COS 232-5 - Tree stress responses across biological scales: a case study in mangroves

Abstract: Trees must surmount the challenge of stress tolerance if they are to survive, and they do so in a variety of ways. Until recently, tree stress responses have primarily been made using physiological, elemental, or morphological measurements. While easy to collect, they are still proximate responses subject to underlying metabolic and genetic controls. Thus, a holistic understanding of stress responses must be cross-scale — from genes to metabolism to physiology to vital rates. To that end, we present a case study of genes-to-phenotype responses of a Pacific mangrove guild to better understand the biological scales at which stress responses manifest. We sampled three focal mangrove species: Rhizophora stylosa, Bruguiera gymnorhiza, and Kandelia obovata across a salinity gradient and characterized their differential gene expression, epigenetic modifications, phytochemical profiles, and physiological performance to ask how salinity stress affects each level. We found clear salinity-mediated effects at all biological scales, most markedly in the plants' gene expression patterns for known salinity tolerance genes — some of which were differentially methylated between environments, highlighting the importance of these dynamic processes for trees that might otherwise be unable to adapt to harsh conditions over a single generation. Focusing on the plants' phytochemical properties, we find that bulk chemical profiles of mangroves, regardless of family, cluster with one another in multidimensional space relative to other nearby non-mangrove tree species. Mangrove leaves, particularly from plants in saline habitats, appeared to contain a lower diversity of secondary metabolites compared to leaves of nearby non-mangrove plants, hinting perhaps at the role of chemically-mediated herbivore defense in these species and/or a tradeoff between secondary metabolite production and salinity tolerance. Beyond the effect of mangrove character and environmental context, both the full and secondary phytochemical profiles of our study mangroves reflected the evolutionary relationships between the study species, suggesting that phylogeny can, in some cases, be used as a proxy for phytochemistry. Since plant chemical traits can have major consequences on herbivory, decomposition, and nutrient cycling, much more work is needed to understand how phytochemistry varies across stress gradients and links to lower-level (e.g., gene expression) and higher-level (e.g., growth rate) processes. Ongoing work in our group will investigate these responses in more taxa across different ecological contexts such as over latitudinal gradients, in native and invasive populations, and under joint biotic/abiotic stressors.