Characterization of Cereal Aphid Resistance in Tetraploid Switchgrass Populations (\u3ci\u3ePanicum virgatum\u3c/i\u3e L.)

Switchgrass, Panicum virgatum L., is a perennial warm-season grass that has been identified as a model species for the development of bioenergy crops in the United States. The objectives of this research were to evaluate selected switchgrass populations for host suitability and differential resistance to potential aphid pests, determine the categories (antibiosis, antixenosis, and/or tolerance) of resistance among selected switchgrass populations, and elucidate Schizaphis graminum (Rondani) feeding behavior on resistant and susceptible switchgrasses. Screens for host suitability of two switchgrass populations, Summer and Kanlow, and two experimental strains, KxS and SxK, revealed all switchgrasses were unsuitable feeding and reproductive hosts to Rhopalosiphum padi (L.), and Diuraphis noxia (Mordvilko). Both Sipha flava (Forbes) and S. graminum were able to establish on all switchgrasses tested with differential levels of resistance among the switchgrasses. Two no-choice studies, performed to characterize the categories of resistance (antibiosis and tolerance) to S. flava and S. graminum, demonstrated that Kanlow possesses high levels of antibiosis to both aphids, while KxS possesses low-to-moderate levels of antibiosis to S. flava. Functional plant loss indices indicated that tolerance is an important category of resistance for Summer to S. graminum. Two choice studies evaluated S. graminum and S. flava preference for switchgrass populations, with a third study to assess S. graminum feeding behavior using the electrical penetration graph (EPG) technique. Choice studies for S. flava indicated no preference by aphids for any of the switchgrass populations. However, S. graminum displayed a preference for KxS at 24 h after aphid introduction. Feeding behavior studies for S. graminum on switchgrass indicated that aphids had significantly less phloem ingestion on Kanlow than both KxS and Summer, suggesting that resistance factors in Kanlow are associated with the phloem tissue. These studies are the first attempt to analyze the categories of resistance in switchgrass and provide critical information for characterizing the mechanisms of resistance and improving our knowledge of the plant-insect interactions within this system. Advisers: Tiffany Heng-Moss and Jeff Bradsha

Characterization of Cereal Aphid Resistance in Tetraploid Switchgrass Populations (\u3ci\u3ePanicum virgatum\u3c/i\u3e L.)

Switchgrass, Panicum virgatum L., is a perennial warm-season grass that has been identified as a model species for the development of bioenergy crops in the United States. The objectives of this research were to evaluate selected switchgrass populations for host suitability and differential resistance to potential aphid pests, determine the categories (antibiosis, antixenosis, and/or tolerance) of resistance among selected switchgrass populations, and elucidate Schizaphis graminum (Rondani) feeding behavior on resistant and susceptible switchgrasses. Screens for host suitability of two switchgrass populations, Summer and Kanlow, and two experimental strains, KxS and SxK, revealed all switchgrasses were unsuitable feeding and reproductive hosts to Rhopalosiphum padi (L.), and Diuraphis noxia (Mordvilko). Both Sipha flava (Forbes) and S. graminum were able to establish on all switchgrasses tested with differential levels of resistance among the switchgrasses. Two no-choice studies, performed to characterize the categories of resistance (antibiosis and tolerance) to S. flava and S. graminum, demonstrated that Kanlow possesses high levels of antibiosis to both aphids, while KxS possesses low-to-moderate levels of antibiosis to S. flava. Functional plant loss indices indicated that tolerance is an important category of resistance for Summer to S. graminum. Two choice studies evaluated S. graminum and S. flava preference for switchgrass populations, with a third study to assess S. graminum feeding behavior using the electrical penetration graph (EPG) technique. Choice studies for S. flava indicated no preference by aphids for any of the switchgrass populations. However, S. graminum displayed a preference for KxS at 24 h after aphid introduction. Feeding behavior studies for S. graminum on switchgrass indicated that aphids had significantly less phloem ingestion on Kanlow than both KxS and Summer, suggesting that resistance factors in Kanlow are associated with the phloem tissue. These studies are the first attempt to analyze the categories of resistance in switchgrass and provide critical information for characterizing the mechanisms of resistance and improving our knowledge of the plant-insect interactions within this system. Advisers: Tiffany Heng-Moss and Jeff Bradsha

