C(3)–C(4) intermediacy in grasses: organelle enrichment and distribution, glycine decarboxylase expression, and the rise of C(2) photosynthesis

Photorespiratory glycine shuttling and decarboxylation in bundle sheath (BS) cells exhibited by C(2) species is proposed to be the evolutionary bridge to C(4) photosynthesis in eudicots. To evaluate this in grasses, we compare anatomy, cellular localization of glycine decarboxylase (GDC), and photosynthetic physiology of a suspected C(2) grass, Homolepis aturensis, with these traits in known C(2) grasses, Neurachne minor and Steinchisma hians, and C(3) S. laxum that is sister to S. hians. We also use publicly available genome and RNA-sequencing data to examine the evolution of GDC subunits and enhance our understanding of the evolution of BS-specific GDC expression in C(2) and C(4) grasses. Our results confirm the identity of H. aturensis as a C(2) species; GDC is confined predominantly to the organelle-enriched BS cells in H. aturensis and S. hians and to mestome sheath cells of N. minor. Phylogenetic analyses and data obtained from immunodetection of the P-subunit of GDC are consistent with the hypothesis that the BS dominant levels of GDC in C(2) and C(4) species are due to changes in expression of a single GLDP gene in M and BS cells. All BS mitochondria and peroxisomes and most chloroplasts in H. aturensis and S. hians are situated centripetally in a pattern identical to C(2) eudicots. In S. laxum, which has C(3)-like gas exchange patterns, mitochondria and peroxisomes are positioned centripetally as they are in S. hians. This subcellular phenotype, also present in eudicots, is posited to initiate a facilitation cascade leading to C(2) and C(4) photosynthesis.