Establishment of in vitro soybean aphids, Aphis glycines (Hemiptera: Aphididae): a tool to facilitate studies of aphid symbionts, plant-insect interactions and insecticide efficacy

Comparing the cost-benefit probability of management based on early-stage and late-stage economic thresholds with that of seed treatment of Aphis glycines

BACKGROUND

The current integrated pest management (IPM) curative strategy for soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), relies on responsive spraying foliar insecticides during the R1-R5 soybean stage when aphid abundance reaches the economic threshold (ET) of 250 aphids plant-1 (traditional IPM). By analyzing the relationship between aphid abundance and yield loss before the R1 stage, we developed an early-stage ET. We propose to spray foliar insecticides on plants colonized with aphids using the early-stage ET as a trigger (improved IPM), together with seed treatment to manage A. glycines and delay them exceeding the ET of 250 aphids plant-1 in the late stage for whole-field spraying (traditional IPM). Finally, we compared the cost-benefit probabilities of the three management approaches.

RESULTS

The early-stage ET over all potential yields, market prices, and control costs was 64 aphids plant-1 , providing growers 7 days of preparation time to spray foliar insecticides before the economic injury level of 187 aphids plant-1 was reached. Improved IPM achieved the highest cost-benefit probabilities followed by traditional IPM, and the seed treatment achieved the lowest. However, in fields where the pressure from white grubs was high, the probability of achieving a positive net return with seed treatment was higher than that in other locations.

CONCLUSION

Improved IPM based on early-stage ET of 64 aphids plant-1 was the most cost-effective of all the three approaches. Neonicotinoid seed treatment can be applied as an insurance strategy to supplement A. glycines IPM in Liaoning, China. © 2022 Society of Chemical Industry.

Seed Imbibition and Metabolism Contribute Differentially to Initial Assembly of the Soybean Holobiont

Seed germination critically determines successful plant establishment and agricultural productivity. In the plant holobiont's life cycle, seeds are hubs for microbial communities' assembly, but what exactly shapes the holobiont during germination remains unknown. Here, 16S rRNA gene amplicon sequencing characterized the bacterial communities in embryonic compartments (cotyledons and axes) and on seed coats pre- and post-germination of four soybean (Glycine max) cultivars, in the presence or absence of exogenous abscisic acid (ABA), which prevented germination and associated metabolism of seeds that had imbibed. Embryonic compartments were metabolically profiled during germination to design minimal media mimicking the seed endosphere for bacterial growth assays. The distinction between embryonic and seed coat bacterial microbiomes of dry seeds weakened during germination, resulting in the plumule, radicle, cotyledon, and seed coat all hosting the same most abundant and structurally influential genera in germinated seeds of every cultivar. Treatment with ABA prevented the increase of bacterial microbiomes' richness, but not taxonomic homogenization across seed compartments. Growth assays on minimal media containing the most abundant metabolites that accumulated in germinated seeds revealed that seed reserve mobilization promoted enrichment of copiotrophic bacteria. Our data show that seed imbibition enabled distribution of seed-coat-derived epiphytes into embryos irrespective of germination, while germinative metabolism promoted proliferation of copiotrophic taxa, which predominated in germinated seeds.

Validation of a hairy roots system to study soybean-soybean aphid interactions

The soybean aphid (Aphis glycines) is one of the main insect pests of soybean (Glycine max) worldwide. Genomics approaches have provided important data on transcriptome changes, both in the insect and in the plant, in response to the plant-aphid interaction. However, the difficulties to transform soybean and to rear soybean aphid on artificial media have hindered our ability to systematically test the function of genes identified by those analyses as mediators of plant resistance to the insect. An efficient approach to produce transgenic soybean material is the production of transformed hairy roots using Agrobacterium rhizogenes; however, soybean aphids colonize leaves or stems and thus this approach has not been utilized. Here, we developed a hairy root system that allowed effective aphid feeding. We show that this system supports aphid performance similar to that observed in leaves. The use of hairy roots to study plant resistance is validated by experiments showing that roots generated from cotyledons of resistant lines carrying the Rag1 or Rag2 resistance genes are also resistant to aphid feeding, while related susceptible lines are not. Our results demonstrate that hairy roots are a good system to study soybean aphid-soybean interactions, providing a quick and effective method that could be used for functional analysis of the resistance response to this insect.