The chemical ecology of landscapes: Distribution and dynamics of small molecules mediating species interactions

Research Team

Meredith Schuman

Michael Schaepman

Ewa Czyż

Summary
Plants are highly accomplished chemists and produce thousands of specialized metabolites to mediate their interactions with other organisms. In particular, by releasing volatiles and other exudates, sessile plants can interact with and manipulate even their remote surroundings. The production of most specialized metabolites is specifically triggered by certain environmental factors, including biotic interactions such as herbivory as well as abiotic factors such as light quality and water availability, and their distribution is a signature. In turn, this distribution will affect future species interactions by making plants more or less attractive or susceptible for example to herbivores, natural enemies of herbivores, and microorganisms. Yet these distributions are rarely measured, because most established methods to measure plant specialized metabolites require destructive sampling and sample processing. This renders temporally and spatially resolved analyses laborious and introduces substantial time lags between collecting samples and interpreting results. In this project, we will (1) develop remote sensing and environmental sampling methods to efficiently measure distributions of plant specialized metabolites in landscapes, (2) determine the contribution of plant genetic variation to distribution patterns of plant specialized metabolites, and (3) investigate the consequences of these distributions for ecological interactions. This project will result in maps of plant diversity, chemical landscapes, and ecological interactions, and will investigate predictive relationships among these “maps”.

Research
Background
Plant specialized metabolites comprise thousands of small molecules derived from all pathways of general metabolism (e.g. carbohydrate, amino acid, and lipid biosynthesis), and are defined by their diversity and variability both within and between species, which results from both genetic and environmental factors (1). They are thought to evolve in response to diverse ecological pressures (2), and many have direct physiological or behavior-modifying effects on other organisms (including humans), such as medicinal or toxic effects, attraction or repellence (3, 4). The production and distribution of plant specialized metabolites can thus strongly affect the fitness, activity, and distribution of other ecological community members, including other plants, herbivores and their predators and parasitoids, and microbial communities. Furthermore, specialized metabolites may be taken up or modified by other community members and co-opted for other functions, such as when insect herbivores sequester toxic plant metabolites to defend themselves against predators and parasites (5). However, the distribution of plant specialized metabolites in landscapes is challenging to capture: destructive sampling and laborious tissue and data analyses are generally required to characterize metabolomes. We will investigate remote sensing and environmental sampling methods to determine the distribution of these biologically active metabolites on spatiotemporal scales relevant for ecological interactions, and we will use these approaches to study the ecological effects of changing distributions.

Approach
The following technologies show promise for resolving the distribution of plant specialized metabolites in landscapes. We will not hesitate to investigate promising alternatives.

Imaging spectroscopy (400-2400 nm) can be a high-throughput, non-destructive tool to simultaneously detect variation in multiple important plant traits (6). Well-developed models assign specific spectral regions to tissue pigment, dry matter, and water content, and structure, based on physical principles and observed correlations (7–9). A few studies have associated spectral variation with concentrations of specific plant specialized metabolites (10, 11). These associations have rarely been tested using manipulations of the underlying traits. We will employ both manipulation and association studies to determine the contribution of metabolic variation within plant species to spectral variation, and to develop spectroscopic imaging as a tool to measure metabolic diversity non-destructively at different scales. However, the resolution of imaging spectroscopy to determine variation in plant specialized metabolites is not yet known. We will also investigate other optical spectroscopy methods, as well as environmental sampling techniques which can be easily adapted for sampling distributions within populations and communities.

Molecular spectroscopy techniques using optical frequency combs have rapidly advanced in the last 10 years (12). A recent study by Scholten and colleagues developed an optical frequency comb device for gas sample analysis using CO2 as a test analyte (13), and Stern and colleagues have developed a small, robust, low-power optical frequency comb device which could be incorporated in battery-operated precision portable spectrometers (14).

The silicone polymer polydimethylsiloxane (PDMS) has high affinity for many plant specialized metabolites, including volatiles and exudate components, and has already been developed for environmental sampling applications (15). PDMS sampling has the advantage that samples can be collected from well-defined spatial locations and over a range of time scales, and analyzed using high-resolution chromatography instruments without the need for extraction (gas chromatography), or following a simple high-throughput extraction (liquid chromatography) (16, 17). Another established approach, proton transfer reaction-mass spectrometry (PTR-MS), is limited in its ability to resolve chemical structures, but captures real-time dynamics of volatiles. PTR-MS instruments can be transported on vehicles and used to acquire measurement of volatiles by directly sampling the atmosphere (18).

