Loss of the M-box from the glycine decarboxylase P-subunit promoter in C2 Moricandia species

C2 photosynthesis operates by shuttling photorespiratory glycine (C2) from mesophyll (M) to bundle sheath (BS) cells, followed by decarboxylation and release of CO2 around RubisCO. C2 plants are characterized by low apparent photorespiration and enhanced refixation of photorespiratory CO2 and the C2 pathway is thought to represent an intermediate step for the evolution from C3 to C4 photosynthesis. Restriction of glycine decarboxylation to the BS cells is considered to be a prerequisite for C2 photosynthesis. In the C3 plant species Arabidopsis thaliana, a cis-element required for expression of the P-subunit of glycine decarboxylase (GDC—P) in M cells (termed the M-box) was previously identified in the promoter of A. thaliana glycine decarboxylase P-subunit 1 (AtGldp1). Consequently, the loss of this element restricted Gldp1 expression to the BS cells. To investigate conservation, Gldp promoter sequences from another C3 and two additional C2 Moricandia species were isolated by genome walking. In comparison to AtGldp1, the M-box was conserved in the promoter of C3 Moricandia moricandioides, but was not found in the promoters of M. nitens, M. suffruticosa, and M. arvensis, indicating the loss of the M-box from several C2 Moricandia species. The AtGldp1 M-box was further analyzed in detail using promoter::GUS fusions. Results show that interaction between two promoter regions containing predicted CAAT and GATA elements are required for expression of the GUS reporter in M cells and these elements including their spacing are conserved in the promoters of different members of the Brassicaceae.