|LAPPE, RYAN - Iowa State University|
|BAIER, JOHN - University Of Florida|
|BOEHLEIN, SUSAN - University Of Florida|
|HUFFMAN, RYAN - Iowa State University|
|QIAOHUI, LIN - Iowa State University|
|SETTLES, MARK - University Of Florida|
|HANNAH, CURTIS - University Of Florida|
|STEWART, JON - University Of Florida|
|MYERS, ALAN - Iowa State University|
|HENNEN-BIERWAGEN, TRACIE - Iowa State University|
Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2017
Publication Date: 12/18/2017
Citation: Lappe, R.R., Baier, J.W., Boehlein, S.K., Huffman, R., Qiaohui, L., Settles, M.A., Hannah, C.L., Stewart, J.D., Scott, M.P., Myers, A.M., Hennen-Bierwagen, T.A. 2017. Functions of maize genes encoding pyruvate phosphate dikinase in developing endosperm. Proceedings of the National Academy of Sciences. 115(1):E24-E33. https://doi.org/10.1073/pnas.1715668115.
Interpretive Summary: Maize grain is one of the most important ingredients in animal feed in the United States. It is also used to produce ethanol, an important component of liquid fuel. It has many other uses in food and industrial processes. Endosperm is a tissue that makes up the majority of maize grain. Understanding how endosperm is formed will enable us to improve corn yield as well as alter the composition of grain to make it better suited to certain end uses. We examined a gene called PPDK and determined that it has a role in regulating endosperm metabolism in response to the availability of energy. This research will allow development of new hypotheses about endosperm metabolism and will allow researchers to better understand how grain is produced by plants. The public will benefit by decreased prices for food and fuel and improved grain-based products.
Technical Abstract: Pyruvate phosphate dikinase reversibly converts AMP, pyrophosphate and phosphoenolpyruvate (PEP) to ATP, orthophosphate and pyruvate. Maize PPDK functions in mesophyll in C4 photosynthesis, yet also is highly abundant in starchy endosperm during grain fill where its function is unknown. To investigate the non-photosynthetic functions of PPDK the two homologous genes that encode the enzyme, pdk1 and pdk2, were inactivated individually by transposon insertions, and together by tissue specific RNAi. Complete PPDK knockout in RNAi transgenic plants was demonstrated by undetectable protein or enzyme activity. Using these tools pdk1 was shown to provide the great majority of PPDK in leaf and pdk2 was found to generate approximately 90% of endosperm PPDK. Function of pdk1 was essential beyond the three leaf stage when C4 metabolism is acquired, indicating CO2 fixation by RUBISCO is insufficient to support viability. Mutants lacking pdk2 function did not display any obvious growth defect. Knockout kernels weighed the same on average as non-mutant kernels, indicating PPDK is not required for net storage compound synthesis. Complete loss of PPDK conditioned an opaque phenotype with associated changes in kernel composition and density predicted by near infrared reflectance spectroscopy. Total starch, nitrogen, and zein content were not altered, however, in transgenic kernels. Metabolite analysis indicated net endosperm PPDK activity is in the glycolytic direction and energy charge is reduced when the enzyme is absent. The data suggest PPDK modulates fluxes through catabolic and anabolic pathways to achieve optimal seed function.