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The ecological consequences of chemotypic variation of damage-induced volatile organic compounds in sagebrush (Artemisia tridentata)

Periodic Reporting for period 1 - CVVOC (The ecological consequences of chemotypic variation of damage-induced volatile organic compounds in sagebrush (Artemisia tridentata))

Reporting period: 2018-06-01 to 2020-05-31

Unlike animals that can run away from predators, plants are immobile and must stand their ground when attacked by herbivores. Consequently, plants have evolved an impressive defense system. These defenses include physical structures like spines and thorns as well as biomolecules that have toxic, repellent, or antinutritional effects on consumers. While many plant defenses are
constitutively expressed, some are induced in response to herbivore damage. Damaged plants emit volatile organic compounds (VOCs) into the environment that may induce defenses in adjacent, undamaged tissue or may be eavesdropped by neighboring plants, enabling them to prime their own resistance response prior to attack. This exchange of information is commonly referred to as plant-to-plant communication and has been documented in more than 50 species.

Previous work by my host and collaborators found consistent emission patterns of VOCs in sagebrush (A. tridentata), such that plants could be assigned to two heritable ‘chemotypes’. Chemotypes are a distinct and consistent motif of VOCs, dominated by a few compounds that enable the overall floral bouquet to be distinguishable from others. Communication between plants of the same chemotype resulted in less damage by herbivores compared to that between plants of different chemotypes. The motivation for our CVVOC proposal was based on the discovery of several additional chemotypes providing an opportunity to rigorously test and explore the community-wide effects and mechanisms involved in the maintenance of chemotypic diversity. To achieve this primary objective, a secondary objective was to develop fast and reliable markers of volatile-mediated communication and subsequent induced resistance in sagebrush.

In addition to understanding the ecological importance of chemotype-dependent communication and the evolutionary processes that have selected for and maintain it, this work has direct applications in sustainable agriculture. The identification of plant traits that increase resistance to pests is important for ensuring food security and may decrease our dependence on pesticides.

Through combing ecological theory with experiments both in the field and laboratory settings, CVVOC has developed new tools to more rapidly assess plant communication in sagebrush, expanded on the previous model predicting the efficacy of communication between two chemotypes and provided additional support for an evolutionary model explaining the mechanisms underpinning chemotypic diversity.
Below we provide a list the work performed over the two-year reporting period and the main findings of each. As noted, two publications have resulted from this work with more being prepared to submit for peer review. This work has been discussed at several seminars and one congress in 2018 Due to covid-19, planned congresses in the summer of 2020 were canceled.

• A two-season field experiment to study intraspecific communication. The design consisted of incubating branches of a sagebrush plant of a predetermined chemotype with damage-induced plant volatiles (DIPVs) from sagebrush with the same or different chemotypes. Leaves from experimental branches were used for 1 of 4 assays: 1) enzyme activity, 2) gene expression 3) a bioassay with a sagebrush specialist beetle and 4) a total herbivore damage assay at the end of the growing season.

o We observed a 25% and 40% decrease in herbivory when exposed to DIPVs from different or the same chemotype, respectively, when compared to an ambient air control.

o The total herbivory assay agreed with gene expression levels and enzymatic activity.

o We found interactive effects between emitting and receiving chemotypes, indicating the complexity of plant-to-plant communication. For example, we observed non-reciprocal chemotypic perception. This non-reciprocity occurs in interspecies communication, but this is the first documentation of this phenomenon occurring between individuals of the same species.

• A single season field experiment studying interspecific communication between sagebrush and tobacco.

o We did not detect an effect of sagebrush DIPVs on glandular exudate production in receiver tobacco plants, nor decreased leaf damage by herbivores. Sagebrush is known to induce resistance in tobacco plants. Our inability to detect an effect of DIPVs regardless of chemotype likely stems from the low abundance of the primary tobacco herbivore, Manduca quinquemaculata. Because this work was done in collaboration within a larger study, the results were still published.

• A major objective of CVVOC was to develop relatively fast and reliable indicators of volatile-mediated communication and induced resistance. To this end, we developed 3 different assays: 1) bioassay with herbivores, 2) enzymatic assays and 3) gene expression assays using RT-PCR.
o All assays were able to distinguish between leaves exposed to DIPVs and control leaves. To date, only the bioassay has been published.

• We conducted a laboratory experiment in growth chambers using sagebrush grown in a glasshouse from seeds collected at our field sites in California, USA. Here we utilized air pumps to deliver DIPVs from emitter branches to receiver plants of known chemotypes under controlled conditions.

o The results largely reflected those seen in the field experiment with one major exception: we were able to detect a weak effect of DIPVs on VOC emissions of undamaged plants. Though attempted, we were unable to detect this in the field, likely due to the weak nature of the effect.
Overall CVVOC has been successful. To a large extent, we have achieved our research goals. To date, two publication have resulted from this project with more in the process of being submitted for peer review. Moreover, this research has led to several new lines of investigation.

Through the development of various assays to detect volatile-mediated communication and induced resistance, this project has provided tools for future researchers to investigate similar questions in the field and laboratory settings. Using these tools, we were able to test, confirm, and expand upon the current models of volatile-mediated communication and evolutionary process generating and maintaining chemotypic diversity.

Demonstrating the role chemotypes play in inducing resistance in sagebrush begs the question if similar processes are occurring in economically important plants. Selecting for plant traits that increase yield is a common practice. Results from this project indicate that more studies assessing the VOC emission profiles, inducibility, and apparency to herbivores via volatile cues of novel genetic crop lines are warranted.