Gene expression in sugarcane aphid-resistant sorghum varieties at times when they are most prevalent in the Texas Panhandle were the focus of a recent Texas A&M AgriLife Research study.
Sugarcane aphids remain the most significant threat to sorghum production, and their outbreaks can arise quickly and unexpectedly, especially in the Southern High Plains where infestations commonly coincide with sorghum bloom, said Dr. Ada Szczepaniec, AgriLife Research entomologist in Amarillo.
Sugarcane aphid outbreaks in sorghum were first reported in 2013. Previous research has illustrated that the outcomes of interactions between sugarcane aphid and sorghum — and thus the severity of the outbreaks — depend on the sorghum hybrid and potentially the phenology of the sorghum, Szczepaniec says.
To dig further into the mechanisms that drive the severity of aphid attacks and the role planting timing has in management protocols, she designed a study to understand the physiological changes in a commercially available resistant variety and a susceptible variety of sorghum. The varieties were studied at two weeks and six weeks post-emergence and were exposed to sugarcane aphid infections.
“In prior years, we demonstrated that using resistant sorghum varieties is the most effective way to manage these pests,” Szczepaniec says. “We also documented that when sugarcane aphids colonize flowering sorghum or just prior to bloom, their numbers increase extremely fast. This can pose a challenge to timely insecticide applications.”
The effects of sorghum age and genotype on the daily change in aphid densities were also evaluated in separate greenhouse experiments in her study.
“We found that the seedling sorghum expressed significantly more genes involved in natural plant resistance to pests than sorghum at the cusp of panicle emergence. This was true across varieties,” Szczepaniec says.
“More importantly, we found a suite of transcriptional changes in the resistant variety that were weak or absent in the susceptible sorghum. Specifically, the aphid-resistant variety exposed to sugarcane aphids bolstered several genes involved in natural plant resistance to pests, and this response was particularly robust in the two-week plants.”
She said her new research links the sugarcane aphid population dynamics noted in the field studies with gene expression data, which provide strong support for previous management recommendations.
“We found specific pathways in the plants that explain how resistant sorghum can mount powerful natural defenses against the aphids, and we discovered mechanisms responsible for greater susceptibility of sorghum in reproductive stages,” Szczepaniec says.
“These findings further support our recommendations for planting early, using resistant sorghum varieties, and intensifying scouting and sampling, particularly when sorghum is flowering.”
Szczepaniec says outcomes of this research were also of interest to other scientists as research on sugarcane aphid association with sorghum is still fairly new. Prior to this publication, she said, not much had been known about the molecular mechanisms that drive their interactions and result in sugarcane aphid outbreaks.
The gene expression changes in the younger resistant sorghum included induction of powerful plant hormones that govern how sorghum responds to pests, and more effective deployment of genes that help sorghum alleviate the harmful effects of oxidative stress following sugarcane aphid herbivory.
“The suite of physiological changes we quantified in sorghum was mirrored in the aphid population growth, which was significantly faster in the susceptible and older sorghum than in the resistant and younger plants,” Szczepaniec says.
“We believe this research is providing the first insights into molecular mechanisms underlying lower population growth of sugarcane aphids on the resistant sorghum variety,” she says. “It also appears that the younger resistant sorghum was able to mount a robust defense response following aphid infection, which was much weaker in the older sorghum.”
She says there are several pathways and specific genes that provide specific clues into the mechanisms underlying host-plant resistance to this invasive insect, and that will be a part of her continued study.
“We believe this knowledge will inform future sorghum breeding programs and contribute to the development of more varieties that can combat sugarcane aphid infestations,” Szczepaniec says. “It also helps us understand the mechanisms responsible for sugarcane aphid outbreaks and provides further support for specific integrated management tactics producers can take to help reduce the economic costs of these pests.”
This research was published in BMC Genomics and presented at several national and international conferences.
Funding came in part from the U.S. Department of Agriculture’s National Institute of Food and Agriculture, USDA-Agricultural Research Service, Areawide Pest Management of the Invasive Sugarcane Aphid in Grain Sorghum and Texas A&M AgriLife Genomics and Bioinformatics Center.