Identification of bundle sheath cell fate factors provides new tools for C3-to-C4 engineering.

Spatial compartmentation of the photosynthetic process between bundle sheath (BS) cells and mesophyll cells is one of the features that increase the productivity of C4 plants. To introduce C photosynthesis into C3 plants therefore calls for the identification of factors that control BS cell fate and promoter sequences that confer gene expression specifically in the BS and mesophyll cells. We recently demonstrated that three GRAS family transcription factors, SHORT-ROOT (SHR), SCARECROW (SCR) and SCR-LIKE 23 (SCL 23), are required for BS cell fate specification in Arabidopsis thaliana. Homologs to these genes are present in other plant species, C3 and C4, suggesting a conserved mechanism for BS cell fate specification. Interestingly, initially SCR and SCL23 are expressed uniformly in BS cells, but at later stages of leaf development SCR expression becomes restricted to the BS cells associated with the phloem, whereas SCL23 is preferentially expressed in the BS cells abutting the xylem. Characterization of the functions and expression patterns of SHR, SCR and SCL23 homologs in other plants, especially C3 crops, will not only advance the knowledge about BS cell development but also provide new tools for manipulating the number and physiology of BS cells, a critical prerequisite for C3-to-C4 engineering.