Research Grants for 2020 were awarded to:
Mosses are small, non-vascular plants that are poikilohydric and desiccation tolerant, which means their tissues quickly equilibrate to ambient water content and they are able to recover from being completely dry. Terrestrial mosses will dehydrate and go dormant between precipitation events. While many mosses are found in cool, low-light environments, several species are abundant in deserts where they are desiccated for much of the year. Mosses that live in the desert spend most of their time in a desiccated, quiescent state exposed to high solar radiation due to low atmospheric water vapor with no ability to actively protect from or repair damage by UV radiation. My research investigates UV protection mechanisms used by two Mojave Desert mosses S. ruralis and S. caninervis and will contribute to understanding how these plants tolerate high levels of UV radiation while desiccated as well as the evolutionary history of UV tolerance in land plants.
Syntrichia caninervis and S. ruralis are highly desiccation tolerant desert mosses; they can lose almost all of their cellular water and recover after rehydration. In nature, desert mosses not only have to withstand the damage of desiccation itself but must also be able to recover from any damage incurred while quiescent or have adequate mechanisms for damage from desiccation. Mosses have no ability for active repair when dry and face risk of damage to sensitive molecules, including those in the photosynthetic apparatus and DNA which absorb wavelengths in the UV spectrum. Strategies for passive photoprotection in mosses are poorly understood. Mosses lack many of the physical protective structures seen in vascular plants, such as waxy cuticles that strongly absorb UV radiation.
For part of my dissertation I am testing the acute and chronic transcriptomic response to UV in S. caninervis and S. ruralis. For chronic effects, I have grown clonal lineages from fragments to maturity in high and low UV environments, with or without a desiccation effect. I then flash-froze the samples in liquid nitrogen, sequenced RNA, and compared transcript abundances between treatments and species. This experiment proved insightful and provided concrete answers about the genes involved in the long-term, acclimated response to UV exposure under different hydration environments. Yet, an important question remains: what is the ‘early’ or acute transcriptomic response to UV in a plant that has never been exposed to it before? Literature suggests I can expect interesting and insightful differences in the two timelines and answering this question would greatly strengthen my research and findings. To proceed towards this goal, I have performed a companion experiment.
Clonal lineages of the two study-species were cultured in white fluorescent lights with no UV radiation (n = 18). When mature, the samples were exposed to 0, 10, or 30 minutes of broad range UV-A and UV-B radiation; the same high-UV environment as in the first half of this project. All samples were immediately flash-frozen and stored in -80 C after UV exposure to preserve their transcriptomes for analysis. To continue this experiment, total RNA will be extracted, purified, and sent to QB3 for library prep and sequencing. Transcriptomes will be analyzed for differential expression and identification of candidate UV protection genes to understand the early exposure timeline and compare it to the previous ‘chronic UV exposure’ results.
My objective is to investigate the evolution of cold hardiness in a California native plant, Cephalanthus occidentalis, which has a native range well above and below the frost line. Through the examination of tradeoffs between cold tolerance and competition (growth rate and herbivory resistance), supplemented with phylogeographic inquiry, I will uncover estimated divergence times and population dynamics to learn strategies woody angiosperms use to expand from tropical to temperate biomes.
Cephalanthus occidentalis (Rubiaceae) is the only Neotropical woody plant that has a geographic range from deep within the tropics to the temperate-boreal border, but the reasons for this broad range have not been studied. This geographic distribution is very unusual because the energy that plants invest in frost tolerance is thought to be a tradeoff with competitive ability in tropical habitats, and this tradeoff is thought to be responsible for enforcing range limits within biomes for woody plants.
Populations in California represent a distinct group from the rest of the continuous population that spans from southern Mexico into northern Maine.
Functional morphological data matched with phylogeographic information of many habitats occupied by Cephalanthus will provide opportunity to investigate specific traits that correlate with the cold hardiness. This species provides a chance to explore the role of diverse evolutionary and ecological drivers involved in lineage divergence, i.e. frost, herbivore resistance. Cephalanthus occidentalis may have recently evolved frost tolerance and dispersed into the temperate zone, facilitating divergence between morphologically indistinguishable temperate and tropical populations. Alternatively, the species may have evolved a low-cost mechanism of frost tolerance throughout its range that does not tradeoff with competitive ability and may represent a single species living in disparate biomes.
I will investigate and compare phylogeographic variation at the population level of C. occidentalis in the California disjunct range and the continuous range from central Mexico into northern Maine (Fig. 1). Estimating the time of these diversification events and population dynamics will elucidate strategies needed for woody plants to expand from tropical to temperate biomes. I will sample at least 10 populations of C. occidentalis in California, the east coast of the United States and in Mexico; focusing collections toward extreme elevational, latitudinal and water availability boundaries. I will test if these environmentally distinct populations represent genetically diverged populations and have traits that correlate along these gradients. DNA extraction, library prep, and RADSeq sequencing will follow standard approaches. Sequencing reads will be cleaned and assembled using custom Perl scripts. Reads will be aligned to the reference Coffea arabica genome using Novoalign. Phylogenetic inference will use the maximum likelihood criterion. I will also conduct reciprocal-transplant and common garden experiment, to test whether tropical, subtropical, and temperate populations of C. occidentalis have variation in growth rates, frost tolerance and defense against herbivores. I aim to set the northern reciprocal transplant/common garden along the four stations associated with the University of California Natural Reserve System at the White Mountain Research Center and the southern along the Sierra Madre del Sur in Oaxaca Mexico. These two relative locations will enable elevational and latitudinal comparison, and test phenotypic plasticity vs local adaptation in growth, herbivory defense, and frost tolerance. For this project, I will collect both seed and propagate 10 (1-2cm x 20 cm) cuttings from at least five populations in California, Mexico and the east coast of the United States. I will compare basal stem circumference, maximum stem height and number of stems of plants grown in herbivore exclosures vs unprotected controls and measure the effect of herbivory.
Background & rationale
In an era of degrading ecosystems, innovative conservation practices are essential to maintaining the biodiversity of natural environments and the services they provide. Traditional conservation techniques include invasive species removal, seed banking, cryo-conservation, and out-planting. Recently, researchers have been studying the beneficial microbes living in and on plant tissue and investigating their potential to improve plant health in natural habitats. Plant microbes are ubiquitous in all wild plants and play a critical role in essential plant functions such as the promotion of growth, stress tolerance, and disease resistance. Most recently, studies have shown that inoculating threatened and endangered plants with these beneficial microbes may be a viable means of improving conservation of these species in the wild. In my dissertation research, I am specifically interested in the bacteria and microfungi living within and on seed tissue and understanding how we can harness these microbes for conservation efforts. Seed microbes are of particular importance because they can be transmitted from both the surrounding environment (horizontal transmission) as well as the mother plants (vertical transmission). This paternal inheritance of seed-associated microbes gives them priority in the colonization of plant tissue once germination occurs. This is significant considering vulnerability of the seed and seedling life stage. In some species, an average of 90% of seedlings do not survive. This makes the seed structure an important unit of microbial selection and can determine the trajectory for future adult plant microbiomes and health.
Despite the crucial role of seed-microbes in seedling survival and initial microbiome establishment, little is known about how they are transmitted, the assembly of their communities, and how they affect early plant health. Understanding these dynamics will be imperative for the future implementation of these microbes in conservation. Therefore, the proposed project asks three specific questions:
- How does the composition of seed microbes differ between plant species,
- How much of the variation in microbial composition can be explained by specific seed traits, and
- How does the combination of seed trait and microbe composition affect seedling health following out-planting?