Adaptable Planet

As Earth’s climate changes, will plants be able to adapt?
Plants are sensitive to temperature, as are humans and animals. In fact, the survival, growth, and reproduction of all living organisms are affected by the ambient temperatures of their environment. However, unlike humans and animals, plants aren’t mobile and consequently, must find other ways to adapt to a changing environment. Plants play an important role in our health and well-being. They are a mainstay in human and animal diets, are a natural source of many medicinal compounds, and are becoming an increasingly critical part of bioenergy production.

Kathleen Engelmann, Ph.D. in Ecology and Evolutionary Biology, was interested in studying how genes play a role in determining a plant’s response to surrounding temperature fluctuations. The findings of such a study are especially important for economically important species. As Engelmann explains, “This research is likely to become more important as global temperatures increase.”

With the support of a UB Seed Money Grant, Engelmann conducted the study, Genetics of Variation in Thermal Tolerance, to address several fundamental questions in ecological genetics, using the small flowering plant, Arabidopsis thaliana. The plant was selected for its widespread use as a model genetic organism. Major research questions included: What are the major regions of the Arabidopsis genome that control the response to thermal variation? Is there standing genetic variation to thermal tolerance in natural accessions of Arabidopsis? If so, is that variation geographically distributed in such a way as to be consistent with adaptive evolution in this species? Different Arabidopsis strains were studied to determine whether or not responses to temperature variation were gene dependent.

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Engelmann notes that while it is widely appreciated that ambient temperature in most terrestrial systems varies, often substantially, the majority of studies on plant growth and physiology have been conducted under steady or minimally fluctuating temperature regimes. In their natural environment, plants and other organisms with limited mobility must cope with temperature fluctuations by altering their growth and development. This involves the integration of multiple external signals, including temperature and light, for the regulation of development from germination to growth to flowering stages.

Plants depend on the perception of both high and low temperatures, both for their survival and for the regulation of key developmental events. In fact, temperature signals play a major role in the regulation of seed dormancy, growth rates, morphology, and competence to flower. Plant responses are sensitive to the genetic background of the ecotype being tested, the vernalization (the long cool winter periods required to induce flowering) status of the plant, and both the mean daily temperature and the daily highs and lows. Recent studies in Arabidopsis and other plants have shown that growth and reproduction under fluctuating temperature differs from growth under steady temperature conditions and that the response to fluctuating temperatures is under genetic control, although the regulatory mechanisms are yet to be defined.

Previous research conducted by Engelmann demonstrated that a Spanish ecotype of Arabidopsis, Ts-5 (Tossa de Mar, Spain), had much greater seedling survivorship than the common lab strain Col g(1) (Columbia, glabrous mutant, also the parent strain of a well mapped set of recombinant inbred lines) under elevated ambient temperatures and at temperatures that range from 35ºC during the daytime to 4ºC at night. This finding and those of other researchers led Engelmann to conclude, “Clearly understanding temperature response pathways requires an understanding of how gene action interacts with hormone regulators to produce phenotypic variation. Evidence to date strongly suggests there are multiple pathways involved in temperature response. However, few studies have set out to test the response to variable temperature and relatively few address the degree of standing natural variation.”

“This research is likely to become more important as global temperatures increase.” – Engelmann

Engelmann’s research consisted of two experiments, using environmental chambers to compare growth under a temperature regime that fluctuated daily from 12ºC to 32ºC versus a constant 22ºC growth environment. The first experiment employed two Arabidopsis strains: (1) a panel of recombinant inbred lines derived from a cross between Kas-1, a wild ecotype from Kashmir, and (2) Col g1, a commonly used lab strain. These strains are known to segregate for several genes of known importance to Arabidopsis growth and development, some of which are temperature sensitive. The second experiment analyzed the expression of genes found in regions of the plant genome that may be involved in the response to fluctuating temperature.

Plants were checked daily for production of new leaves and mortality. Data was recorded when the first, second, and fourth pair of true leaves were visible, and when flowering was initiated. For each time point, the date was recorded and, where appropriate, length, width and petiole length of the largest leaf, leaf angle, number of leaves, and rosette diameter were recorded. Analyses of survivorship, growth rate (measured as leaf number doubling time), flowering time, and shade avoidance (leaf shape and angle) were analyzed for significant differences.

Results demonstrated that many of the major regulatory genes controlling growth and flowering did not appear to regulate these functions under summer-like field conditions and, in fact, there appeared to be far fewer regulatory Quantitative Trait Loci (QTL) under these conditions. This loss of regulatory QTLs was correlated with a loss of phenotypic correlation between traits and a loss of fitness observed under these conditions. Furthermore, there are several important regulatory genes that have different expression profiles under fluctuating versus constant temperatures.

Biology undergraduates had the opportunity to work closely with Engelmann throughout the study and co-present the findings at an annual meeting of Society for the Study of Evolution.