Plant Biotic Interactions: plants aren’t so defenseless after all

Plants are sessile organisms and offer an abundance of food for hungry, relentless herbivores. To defend themselves, plants fight a silent chemical war with their herbivore foes. To do this, first plants must recognize mechanical damage inflicted by herbivores so that they can “decide” whether or not to make the costly investment of preparing and unleashing their chemical arsenal. Dr. Simon Zebelo from the University of Maryland, Eastern Shore has been researching how exactly plants do this. During his research, he discovered that oral secretions from beet armyworm caterpillars (Spodoptera exigua) combined with mechanical damage of plant tissue leads to induction of defense related responses in tomato plants. (Felton, 2008)

These oral secretions contain chemical compounds, called elicitors, which alert the plant to the herbivores presence. Where do these elicitors come from? The ventral eversible gland (VEG), which plays a role in caterpillar defense, seemed like a likely candidate because it was a secretory structure that came into contact with the plant while feeding. Using a heated pin, Dr. Simon and his co-workers destroyed the VEG and compared the ablated beet armyworm caterpillars to those with their VEG intact. They confirmed that the VEG secretions induce defenses in tomato plants (Solanum lycopersicum) that include increased expression of defense-related genes and emission of volatile organic compounds (VOCs). Thus, plants defend themselves by recognizing VEG originated elicitors and triggering their defense machineries.  

Next, Dr. Zebelo shifted his attention towards investigating a unique relationship between plants and plant-growth promoting rhizobacteria (PGPR). PGPR colonize the roots of plants and have been shown to promote plant growth through nitrogen fixation, hormone synthesis, nutrient uptake, and reduced plant disease. Dr. Zebelo and his co-workers are interested in how rhizobacteria mediate plant defense against insects. Dr. Zebelo and his colleagues were interested in seeing if PGPRs change the composition of VOCs. VOCs can deter herbivores and attract parasitic wasps which gladly parasitize the herbivores. They used a model system consisting of cotton (Gossypium hirsutum), beet armyworm (S. exigua), and a parasitoid wasp (Microplitis croceipes) to investigate the effect of treatment of cotton plants with single strain (INR7) or mixture of strains (Blend 8 and Blend 9) of PGPR (Bacillus spp.) on plant chemistry and plant-insect interactions. By inoculating cotton plants with strains of PGPR, collecting volatiles from collecting chambers (Fig 2), and then analyzing the quantity and quality of VOCs, it was became clear that PGPR cause the plants to produce more VOCs.

Following these results Dr. Zebelo and his co-workers tested whether the VOCs emitted by PGPR inoculated cotton plants affect the oviposition behavior of armyworm moths and the activity of parasitic wasps. Interestingly, beet armyworm moths laid fewer eggs on PGPR treated cotton plants and these PGPR treated plants were more attractive to parasitoids.

Gossypol is a secondary metabolite which has been linked with helping cotton plants protect themselves against herbivory by decreasing herbivore fitness. Dr. Zebelo and his co-workers questioned how PGPR affects the biosynthesis of gossypol and affect the feeding behavior of armyworm caterpillars. In a series of laboratory and greenhouse investigations, they demonstrated that PGPR treatment elicits the induction gossypol-related gene expression and this leads to increased levels of gossypol in cotton plants, which reduced herbivory by beet armyworm caterpillars.
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This fascinating world of microbe-plant-insect interactions has important implications for agriculture. By exploring the largely unknown world of volatile ecology, we can identify new compounds that people can use to manage pests. However, Dr. Zebelo cautions against making too many generalizations at this early stage.  He reminds us that these interactions vary across plant, insect, and PGPR communities and are extremely complex, meriting additional research in this promising field.
           
References
 
Felton G. (2008) Caterpillar secretions and induced plant responses. In: Schaller, A., editor. Induced Plant Resistance to Herbivory. Netherlands: Springer. Chapter 18, 369–387.
 
Zebelo S., Piorkowski J., Disi J., & Fadamiro H. (2014). Secretions from the ventral eversible gland of Spodoptera exigua caterpillars activate defense-related genes and induce emission of volatile organic compounds in tomato, Solanum lycopersicum. BMC Plant Biology, 14(1), 140.
 
Zebelo S., Song Y., Kloepper J. W., & Fadamiro H. (2016). Rhizobacteria activates (+)-δ-cadinene synthase genes and induces systemic resistance in cotton against beet armyworm (Spodoptera exigua). Plant, Cell & Environment, 1-9.
 
 
About the Authors:
 
Hanna Kahl is a first year master’s student in Cerruti Hooks’ lab researching the effects of intercropping on insect feeding guilds.
 
Jonathan Wang is a PhD student in Raymond St. Leger’s lab. He is studying Drosophila immunity and fungal pathogenesis.

Figure 1 Anatomical diagram of Spodoptera exigua pointing out the ventral eversible gland (VEG)

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Figure 2 Plants in volatile collection chambers