We will choose appropriate representative plant species for initial investigations: tree species for aerial imaging spectroscopy, and faster-growing annual plants for measurements which can be applied at a smaller scale. Within each species, we will perform both associational and manipulative studies. When possible, we will employ precise genetic and chemical tools to manipulate individual metabolites and pathways and to determine associations. We plan measurements at URPP-GCB sites (Laegern, Borneo), in common garden experiments and controlled environments, and collaborative projects at external field sites. Chemical “maps” will be compared to distributions resulting from destructive harvests and analyses of samples from individual plants. Genetic diversity measurements from parallel projects will be used to determine associations between genetic and chemical diversity. We will cooperate with collaborators to survey the distribution of other ecological community members, and we will perform experiments manipulating ecological communities. These experiments will be used to associate “chemical maps” with maps of community composition and ecological interactions.

Conclusion
This project will integrate both established and state-of-the-art spectroscopy techniques and environmental sampling methods to investigate the distribution of plant specialized metabolites in real environments on a scale relevant for ecological interactions. We will focus on the variation which may represent adaptive responses to ecological pressures, rather than aiming to establish species-typical profiles. In addition to associational studies, we will use chemical and genetic techniques to manipulate distributions in order to test the sensitivity and accuracy of our detection methods, and to determine consequences for ecological interactions.

References

1. Schuman MC, Van Dam NM, Beran F, Harpole WS (2016) How does plant chemical diversity contribute to biodiversity at higher trophic levels? Curr Opin Insect Sci 14:46–55.

2. Chae L, Kim T, Nilo-Poyanco R, Rhee SY (2014) Genomic signatures of specialized metabolism in plants. Science 344:510–3.

3. Schuman MC, Baldwin IT (2018) Field studies reveal functions of chemical mediators in plant interactions. Chem Soc Rev 47:5338–5353.

4. Jassbi ARAR, Zare S, Asadollahi M, Schuman MC (2017) Ecological Roles and Biological Activities of Specialized Metabolites from the Genus Nicotiana. Chem Rev 117(19):12227–12280.

5. Erb M, Robert CA (2015) Sequestration of plant secondary metabolites by insect herbivores: molecular mechanisms and ecological consequences. Curr Opin Insect Sci 14:8–11.

6. Schneider FD, et al. (2017) Mapping functional diversity from remotely sensed morphological and physiological forest traits. Nat Commun 8:1441.

7. Jacquemoud S, Baret F (1990) PROSPECT: A model of leaf optical properties spectra. Remote Sens Environ 34:75–91.

8. Féret JB, Gitelson AA, Noble SD, Jacquemoud S (2017) PROSPECT-D: Towards modeling leaf optical properties through a complete lifecycle. Remote Sens Environ 193:204–15.

9. Zhang Y, et al. (2017) An extended PROSPECT: Advance in the leaf optical properties model separating total chlorophylls into chlorophyll a and b. Sci Rep 7:6429.

10. Couture JJ, Serbin SP, Townsend PA (2013) Spectroscopic sensitivity of real-time, rapidly induced phytochemical change in response to damage. New Phytol 198:311–9.

11. Couture JJ, et al. (2016) Spectroscopic determination of ecologically relevant plant secondary metabolites. Methods Ecol Evol 7:1402–12.

12. Dutt A, et al. (2018) On-chip dual-comb source for spectroscopy. Sci Adv 4:e1701858.

13. Scholten SK, et al. (2018) Number-Density measurements of CO2 in real time with an optical frequency comb for high accuracy and precision. Phys Rev Appl 9:054043.

14. Stern B, Ji X, Okawachi Y, Gaeta AL, Lipson M (2018) Battery-operated integrated frequency comb generator. Nature 562:401–5.

15. Seethapathy S, Górecki T (2012) Applications of polydimethylsiloxane in analytical chemistry: a review. Anal Chim Acta 750:48–62.

16. Kallenbach M, et al. (2014) A robust, simple, high-throughput technique for time-resolved plant volatile analysis in field experiments. Plant J 78:1060–1072.

17. Kumar P, Pandit SS, Steppuhn A, Baldwin IT (2014) Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore  defense. Proc Natl Acad Sci U S A 111:1245–52.

18. Schaub A, et al. (2010) Real-time monitoring of herbivore induced volatile emissions in the field. Physiol Plant 138:123–33.