The soybean aphid, Aphis glycines Matsumura made its first recorded appearance in the U.S. in Wisconsin during the summer of 2000. A. glycines uses piercing and sucking mouthparts to penetrate plant phloem, and as a result depletes photosynthates potentially decreasing soybean yield by as much as 50-70%. Summer A. glycines morphs reproduce clonally on soybean plants causing rapid colony establishment. Various resistance genes have been identified in soybean germplasm, however commercial lines containing most recently developed resistance genes have not yet been released. In ten short years since the arrival of A. glycines in the U.S., three different biotypes have been identified. Causal factors of biotype development are not well understood. The majority of soybean acres in the U.S. are planted to susceptible varieties that would not typically trigger new biotype establishment. A. glycines is a devastating pest of soybeans. The race is on to identify novel sources of resistance and understand the mode of resistance in varieties that are able to deter aphid colonization. Grower prosperity and the food and feed sectors depend on it.
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Soybean researchers speculate that A. glycines was present in the U.S. prior to 2000, and accounts of infestation supporting this theory exist, yet no physical specimens are available to confirm this theory. By the close of summer in 2000, A. glycines infestations were reported in as many as ten Midwestern states. The distribution of A. glycines in the U.S. corresponds to the distribution of various species of buckthorn. This area includes temperate regions of the U.S. For example, the Midwest, Nebraska, Kansas, Missouri, parts of Ontario, etc. are all plagued by aphid infestations. A. glycines may be found in warmer southern areas due to the long distances they are capable of traveling when they get caught in wind patterns and distributed to other areas. The southern U.S. does not have the overwintering host, and temperatures may exceed optimum levels for the pest to flourish.
Across the Midwest, A. glycines establishment in the field occurs around mid-June when soybeans are in reproductive stages. Perhaps volatiles associated with flower set act as attractants. Although this timeframe exists in nature, it does not indicate that only reproductive stage soybeans are colonized by A. glycines. Literature illustrates that successful resistance screening can be performed at any stage. A. glycines is a heteroecious holocyclic pest. Sexual reproduction occurs on Rhamnus (Buckthorn) species and eggs are deposited and overwinter near the buds. In the summer, aphids emerge and fly to soybeans where the summer morphs-the asexual stage of the lifecycle is characterized by rapid clonal reproduction. Forms present during this stage include apterous (non-winged) aphids and alate (winged) aphids. Aphids transition into alate form under overpopulation circumstances, and also may emerge when food sources are not ideal.
Often newest plant growth is the first area of soybeans to be colonized by A. glycines. Under high infestation levels, stems and older leaves may also be colonized. Stems are often very heavily infested in high pressure situations. The underside of older leaves is often colonized preferentially to top surface of the leaves. This may be due to the arrangement of cell types within the leaf. The upper surface of leaf is where most specialized photosynthesizing cells are be located. These cells may be larger or tougher and harder for aphids to penetrate as compared to the lower leaf surface which does not commonly receive direct sunlight therefore lacks specialized cells for harnessing sunlight on the surface.
Soybean resistance to A. glycines has been characterized as either antibiosis or antixenosis. Both antibiosis and antixenosis resistance testing are common in soybean germplasm screening programs. There are different approaches to measuring the effect of resistance on A. glycines. Common approaches include measurements of: fecundity, feeding duration, stylet probing patterns, honeydew production, behavioral patterns, rate of growth to maturity, photosynthesis rates, and chlorophyll loss.
Antixenosis, or "choice" testing presents A. glycines with a choice of food source. The desired effect is a non-preference reaction by the pest where very low infestation levels are sustained by the genotype. Antixenosis testing is often performed in large field cages. This type of screening is typically used as a preliminary step in resistance screening workflows allowing large amounts of susceptible material to be eliminated rapidly. Remaining resistant or moderately resistant materials are advanced to more intensive testing.
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For antixenosis screening, plot sizes are often small and may or may not be replicated. A. glycines samples are collected from areas surrounding the test site approximately one month prior to the anticipated infestation date. Small colonies are increased on susceptible fodder varieties, and small infested leaf discs containing roughly five apterous aphids per piece are placed on the new growth of each plant in the cage. Plots are evaluated based on the infestation level of the plot as a whole. Trials are often replicated in various geographies in order to evaluate germplasm performance in the presence of different biotypes. Antixenosis tests may also be performed in greenhouses or growth chambers in a process similar in nature to the preference-type testing in the large field cage, only under a more controlled environment. Evaluation systems assign a simplified rating corresponding to a numerical range of aphids present per plant.
Soybean varieties displaying antibiosis resistance directly impact the ability of A. glycines to survive and flourish on those particular genotypes. Here, instead of testing preferential feeding and colonization as in the case of antixenosis, antibiosis testing involves isolating aphids on a single genotype to measure whether A. glycines will be negatively impacted by the genetic composition of the genotype. One of the most common is a whole plant or whole plot antibiosis assay where each genotype is caged, infested, and evaluated based on total colonization after a given period. This method is very straightforward and simple to perform. It does not require a lot of expensive equipment to generate quality data using this method.