Plant Tolerance: A Unique Approach to Control Hemipteran Pests

Plant tolerance to insect pests has been indicated to be a unique category of resistance, however, very little information is available on the mechanism of tolerance against insect pests. Tolerance is distinctive in terms of the plant’s ability to withstand or recover from herbivore injury through growth and compensatory physiological processes. Because plant tolerance involves plant compensatory characteristics, the plant is able to harbor large numbers of herbivores without interfering with the insect pest’s physiology or behavior. Some studies have observed that tolerant plants can compensate photosynthetically by avoiding feedback inhibition and impaired electron flow through photosystem II that occurs as a result of insect feeding. Similarly, the up-regulation of peroxidases and other oxidative enzymes during insect feeding, in conjunction with elevated levels of phytohormones can play an important role in providing plant tolerance to insect pests. Hemipteran insects comprise some of the most economically important plant pests (e.g., aphids, whiteflies), due to their ability to achieve high population growth and their potential to transmit plant viruses. In this review, results from studies on plant tolerance to hemipterans are summarized, and potential models to understand tolerance are presented

Feeding behavior of \u3ci\u3eAphis glycines\u3c/i\u3e (Hemiptera: Aphididae) on soybeans exhibiting antibiosis, antixenosis, and tolerance resistance

Aphis glycines Matsumura (Hemiptera: Aphididae) is a serious pest of soybean in North America. Plant resistance is a valuable tool for the management of this pest, and a better understanding of the interactions between aphid and soybeans expressing varying levels and different categories of resistance can assist in the development of aphid resistant or tolerant genotypes. The electrical penetration graph (EPG) technique was used to evaluate the feeding behavior of A. glycines (biotype 1) on 4 soybean genotypes: (1) ‘Dowling’ (contains Rag1 gene and exhibits antibiosis); (2) PI 200538 (contains Rag2 gene and exhibits antixenosis); (3) KS4202 (exhibits tolerance); and (4) ‘SD76RR’ (susceptible to aphid). Aphids spent shorter periods in the sieve element phase on ‘Dowling’ and exhibited a greater number of pathway phases, non-probing events, and a longer time in nonprobing events in PI 200538 and ‘Dowling.’ For ‘SD76RR’ and KS4202, aphids demonstrated more sustained phloem ingestion, spent shorter time in non-probing events, and exhibited fewer pathway phases and potential drops. These results indicate that resistance factors are present in the phloem of ‘Dowling.’ For PI 200538, it is suggested that antixenotic factors are involved in resistance to A. glycines. Because KS4202 is tolerant to biotype 1 of A. glycines, the suitability of this genotype was expected already. This study provides important data that contribute to the understanding of how soybean aphids (biotype 1) feed on soybean genotypes with various aphid resistant genes and categories. In addition to assisting in the distinction between resistance categories, these results are useful in soybean breeding programs focusing on developing genotypes with greater resistance to insects. Aphis glycines Matsumura (Hemiptera: Aphididae) é uma das principais pragas da cultura da soja na América do Norte. Dentre as técnicas de manejo da praga, a resistência de plantas a insetos se destaca como uma ferramenta valiosa. Assim, uma melhor compreensão sobre as interações entre o afídeo e plantas de soja que expressam variáveis níveis e diferentes categorias de resistência, pode auxiliar no desenvolvimento de genótipos resistentes ao inseto. A técnica de EPG foi utilizada para avaliar o comportamento alimentar de A. glycines (biótipo 1) em 4 genótipos de soja: (1) ‘Dowling’ (contém gene Rag1 e expressa antibiose); (2) PI 200538 (contém gene Rag2 e expressa antixenose); (3) KS4202 (expressa tolerância); e (4) ‘SD76RR’ (suscetível ao afídeo). Os afídeos demonstraram curtos períodos de alimentação na fase de seiva em ‘Dowling’ e exibiram uma grande quantidade de fases de caminhamento estiletar, número de períodos de não-prova e longos período de não-prova em PI 200538 e ‘Dowling.’ Para ‘SD76RR’ e KS4202, os afídeos mostraram um maior período de alimentação em vasos do floema, apresentaram curtos períodos de não-prova e exibiram poucas fases de caminhamento estiletar, além de menor número de quedas de potencial. Os resultados indicam que fatores de resistência estão presentes nos vasos floemáticos de ‘Dowling.’ Para PI200538, sugere-se que fatores antixenóticos estão envolvidos na resistência a A. glycines. Uma vez que KS4202 expressa tolerância ao biótipo 1 de A. glycines, a adequabilidade deste genótipo como fonte de alimento para o inseto já era esperada. Este estudo fornece importantes dados que contribuem para um melhor entendimento de como o pulgão-da-soja (biótipo 1) se alimentam de plantas de soja portadoras de diferentes genes e categorias de resistência. Além de auxiliar na distinção entre as categorias de resistência, esses resultados podem ser úteis em programas de melhoramento de soja, com intuito de selecionar genótipos mais resistentes a insetos

