A multi-cell model for the C4 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.