Assays in which entire plants are infested for data collection typically utilize a rating scale developed by Mensah, et al. in 2005. The rating scale assigns a Damage Index (DI) from zero to four corresponding to the infestation level of the plants. A DI rating of zero is assigned to plants or plots that have zero aphids present after the test period. This occurs infrequently if ever, as even the most resistant varieties tend to have at least a few aphids present on them. A DI of one describes plants with less than 100 aphids per plant. Plants with a DI of two have 100-300 aphids present typically on young leaves. Varieties having 301-800 aphids per plant with shiny, honeydew covered leaves that are slightly yellow and curled are assigned a DI value of 3. The most susceptible rating for these assays is a rating of four, where each plant has more than 800 aphids present after the test period. Entries assigned a DI of four are severely curled and covered with sooty mold and many cast skins.
Another method of antibiosis screening uses adhesive foam microcages to isolate A. glycines on a very small oval shaped part of the leaf surface. This technique is very similar to that previously mentioned; however here testing occurs on a much smaller scale, and it is not a dead-end assay. By using a single leaf only, the remainder of the plant is available for use in different assays, or to fast-track resistant germplasm identified by the assay. Plants can be grown out to increase seed, and also plant tissue can be sent to the genotyping lab to determine the genetic makeup underlying the resistance to A. glycines.
A measurement of aphid colonization, for example going from 5 aphids at assay setup to hundreds per plant for a susceptible entry compared to less than 50 aphids per plant for a resistant entry at rating is a very clear cut way to differentiate between resistance and susceptibility. For identifying resistance, this method is very accurate, but identifying the bigger picture physiologically is a different piece. What is the mode of action for resistance? Does rate of feeding remain the same, or are there reactions by the plant that make it physically impossible for aphids to actually feed? Essentially, phytochemicals in the soybean may be having antibiotic effects on A. glycines to the extent where reproduction is slowed or ceases, or A. glycines may be unable to feed due to reduced phloem sap flow rendering it physically impossible for A. glycines to acquire adequate nourishment to colonize an entry. Aphid salivary secretions, if recognized by the plant, are thought to trigger a response system in the plant that causes phloem sap to coagulate, and sap flow to the aphid is much reduced.
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Methods exploring mode of resistance can effectively be used to test for resistance or susceptibility; however, many of them are very tedious and require the use of expensive equipment. The efficiency of these processes does not outshine antibiosis and antixenosis methods described above. To determine feeding duration and patterns, electrical penetration graphs may be used to determine how aphids feed when they are on resistant and susceptible varieties. In another technique to determine amount of phloem sap ingested, honeydew is captured on filter paper and stained with Ninhydrin diluted in ethanol-a method commonly used by police departments in fingerprinting technologies. Ninhydrin binds to amino acids in the honeydew and when dry results in blue stained areas on the filter paper. Amount of honeydew excretion is directly correlated to how much phloem sap is removed during feeding. The more intense the blue color, the more honeydew present and in turn the more susceptible the variety.
Assays measuring photosynthesis and chlorophyll loss seek understanding of the mode by which aphids cause 50-70% yield loss. Li-Cor 6400 photosynthesis meters are capable of measuring the rate at which plants photosynthesize when under aphid pressure. SPAD 502 chlorophyll meters can be used to measure potential loss of chlorophyll lost through aphid feeding. For most successful testing, it is recommended that a workflow be created to best utilize resources and screen most efficiently.
Resistance screening can take place in the greenhouse, growth chamber or field. Field testing is more realistic to the situations that will occur during crop growth. Many secondary interactions occur in the field that may affect resistance-complexes of pathogens and secondary pests are present in the field and likely affect plant chemical profiles. In turn, these interactions may affect the decision of A. glycines to feed and/or colonize. Field weather conditions may be suboptimal. In a temperate climate this is a risk-researchers have one opportunity per year to collect data. If weather is severe and unfavorable, all data and an entire year may be lost. Greenhouse and growth chamber environments on the other hand can often be controlled to perfection. Plants are grown in best possible environment free of secondary pests or pathogens. This allows the effects of the genotype to be observed without confounding circumstances; however conditions this perfect will rarely if ever occur in nature.
Greenhouse or lab testing often requires A. glycines colonies to be maintained, which may add some inaccuracies unless steps are taken to maintain genetic diversity in the colony. This can be achieved by collecting samples from nature at least annually to be sure colony is as close as possible to what is present in nature. If this colony refreshing does not occur, a genetic bottleneck will occur due to the absence of the sexual lifecycle phase. If the colony is lacking diversity to a high degree, the actual resistance screen may not be powerful due to the fact that soybean test genotypes may not perform the same way against the diverse aphids present in the natural environment.
Progress continues toward the quest for novel sources of resistance. Resistance screening methods are frequently optimized to allow for processing of increased numbers and efficiency of data points generated. The emergence of new biotypes continues, resulting in an increased essentiality to discover new resistance genes. Stacking strategies may be the best option for winning the battle. The world relies on soybeans for many purposes. Immense decreases in yield caused by A. glycines have negative implications on agriculture. Resistant varieties would greatly decrease the amount of harmful insecticides that must be applied resulting in a greener situation and saving growers the expense of purchasing chemicals, and yield losses due to transmission of virus vectored by A. glycines would be reduced. Researchers are working hard and making advances that will benefit soybean production nationwide. Many companies and universities have prioritized projects working toward these efforts, and with additional focus and funding there is hope the situation will be improved within the next few years.
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