Feeding behavior of \u3ci\u3eAphis glycines\u3c/i\u3e (Hemiptera: Aphididae) on soybeans exhibiting antibiosis, antixenosis, and tolerance resistance

Aphis glycines Matsumura (Hemiptera: Aphididae) is a serious pest of soybean in North America. Plant resistance is a valuable tool for the management of this pest, and a better understanding of the interactions between aphid and soybeans expressing varying levels and different categories of resistance can assist in the development of aphid resistant or tolerant genotypes. The electrical penetration graph (EPG) technique was used to evaluate the feeding behavior of A. glycines (biotype 1) on 4 soybean genotypes: (1) ‘Dowling’ (contains Rag1 gene and exhibits antibiosis); (2) PI 200538 (contains Rag2 gene and exhibits antixenosis); (3) KS4202 (exhibits tolerance); and (4) ‘SD76RR’ (susceptible to aphid). Aphids spent shorter periods in the sieve element phase on ‘Dowling’ and exhibited a greater number of pathway phases, non-probing events, and a longer time in nonprobing events in PI 200538 and ‘Dowling.’ For ‘SD76RR’ and KS4202, aphids demonstrated more sustained phloem ingestion, spent shorter time in non-probing events, and exhibited fewer pathway phases and potential drops. These results indicate that resistance factors are present in the phloem of ‘Dowling.’ For PI 200538, it is suggested that antixenotic factors are involved in resistance to A. glycines. Because KS4202 is tolerant to biotype 1 of A. glycines, the suitability of this genotype was expected already. This study provides important data that contribute to the understanding of how soybean aphids (biotype 1) feed on soybean genotypes with various aphid resistant genes and categories. In addition to assisting in the distinction between resistance categories, these results are useful in soybean breeding programs focusing on developing genotypes with greater resistance to insects. Aphis glycines Matsumura (Hemiptera: Aphididae) é uma das principais pragas da cultura da soja na América do Norte. Dentre as técnicas de manejo da praga, a resistência de plantas a insetos se destaca como uma ferramenta valiosa. Assim, uma melhor compreensão sobre as interações entre o afídeo e plantas de soja que expressam variáveis níveis e diferentes categorias de resistência, pode auxiliar no desenvolvimento de genótipos resistentes ao inseto. A técnica de EPG foi utilizada para avaliar o comportamento alimentar de A. glycines (biótipo 1) em 4 genótipos de soja: (1) ‘Dowling’ (contém gene Rag1 e expressa antibiose); (2) PI 200538 (contém gene Rag2 e expressa antixenose); (3) KS4202 (expressa tolerância); e (4) ‘SD76RR’ (suscetível ao afídeo). Os afídeos demonstraram curtos períodos de alimentação na fase de seiva em ‘Dowling’ e exibiram uma grande quantidade de fases de caminhamento estiletar, número de períodos de não-prova e longos período de não-prova em PI 200538 e ‘Dowling.’ Para ‘SD76RR’ e KS4202, os afídeos mostraram um maior período de alimentação em vasos do floema, apresentaram curtos períodos de não-prova e exibiram poucas fases de caminhamento estiletar, além de menor número de quedas de potencial. Os resultados indicam que fatores de resistência estão presentes nos vasos floemáticos de ‘Dowling.’ Para PI200538, sugere-se que fatores antixenóticos estão envolvidos na resistência a A. glycines. Uma vez que KS4202 expressa tolerância ao biótipo 1 de A. glycines, a adequabilidade deste genótipo como fonte de alimento para o inseto já era esperada. Este estudo fornece importantes dados que contribuem para um melhor entendimento de como o pulgão-da-soja (biótipo 1) se alimentam de plantas de soja portadoras de diferentes genes e categorias de resistência. Além de auxiliar na distinção entre as categorias de resistência, esses resultados podem ser úteis em programas de melhoramento de soja, com intuito de selecionar genótipos mais resistentes a insetos

