Untranslated regions of FbRbcS1 mRNA mediate bundle sheath cell-specific gene expression in leaves of a C₄ plant

C₄ photosynthesis typically requires two specialized leaf cell types, bundle sheath (bs) and mesophyll (mp), which provide the foundation for this highly efficient carbon assimilation pathway. In leaves of Flaveria bidentis, a dicotyledonous C₄ plant, ribulose 1,5-bisphosphate carboxylase (rubisco) accumulates only in bs cells surrounding the vascular centers and not in mp cells. This is in contrast to the more common C ₃ plants, which accumulate rubisco in all photosynthetic cells. Many previous studies have focused on transcriptional control of C₄ cell type-specificity; however, post-transcriptional regulation has also been implicated in the bs-specific expression of genes encoding the rubisco subunits. In this current study, a biolistic leaf transformation assay has provided direct evidence that the 5′- and 3′-untranslated regions (UTRs) of F. bidentis FbRbcS1 mRNA (from a nuclear gene encoding the rubisco small subunit), in themselves, confer strong bs cell-specific expression to gfpA reporter gene transcripts when transcribed from a constitutive CaMV promoter. In transformed leaf regions, strong bs cell-specific GFP expression was accompanied by corresponding bs cell-specific accumulation of the constitutively transcribed FbRbcS1 5′-UTR-gfpA-3′-UTR mRNAs. Control constructs lacking any RbcS mRNA sequences were expressed in all leaf cell types. These findings demonstrate that characteristic cell type-specific FbRbcS1 expression patterns in C₄ leaves can be established entirely by sequences contained within the transcribed UTRs of FbRbcS1 mRNAs. We conclude that selective transcript stabilization (in bs cells) or degradation (in mp cells) plays a key role in determining bs cell-specific localization of the rubisco enzyme.