Plant Cell 28, 454–465 (2016)

The evolution of C4 photosynthesis from the C3 pathway involves increased expression of multiple enzymes and their compartmentalization into mesophyll or bundle sheath cells. Current knowledge regarding the mechanism, however, remains limited to a few regulatory elements that each affect a single gene in a specific lineage. Ben Williams, at the University of Cambridge, UK and colleagues have now discovered an untranslated region (UTR) motif that directs multiple C4 enzymes to accumulate preferentially in the mesophyll cells of Gynandropsis gynandra, the closest C4 relative of Arabidopsis.

To understand the mechanisms regulating multiple enzymes in the C4 pathway, the researchers used a GUS reporter system to examine the cis-elements on CARBONIC ANHYDRASE4 (CA4) of G. gynandra for their ability to affect CA4 abundance in C4 leaves. Both the promoter and introns of this gene elevated the abundance of the GUS reporter, presumably through loss of cis-elements that repress CA4 expression in C3 species.

The UTRs of CA4 and two other C4 enzymes caused strong preferential protein accumulation in mesophyll cells. Deletion analysis combined with computational motif detection identified a nine-nucleotide motif shared by all the UTRs (MEM2) that directs the preferential enzyme accumulation with another element immediately upstream of the start codon.

However, MEM2 does not cause differential expression of transcripts between mesophyll and bundle sheath cells, suggesting that the preferential enzyme accumulation was triggered by MEM2 post transcription.