A transcription factor quintet orchestrating bundle sheath expression in rice

Abstract

The C4pathway has evolved in over sixty lineages of plants and improves photosynthetic efficiency by ∼50%. One unifying character of C4plants is photosynthetic activation of a compartment such as the bundle sheath, but gene regulatory networks controlling this cell type are poorly understood. For example, inArabidopsis thalianaa bipartite MYC-MYB transcription factor module restricts gene expression to this cell type but in grasses the regulatory logic allowing bundle sheath gene expression has not been defined. Using the global staple and C3crop rice we identified theSULFITE REDUCTASEpromoter as sufficient for strong expression in this cell type. This promoter encodes an intricatecis-regulatory logic with multiple activators and repressors acting in a combinatorial manner. However, within this landscape we identified a distal enhancer comprising a module activated by quartet of transcription factors from the G2-like, MYB-related, IDD and bZIP families. This module is necessary and sufficient to pattern gene expression to the rice bundle sheath, and oligomerisation allowed activity to be increased 23-fold. In summary, we identify a short tunable enhancer patterning expression to the rice bundle sheath that we anticipate will be useful for engineering this cell type. The transcription factor module responsible for this patterning is distinct from that previously reported inA. thaliana.