Evolutionary diversification of C2 photosynthesis in the grass genus Homolepis (Arthropogoninae)

Abstract

• Background and Aims To better understand C<inf>4</inf> evolution in monocots, we characterized C<inf>3</inf>–C<inf>4</inf> intermediate phenotypes in the grass genus Homolepis (subtribe Arthropogoninae). • Methods Carbon isotope ratio (δ¹³C), leaf gas exchange, mesophyll (M) and bundle sheath (BS) tissue characteristics, organelle size and numbers in M and BS tissue, and tissue distribution of the P-subunit of glycine decarboxylase (GLDP) were determined for five Homolepis species and the C<inf>4</inf> grass Mesosetum loliiforme from a phylogenetic sister clade. We generated a transcriptome-based phylogeny for Homolepis and Mesosetum species to interpret physiological and anatomical patterns in an evolutionary context, and to test for hybridization. • Key Results Homolepis contains two C<inf>3</inf> species (H. glutinosa, H. villaricensis), one species with a weaker form of C<inf>2</inf> termed sub-C<inf>2</inf> (H. isocalycia), and two C<inf>2</inf> species (H. longispicula, H. aturensis). Homolepis longispicula and H. aturensis express over 85 % of leaf glycine in centripetal mitochondria within the BS, and have increased fractions of leaf chloroplasts, mitochondria and peroxisomes within the BS relative to H. glutinosa. Analysis of leaf gas exchange, cell ultrastructure and transcript expression show M. loliiforme is a C<inf>4</inf> plant of the NADP-malic enzyme subtype. Homolepis comprises two sister clades, one containing H. glutinosa and H. villaricensis and the second H. longispicula and H. aturensis. Homolepis isocalycia is of hybrid origin, its parents being H. aturensis and a common ancestor of the C<inf>3</inf> Homolepis clade and H. longispicula. • Conclusions Photosynthetic activation of BS tissue in the sub-C<inf>2</inf> and C<inf>2</inf> species of Homolepis is similar to patterns observed in C<inf>3</inf>–C<inf>4</inf> intermediate eudicots, indicating common evolutionary pathways from C<inf>3</inf> to C<inf>4</inf> photosynthesis in these disparate clades. Hybridization can diversify the C<inf>3</inf>–C<inf>4</inf> intermediate character state and should be considered in reconstructing putative ancestral states using phylogenetic analyses.