A multi-cell model for the C 4 photosynthetic pathway in developing wheat grains based upon tissue-specific transcriptome data.

A non-Kranz C4 photosynthesis of the NAD-ME subtype, specifically in developing wheat grains (14 dpa, days post-anthesis) was originally demonstrated using transcriptome-based RNA-seq. Here we present a re-examination of evidence for C4 photosynthesis in the developing grains of wheat and, more broadly, the Pooideae and an investigation of the evolutionary processes and implications. The expression profiles for the genes associated with C4 photosynthesis (C4 - and C3 -specific) were evaluated using published transcriptome data for the outer pericarp, inner pericarp, and endosperm tissues of the developing wheat grains. The expression of the C4 -specific genes across these three tissues revealed the involvement of all three tissues in an orderly fashion to accomplish the non-Kranz NAD-ME-dependent C4 photosynthesis. Based on their expression levels in RPKM (reads per kilobase per million mapped reads) values, a model involving multiple cell- and tissue-types is proposed for C4 photosynthesis involved in the refixation of the respired CO2 from the endosperm tissues in the developing wheat grains. This multi-cell C4 model, proposed to involve more than two cell types, requires further biochemical validation.