Divergent Switchgrass Cultivars Modify Cereal Aphid Transcriptomes

Schizaphis graminum Rondani (Hemiptera: Aphididae) and Sipha flava Forbes (Hemiptera: Aphididae) are two common pests of bioenergy grasses. Despite the fact that they are both considered generalists, they differ in their ability to colonize Panicum virgatum cultivars. For example, S. flava colonizes both P. virgatum cv. Summer and P. virgatum cv. Kanlow whereas S. graminum can only colonize Summer. To study the molecular responses of these aphids to these two switchgrass cultivars, we generated de novo transcriptome assemblies and compared the expression profiles of aphids feeding on both cultivars to profiles associated with feeding on a highly susceptible sorghum host and a starvation treatment. Transcriptome assemblies yielded 8,428 and 8,866 high-quality unigenes for S. graminum and S. flava, respectively. Overall, S. graminum responded strongly to all three treatments after 12 h with an upregulation of unigenes coding for detoxification enzymes while major transcriptional changes were not observed in S. flava until 24 h. Additionally, while the two aphids responded to the switchgrass feeding treatment by downregulating unigenes linked to growth and development, their responses to Summer and Kanlow diverged significantly. Schizaphis graminum upregulated more unigenes coding for stress-responsive enzymes in the Summer treatment compared to S. flava; however, many of these unigenes were actually downregulated in the Kanlow treatment. In contrast, S. flava appeared capable of overcoming host defenses by upregulating a larger number of unigenes coding for detoxification enzymes in the Kanlow treatment. Overall, these findings are consistent with previous studies on the interactions of these two cereal aphids to divergent switchgrass hosts

Evaluation of Greenbug and Yellow Sugarcane Aphid Feeding Behavior on Resistant and Susceptible Switchgrass Cultivars

Switchgrass (Panicum virgatum L.) is an emerging biofuel crop that serves as host for aphids. To discern the effects of plant age and possible resistance mechanisms, the feeding behavior of greenbugs (Schizaphis graminum Rondani.) and the yellow sugarcane aphid (Sipha flava Forbes.) was monitored on three diverse switchgrasses by the electrical penetration graph (EPG) technique. Callose deposition and genes associated with callose metabolism were also analyzed to discern their association with plant resistance. There was a strong host effect on greenbugs feeding on lowland cultivar Kanlow at the V3 stage of development, as compared to the greenbug-susceptible upland cultivar Summer and plants derived from Kanlow (♂) × Summer (♀) (K×S) crosses. These data confirmed that Kanlow at the V3 stage had antibiosis to greenbugs, which was absent in the Summer and K×S plants. In contrast, similar effects were not observed for yellow sugarcane aphids, excluding significant differences in the time to first probe on Kanlow plants at the V1 stage and reduction in time spent on pathway processes on Kanlow plants at the V3 stage. These data demonstrated that Kanlow plants may have multiple sources of resistance to the two aphids, and possibly some were phloem based. Microscopy of leaf sections stained with aniline blue for callose was suggestive of increased callose deposition in the sieve elements in Kanlow plants relative to Summer and K×S plants. RT-qPCR analysis of several genes associated with callose metabolism in infested plants was equivocal. Overall, these studies suggest the presence of multiple defense mechanisms against aphids in Kanlow plants, relative to Summer and K×S plants

