Shu-Hua Cheng - US grants

9420054 Seemann The concentration of carbon dioxide in the earth's atmosphere is projected to double by the middle to end of the 21st century. This change will have substantial effects on natural and agricultural ecosystems, primarily because of the effect of carbon dioxide on photosynthesis, the major biological process in the global carbon cycle. In many plant species, long-term growth at elevated atmospheric carbon dioxide results in the substantial change in both absolute photosynthetic capacity and the protein composition of the photosynthetic apparatus. However, the molecular mechanism(s) by which external carbon dioxide concentration acts at the level of gene expression, are largely unknown. The overall objective of this research is to link carbon dioxide effects on photosynthetic physiological processes (e.g. leaf gas exchange) and biochemical processes (e.g. rubisco protein level and regulation of activity) to changes in both rubisco gene expression and specific carbohydrate levels in order to identify molecular mechanisms which control the response of plants to elevated carbon dioxide. Arabidopsis thaliana (wild-type and existing mutants) will be used as the model plant system. Specific objectives are: (1) Determine the effect of atmospheric carbon dioxide concentration during different developmental stages on rubisco large subunit (rbcL) and small subunit (rbcS) gene expression, rubisco holoenzyme level, activation state, RuBP pool size and the response of photosynthesis to intercellular carbon dioxide partial pressure (Ci) in Arabidopsis thaliana grown at and switched between a number of carbon dioxide concentrations (subambient, ambient, twice ambient, saturating). (2) Identify the cellular signal(s) that function(s) in the signal transduction pathway between atmospheric carbon dioxide concentration and rubisco gene expression by utilizing (1) mutants of Arabidopsis thaliana with altered source or sink activities (altered leaf carbohydra te metabolism, no seed production, altered root:shoot ratio, delayed flowereing time) in studies of carbon dioxide effects on rubisco gene expression and photosynthesis, and; (2) transient expression studies of carbohydrate effects on rbcS promoter-GUS constructs.

Seemann 9808753 Since the start of the industrial revolution, the concentration of C02 in the earth's atmosphere has risen by approximately 30%, primarily as a result of fossil fuel combustion and land-use changes, and is conservatively projected to double by the end of the 21st century. Photosynthesis, the major physiological process in the biosphere and the only biological process that removes a significant amount of C02 from the atmosphere, is itself extremely sensitive to C02 concentration. In the short term (hours to days) following exposure to 21st century C02 concentrations, the rate of photosynthesis is substantially increased, but in the long term (days to weeks) it may be significantly reduced as a result of an "acclimation" of photosynthesis, a process characterized by a reduction in the level of components of the photosynthetic apparatus. This acclimation phenomenon will have significant effects on both the global carbon cycle and ecosystem structure and function, but its biochemical and molecular basis are poorly understood. Furthermore, plant species may differ substantially in the extent that they may acclimate photosynthetic capacity to elevated atmospheric C02. The objectives of this research project are to: (1) understand the biochemical pathway(s) by which plants sense elevated C02 and signal changes in the expression of genes for photosynthesis; (2) identify the molecular mechanisms that in fact alter gene expression and consequently photosynthesis at elevated C02; and (3) determine the biochemical and molecular bases for species differences in photosynthetic acclimation to elevated C02,

Plant calcium-dependent protein kinases (CDPKs) have dual functions as calcium sensors and effectors, and are thought to be involved in multiple cellular responses to external and internal stimuli. However, their precise physiological functions remain elusive. Using a protoplast based functional genomics approach and RNA interference based reverse genetics, a novel function of two Arabidopsis CDPKs in auxin signaling was discovered, providing the first molecular evidence connecting Ca2+ signaling to the action of auxin. A CDPK double knockout mutant shows severe meristem defects, repression of several key meristem regulators, and exhibits abnormal growth throughout the plant life cycle. Based on these results, it is hypothesized that these two CDPKs represent a "missing link" between auxin and Ca2+ signaling, and developmental processes. The long-term goal of this research project is to understand the mechanisms and significance of the CDPK-mediated auxin signal transduction pathway. The data will show that a rapid movement of CDPK from the plasma membrane to the cytosol upon auxin stimulation is a crucial step for CDPK-dependent auxin signaling. The objective of this project is to identify and characterize cellular mechanisms controlling auxin-dependent CDPK translocation. The proposed experiments will use functional assays with an auxin-responsive reporter in both protoplasts and seedlings, combined with fluorescent-protein based bio-imaging to establish if CDPK-mediated auxin signaling involves protein lipid modification, phosphorylation, vesicle trafficking, and/or cytoskeleton remodeling.