Transcriptional analysis of defense mechanisms in upland tetraploid switchgrass to greenbugs

Authors: DONZE-REINER, TERESA - University Of Nebraska; Palmer, Nathan - Nate; Scully, Erin; PROCHASKA, TRAVIS - University Of Nebraska; KOCH, KYLE - University Of Nebraska; Sattler, Scott; HENG-MOSS, TIFFANY - University Of Nebraska; BRADSHAW, JEFF - University Of Nebraska; Sarath, Gautam; AMUNDSEN, KEENAN - University Of Nebraska; TWIGG, PAUL - University Of Nebraska

Submitted to: BMC Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2017
Publication Date: 2/16/2017
Publication URL: http://handle.nal.usda.gov/10113/5642524
Citation: Donze-Reiner, T., Palmer, N.A., Scully, E.D., Prochaska, T.J., Koch, K.G., Sattler, S.E., Heng-Moss, T., Bradshaw, J., Sarath, G., Amundsen, K., Twigg, P. 2017. Transcriptional analysis of defense mechanisms in upland tetraploid switchgrass to greenbugs. Biomed Central (BMC) Plant Biology. 17(1):46.

Interpretive Summary: Switchgrass has been selected as a promising biofuel/biomass crop. However, there is limited knowledge on the types of insect pests that could pose a problem if these grasses are cultivated on a large-scale. Recent reports suggested that several aphids can feed on switchgrass and can significantly reduce plant growth and development. Understanding these plant-insect interactions will be needed to develop sustainable, integrated pest management strategies. Here, the defensive response of an upland switchgrass cultivar Summer was evaluated when infested with greenbugs, a cereal aphid with broad host range, and known to cause significant yield losses in other crops. Using a combination of next- generation sequencing and metabolite analyses the molecular changes underlying the defensive response of switchgrass to aphid herbivory were evaluated. The extensive data yielded by sequencing was analyzed using tools in bioinformatics to develop a detailed picture of genes and gene families that were specifically induced in response to aphid feeding. Many genes that could be important in plant defensive response to aphid feeding were identified. This study provides a good resource for future investigations of switchgrass-insect interactions, and provides clues to specific genes that could be exploited to improve plant resistance to aphids.

Technical Abstract: Background: Aphid infestation of switchgrass (Panicum virgatum) has the potential to reduce yields and biomass quality. Although switchgrass-greenbug (Schizaphis graminum; GB) interactions have been studied at the whole plant level, little information is available on plant defense responses at the molecular level. Results: The global transcriptomic response of switchgrass cv Summer to GB was monitored by RNA-Seq in infested and control (uninfested) plants harvested at 5, 10, and 15 days after infestation (DAI). Differentially expressed genes (DEGs) in infested plants were analyzed relative to control uninfested plants at each time point. DEGs in GB-infested plants induced by 5-DAI included an upregulation of reactive burst oxidases and several cell wall receptors. Expression changes in genes linked to redox metabolism, cell wall structure, and hormone biosynthesis were also observed by 5-DAI. At 10-DAI, network analysis indicated a massive upregulation of defense-associated genes, including NAC, WRKY, and MYB classes of transcription factors and potential ancillary signaling molecules such as leucine aminopeptidases. Molecular evidence for loss of chloroplastic functions was also detected at this time point. Supporting these molecular changes, chlorophyll content was significantly decreased, and ROS levels were elevated in infested plants 10-DAI. Total peroxidase and laccase activities were elevated in infested plants at 10-DAI relative to control uninfested plants. The net result appeared to be a broad scale defensive response that led to an apparent reduction in C and N assimilation and a potential redirection of nutrients away from GB and towards the production of defensive compounds, such as pipecolic acid, chlorogenic acid, and trehalose by 10-DAI. By 15-DAI, evidence of recovery in primary metabolism was noted based on transcript abundances for genes associated with carbon, nitrogen, and nutrient assimilation. Conclusions: Extensive remodeling of the plant transcriptome and the production of ROS and several defensive metabolites in an upland switchgrass cultivar were observed in response to GB feeding. The early loss and apparent recovery in primary metabolism by 15-DAI would suggest that these transcriptional changes in later stages of GB infestation could underlie the recovery response categorized for this switchgrass cultivar. These results can be exploited to develop switchgrass lines with more durable resistance to GB and potentially other aphids.