1a. Objectives (from AD-416):
Objective 1: Define the key regulatory elements controlling photosynthate partitioning and nitrate assimilation and their interactions; develop and begin to test strategies to modify those processes for agricultural purposes. Objective 2: Determine the mechanistic basis for limitations on photosynthetic performance including those imposed by agriculturally significant stresses. Objective 3: Establish the major features controlling the response of photosynthetic productivity in soybean and corn to elevated atmospheric CO2, tropospheric ozone, and their interactions with drought and temperature, explore the bases for genetic variability in responses, and test potential transgenic amelioration strategies. Objective 4: Determine the environmental impacts of land cover change associated with alternative bioenergy crops.
1b. Approach (from AD-416):
Investigate isoform specificity for nitrate reductase (NR) posttranslational modification in vivo, and elucidate the impact of 14-3-3 binding on NR protein degradation. Localize the membrane binding site(s) on sucrose synthase and identify factors that may control the interaction. Use high-resolution spatial and temporal analysis of leaf growth to identify specific areas where leaf growth is occurring. Determine the biochemical factors responsible for the lower activation state of Rubisco, at high temperatures and test potential transgenic amelioration strategies. Further elucidate the role of Rubisco activase in thermal sensitivity/tolerance. Determine the biochemical basis for the "Green Seed Problem" of canola. Perform metabolite analysis of growing leaves under elevated CO2 and O3 to identify key components that may be involved in controlling growth. Determine the factors that lower the activation state of Rubisco under sink- and/or N-limited conditions, which are often encountered when plants are grown under high CO2. Explore the interaction of elevated CO2 with drought on soybean performance. Explore the interaction of elevated CO2 with temperature on soybean and corn performance. Determine if growth at elevated CO2 enhances or ameliorates oxidative stress. Determine the impact of land cover change from row crops to perennial grasses on hydrological cylce and carbon biosequestration.
3. Progress Report:
Significant progress has been made over the course of this project. Similar research will be addressed in a new project during the next fiscal year. The role of tyrosine phosphorylation in the regulation of plant receptor kinases was established. It was also discovered that the binding of calcium/calmodulin mediates regulation of receptor kinase signaling. The molecular basis for decreased productivity in response to elevated ozone was investigated and genetic variation in soybean was established and a mapping population produced to map O3 sensitivity resistance QTLs. Mechanistic models of photosynthesis were employed to predict changes that would result in improved photosynthetic efficiency and proof of concept experiments conducted. It was established that the biochemical basis for the “Green Seed Problem” of canola is due to the interference with the regulation of a key enzyme in the degradation pathway. Techniques were developed for metabolic analysis of growing leaves under elevated CO2 and O3 to identify key components involved in controlling growth and the importance of antioxidant metabolism established. The interactive effects of elevated CO2 and drought on soybean performance was investigated illustrating the elevated CO2 could ameliorate the effects of mild to moderate drought by conserving soil moisture but did not help and in fact exacerbated the impacts of severe drought. The interaction of elevated CO2 and increased temperature on the photosynthetic and agronomic performance of corn and soybean under production field conditions was also investigated. It was demonstrated, as predicted by theory, that elevated CO2 raises the temperature optimum of C3 but not C4 crops. Shorter term heating to mimic heat wave treatments were conducted showing that when applied during different developmental stages in corn and soybean the most severe effects on yield were when the heat wave treatment coincided with reproductive development. The impact of alternative bioenergy crops on hydrological cycle at multiple spatial scales was investigated using leaf chamber techniques and scaling to the canopy and landscape with modeling and eddy covariance measurements. It was discovered that leaves are sufficiently coupled to the atmosphere that leaf level effects extrapolate in predictable ways to the canopy scale. The consequences of land cover change associated with alternative bioenergy crops on greenhouse gas emission was measured showing and quantifying the importance of starting land cover and level of establishment of the new crop.
1. Redefined the ozone dose response of modern soybean lines. Ozone is an atomospheric pollutant and an important emergent agricultural problem as crop plants exposed to ozone can have substantially reduced yield. ARS researchers investigated the physiological response of soybean to exposure to ozone and identified and characterized an exposure response/threshold for soybean under fully open-air agricultural conditions at the SoyFACE research site. On average, soybean yields are reduced by ~35 kg ha-1 per part per billion increase of ozone over ambient concentrations. There is a linear decline in canopy area, photosynthetic capacity and harvest index in increasing ozone concentrations, which together accumulate to reduce yields. The study also provides strong evidence that modern soybean lines are not more ozone tolerant than older lines demonstrating that so far breeding has not selected for ozone tolerance and different strategies are required to select for ozone tolerance in soybean.
2. Chronic ozone exposure worsens the reduction in photosynthesis and early onset of senescence caused by limited N availability. ARS scientists at Urbana, IL showed that nitrogen uptake and allocation by soybean was interactively affected by growth in limiting nitrogen and elevated ozone. Gene expression markers were used to track the progress through senescence, and showed that limiting nitrogen and elevated ozone, separately and in combination, caused an acceleration of senescence. These results suggest that in growing regions with poor soil fertility crop productivity losses due to ozone pollution will be worse than in good soils with adequate fertilizer inputs.
3. Calcium is well known signaling molecule in plants that acts in regulating growth and development as well as responses of plants to environmental stresses. These calcium signals are mediated by several families of calcium-binding proteins, including the calcium-dependent protein kinases (CDPKs) and calcium-binding protein, calmodulin. Although both CDPKs and calmodulin act in calcium signaling in plants, their possible interactions are not known and represent a important knowledge gap in understanding and predicting plant/environment interactions. It was discovered by ARS researchers in Urbana, IL that while the CDPKs do not require calmodulin for their primary function, CDPKs do bind calmodulin in a calcium - dependent manner. These results fundamentally alter two paradigms of CDPK regulation that might be manipulated in future studies to regulate growth, stomatal aperture, hormone signaling, and immune/stress signaling in crop plants to favor productivity.
4. The control of chlorophyll degradation is regulated by the enyzme pheophorbide a oxygenase (PaO). During senescence in aerial plant tissues, chlorophyll must be degraded by a carefully controlled process because free chlorophyll and many of it degradation products will result in severe photodamage if allowed to accumulate. An ARS scientist in Urbana, IL lead work showing that the key enzyme in a pathway is controlled by calcium-dependent protein kinase phosphorylation of specific sites on the enzyme PaO. It was further shown that low temperature interference with this process is the cause of very costly “Green Seed” problem in canola as well as provides important insights into engineering “Stay Green” into crop plants extending the growing season, which has been shown to improve yield.
VanLoocke, A., Twine, T.E., Zeri, M., Bernacchi, C.J. 2012. A regional comparison of water use efficiency for miscanthus, switchgrass and maize. Agricultural and Forest Meteorology. 164:82-95.