Sugars and cuticular waxes impact sugarcane aphid (\u3ci\u3eMelanaphis sacchari\u3c/i\u3e) colonization on different developmental stages of sorghum

Sugarcane aphid (SCA; Melanaphis sacchari) is a devastating pest of sorghum (Sorghum bicolor) that colonizes sorghum plants at different growth stages. Leaf surface characteristics and sugars often influence aphid settling and feeding on host plants. However, how changes in cuticular waxes and sugar levels affect SCA establishment and feeding at different development stages of sorghum have not been explored. In this study, two- and six-weekold BTx623 plants, a reference line of sorghum, was used to evaluate plant-aphid interactions. Monitoring aphid feeding behavior using Electrical Penetration Graph (EPG) technique revealed that aphids spent more time in the sieve element phase of six-week-old plants compared to two-week-old plants. Significant differences were found in the time spent to reach the first sieve element and pathway phases between the two- and six-week-old plants. However, no-choice aphid bioassays displayed that SCA population numbers were higher in two-week-old plants compared to six-week-old plants. Differences in the abundance of wax and sugar contents were analyzed to determine how these plant components influenced aphid feeding and proliferation. Among the cuticular wax compounds analyzed, α-amyrin and isoarborinone increased after 10 days of aphid infestation only in six-weekold plants. Trehalose content was significantly increased by SCA feeding on two- and six-week-old plants. Furthermore, SCA feeding depressed sucrose content and increased levels of glucose and fructose in two-weekold but not in six-week-old plants. Overall, our study indicates that plant age is a determinant for SCA feeding, and subtle changes in triterpenoids and available sugars influence SCA establishment on sorghum plants

Aphid-Responsive Defense Networks in Hybrid Switchgrass

Aphid herbivory elicits plant defense-related networks that are influenced by host genetics. Plants of the upland switchgrass (Panicum virgatum) cultivar Summer can be a suitable host for greenbug aphids (Schizaphis graminum; GB), and yellow sugarcane aphids (Sipha flava, YSA), whereas the lowland cultivar Kanlow exhibited multi-species resistance that curtails aphid reproduction. However, stabilized hybrids of Summer (♀) x Kanlow (♂) (SxK) with improved agronomics can be damaged by both aphids. Here, hormone and metabolite analyses, coupled with RNA-Seq analysis of plant transcriptomes, were utilized to delineate defense networks induced by aphid feeding in SxK switchgrass and pinpoint plant transcription factors (TFs), such as WRKYs that potentially regulate these responses. Abscisic acid (ABA) levels were significantly higher in GB infested plants at 5 and 10 days after infestation (DAI). ABA levels were highest at 15DAI in YSA infested plants. Jasmonic acid levels were significantly elevated under GB infestation, while salicylic acid levels were signifi40cantly elevated only at 15 DAI in YSA infested plants. Similarly, levels of several metabolites were altered in common or specifically to each aphid. YSA infestation induced a significant enrichment of flavonoids consistent with an upregulation of many genes associated with flavonoid biosynthesis at 15DAI. Gene co-expression modules that responded singly to either aphid or in common to both aphids were differentiated and linked to specific TFs. Together, these data provide important clues into the interplay of metabolism and transcriptional remodeling accompanying defense responses to aphid herbivory in hybrid switchgrass