thought to be non-genetic, and of little evolutionary significance historically, and many problems related to adaptive plasticity, its genetic basis and natural selection for plasticity have not been well documented. To address these questions, clones of S. avenae were collected from three plants, and their phenotypic plasticity under alternative environments was evaluated. Our results demonstrated that nearly all tested life-history traits showed significant plastic changes for certain S. avenae clones with the total developmental time of nymphs and fecundity tending to have relatively higher plasticity for most clones. Overall, the level of plasticity for S. avenae clones’ life-history traits was unexpectedly low. The factor ‘clone’ alone explained 27.7–62.3% of the total variance for trait plasticities. The heritability of plasticity was shown to be significant in nearly all the cases. Many significant genetic correlations were found between trait plasticities with a majority of them being positive. Therefore, it is evident that life-history trait plasticity involved was genetically based. There was a high degree of variation in selection coefficients for life-history trait plasticity of different S. avenae clones. Phenotypic plasticity for barley clones, but not for oat or wheat clones, was frequently found to be under significant selection. The directional selection of alternative environments appeared to act to decrease the plasticity of S. avenae clones in most cases. G-matrix comparisons showed significant differences between S. avenae clones, as well as quite a few negative covariances (i.e., trade-offs) between trait plasticities. Genetic basis and evolutionary significance of life-history trait plasticity were discussed.