6th Annual Symposium
November 14 - 16, 2008
in Kansas City
Click here for details!

Johnson, Garrett, and Sara Baer (Southern Illinois University)

The central grasslands of the United States are amongst the most productive rangelands. These grasslands are dominated by big bluestem, Andropogon gerardii, which persists across the sharp and often variable precipitation gradient, ranging from 1200 mm annual precipitation in Illinois to only 400 mm in western Kansas. Changes in amount and timing of precipitation, as predicted from climate change scenarios, are likely to be critical abiotic stressors in determining the future productivity and sustainability of these rangelands. In this proposal we aim to investigate the degree of ecotypic variation across the range of big bluestem, predict the functional response of this species to climate change across its range, and elucidate the genetic basis for differential variation in function across the natural climate gradient and in response to experimentally altered conditions. Our objectives are to: 1) characterize functional attributes and genetic structure of locally adapted, phenotypically distinct populations of big bluestem across a precipitation gradient from KS to IL; 2) quantify differential response of ecotypes to the natural precipitation gradient and simulated changes in amounts and timing of precipitation as predicted by climate change scenarios, and 3) to characterize the ecosystem consequences of the interaction between genetic differentiation (at the level of gene expression) and climate in newly assembled stands consisting of single v. multiple genotypes. The basis for documenting ecotypic variation has historically relied on differences in plant growth rates, stature, and phenology. We hypothesize that the steep precipitation gradient in the Midwest provided selection to favor the development of local ecotypes that will respond differentially to climate change due to variation in functional attributes [e.g., net rates of photosynthesis (A_net), water use efficiency (WUE), nutrient use efficiency (NUE),and carbon allocation above and belowground] that may underpin variation in physical characteristics resulting from local genetic adaptation. We will use a reciprocal common garden experiment, where seeds from three source populations (western KS, eastern KS, and IL) and all populations combined will be sown in replicated plots at each of the 3 source locations. Each reciprocal common garden experiment will also receive a rainfall manipulation, such that simulated droughts will be punctuated by larger, but less frequent precipitation events. Ecophysiological response variables to be measured will include A_net , WUE, NUE, relative growth rate, and above-and belowground biomass. We will also characterize the genetic structure of each of the source populations using AFLP techniques and characterize the functional consequences of genetic differentiation at the level of gene expression using cDNA-AFLP and/or cross-species microarray hybridization using Zea mays oligonucleotide microrray chips.

Thus, this research will provide a mechanistic understanding of how variation in a dominant forage grass is locally adapted to natural and simulated changes in precipitation (drought stress) and the genetic basis for this local adaptation. The research will identify the genetic mechanisms that contribute to drought stress response of an ecologically dominant forage grass under realistic scenarios of altered timing and amount of precipitation in the Great Plains. This research will provide data that will help optimize resource use of the Great Plains rangelands and will ultimately provide for greater agricultural sustainability in the face of predicted environmental change.
 

Environmental and ecological controls on gene expression of root initiation/development in prairie plants

Shah and Johnson

In almost all-terrestrial ecosystems, little knowledge exists about belowground processes in general and root growth and productivity in particular. External biotic and abiotic stimuli are well known to affect root processes as determined from laboratory studies (Schiefelbein and Benfey, 1991). Yet we know little about controls over root processes in natural ecosystems, especially tallgrass prairie where below-ground processes likely regulate ecosystem structure and function. Johnson and Matchett (2001) quantified the controls of fire and grazing over root productivity. While this study shed some light on environmental stimuli affecting root productivity, root growth and development are obviously under control of an intrinsic developmental program as influenced by the environment. Our objectives are to link processes controlling root growth in natural systems to the regulation of gene expression of root initiation. Rarely has such information on root growth in natural systems been linked at the level of gene expression. Such studies are now possible with the recent publication of the genome of the model plant Arabidopsis. Changes in expression pattern of many of these genes, for example, LRX1, GLABRA2 (GL2), AUX1 and ANR1 are associated with developmental changes in root architecture and morphology (Costa and Dolan, 2000; Zhang and Forde, 1998).

We will focus on environmental factors (e.g., grazing, nutrient availability (as mediated by fire), and drought) that may be important environmental stimuli mediating the expression of genes involved in root initiation and development. Because we are interested in prairie plant gene expression, we will first determine the amount of homology between the well-known Arabidopsis genome and dominant prairie grasses (Andropogon gerardii, Sorgastrum nutans) and prairie forbs. We expect several genes will share homology because we expect the root initiation and development to be highly conserved over evolutionary time. We will monitor changes in root hair initiation and number and lateral root growth in response to environmental factors using molecular and microscopic techniques. The molecular approach will involve following expression of the AUX1, GL2, LRX1 and ANR1 genes in roots by in situ and RT-PCR. We expect AUX1, LRX1 and ANR1 to be turned on in response to fire and low nutrients, and result in root hair proliferation. We expect expression of these genes and concomitant root hair emergence will be suppressed in response to grazing, increased nutrients, and drought. A complementary microscopic approach will involve counting the number of emerged and unemerged (buds) root hairs per centimeter of root length. We also expect grasses to be more plastic in their response to ecological stimuli than forbs. This study will shed new light on environmental controls on root growth spanning from genes to ecosystems and provide a genetic mechanistic basis for the environmental regulation of gene expression in roots.