Photosynthetic Transcription Factors that Determine Bundle Cell Fate and Function

Photosynthesis is one of the most important reactions on the earth because its products are the ultimate energy source for all living organisms and the food of the human beings. Depending on the number of carbon atoms in the initial organic compound made in photosynthesis, plants can be grouped into C3 or C4 plants. C4 plants are evolved from C3 plants, but they have several features that make them much more efficient photosynthetically than C3 plants:

1. The enzyme for C02 fixation, called PEP carboxylase, is not inhibited by oxygen. In contrast, the C02 fixation enzyme in C3 plants, called RUBICO carboxylase, has an oxygenase activity, which reverses the photosynthetic reaction. The oxygenase activity is favored at high light and high temperature, making C3 plants perform even worse in warm area where crop yield potential is high.
2. C02 fixation occurs in the mesophyll cells, whereas C3-type photosynthesis is performed in the bundle sheath (BS) cells, which surround the vascular tissue, using the C02 concentrated by and supplied from mesophyll cells.
3. There are more vascular bundles (veins), hence more BS cells, and a greater number of channels between BS and mesophyll cells, which ensures rapid transport between these two cell types. C4 plants are also efficient in water utilization. Because many important crops are C3 plants, such as rice and wheats, huge resources have been invested to introduce C4 photosynthesis into C3 plants. Although BS cells are also present in C3 plants, they generally contribute little to photosynthesis and this cell type has therefore become a primary target for C3-to-C4 bioengineering. Despite extensive research, until now the mechanism that controls BS cell fate is still unknown.

Our work identified three transcription factors (SCR, SCL23 and SHR) that are required for BS cell fate specification in Arabidopsis, a model C3 plant. SCR and SCL23 are both expressed specifically in BS cells. Though they act redundantly in determining the BS cell fate, they have distinct functions. Because similar genes are present in other plant species, including rice and maize, which is a C4 plant, we believe that similar mechanisms control BSC cell fate determination in all C3 and C4 plants. These bundle sheath cell determinants offer a novel and powerful tool for the C3-to-C4 engineering, which is regarded as a key solution to the demand for food and biomass by a rapidly growing world population.