Supported by the USDA Photosynthesis Program


C4 plants partition CO2 fixation in two consecutive steps in the dimorphic photosynthetic cell types, mesophyll and bundle sheath cells, thus concentrating CO2 for more efficient photosynthesis and eliminating energy waste in photorespiration. The acquisition of the C4 pathway enables plants to thrive under severe environmental conditions such as high temperature, high light intensity, and low water availability.


We have used the monocot maize as a model system to elucidate the regulatory mechanisms and signaling pathways controlling the genes involved in C4 photosynthesis. Our studies show that pre-existing genes were recruited for the C4 pathway after acquiring new expression features mediated by diverse molecular mechanisms and multiple signal transduction pathways. The simple maize protoplast transient expression assay is currently the most versatile cell system that allows us to analyze the functions of photoreceptors, protein kinases, protein phosphatases, and transcription factors in controlling photosynthesis genes. Various chemical reagents and maize mutants are also used to deduce the steps and components of diverse signal transduction pathways based on reporter gene expression. Transgenic plant studies generally support and extend our findings in cellular systems. We are also exploring the maize single cell system for more powerful applications such as expression cloning and mutant complementation.


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C4 Pathway Illustrations

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Plant signalling


Sheen, J. 1999. C4 gene expression. Ann Rev Plant Physiol Plant Mol Biol. 50: 187-217. PDF


Kausch A.P., Owen, T.P. Jr, Zachwieja, S.J., Flynn, A.R., Sheen, J. 2001. Mesophyll-specific, light and metabolic regulation of the C4 PPCZm1 promoter in transgenic maize. Plant Mol. Biol. 45: 1-15.

Yanagisawa, S. and Sheen, J. 1998. Involvement of maize Dof zinc finger proteins in tissue-specific and light-regulated gene expression. Plant Cell 10: 75-89.

Sheen, J. 1998. Mutational analysis of two protein phosphatases involved in ABA signal transduction in higher plants. Proc. Natl. Acad. Sci. USA. 95:975-980.

Sheen, J. 1993. Protein phosphatase activity is required for light-inducible gene expression in maize. EMBO J. 12: 3497-3505.

Schäffner, A.R., Sheen, J. 1992. Maize C4 photosynthesis involves differential regulation of maize PEPC genes. Plant J. 2: 221-232.

Schäffner, A.R., Sheen, J. 1991. Maize rbcS promoter activity depends on sequence elements not found in dicot rbcS promoters. Plant Cell 3: 997-1012.

Sheen, J. 1991. Molecular mechanisms underlying the differential expression of maize pyruvate, orthophosphate dikinase genes. Plant Cell. 3: 225-245.

Sheen, J.1990. Metabolic repression of transcription in higher plants. Plant Cell 2: 1027- 1038.