Evolutionary implications of C2 photosynthesis: how complex biochemical trade-offs may limit C4 evolution

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

BACKGROUND AND AIMS

To better understand C4 evolution in monocots, we characterized C3-C4 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 C4 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 C3 species (H. glutinosa, H. villaricensis), one species with a weaker form of C2 termed sub-C2 (H. isocalycia), and two C2 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 C4 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 C3  Homolepis clade and H. longispicula.

CONCLUSIONS

Photosynthetic activation of BS tissue in the sub-C2 and C2 species of Homolepis is similar to patterns observed in C3-C4 intermediate eudicots, indicating common evolutionary pathways from C3 to C4 photosynthesis in these disparate clades. Hybridization can diversify the C3-C4 intermediate character state and should be considered in reconstructing putative ancestral states using phylogenetic analyses.

Leaf transcriptomes from C3, C3-C4 intermediate, and C4Neurachne species give insights into C4 photosynthesis evolution

The C4 photosynthetic pathway is hypothesized to have evolved from the ancestral C3 pathway through progressive changes in leaf anatomy and biochemistry with extant C3-C4 photosynthetic intermediate species representing phenotypes between species demonstrating full C3 and full C4 states. The Australian endemic genus Neurachne is the only known grass group that contains distinct, closely related species that carry out C3, C3-C4 intermediate, or C4 photosynthesis. To explore and understand the molecular mechanisms underlying C4 photosynthesis evolution in this genus, leaf transcriptomes were generated from two C3, three photosynthetic intermediate (proto-Kranz, C2-like, and C2), and two C4Neurachne species. The data were used to reconstruct phylogenetic relationships in Neurachne, which confirmed two independent C4 origins in the genus. Relative transcript abundances substantiated the photosynthetic phenotypes of individual species and highlighted transcriptional investment differences between species, including between the two C4 species. The data also revealed proteins potentially involved in C4 cycle intermediate transport and identified molecular mechanisms responsible for the evolution of C4-associated proteins in the genus.

Tribulus (Zygophyllaceae) as a case study for the evolution of C2 and C4 photosynthesis

C2 photosynthesis is a photosynthetic pathway in which photorespiratory CO2 release and refixation are enhanced in leaf bundle sheath (BS) tissues. The evolution of C2 photosynthesis has been hypothesized to be a major step in the origin of C4 photosynthesis, highlighting the importance of studying C2 evolution. In this study, physiological, anatomical, ultrastructural, and immunohistochemical properties of leaf photosynthetic tissues were investigated in six non-C4 Tribulus species and four C4 Tribulus species. At 42°C, T. cristatus exhibited a photosynthetic CO2 compensation point in the absence of respiration (C*) of 21 µmol mol-1, below the C3 mean C* of 73 µmol mol-1. Tribulus astrocarpus had a C* value at 42°C of 55 µmol mol-1, intermediate between the C3 species and the C2 T. cristatus. Glycine decarboxylase (GDC) allocation to BS tissues was associated with lower C*. Tribulus cristatus and T. astrocarpus allocated 86% and 30% of their GDC to the BS tissues, respectively, well above the C3 mean of 11%. Tribulus astrocarpus thus exhibits a weaker C2 (termed sub-C2) phenotype. Increased allocation of mitochondria to the BS and decreased length-to-width ratios of BS cells, were present in non-C4 species, indicating a potential role in C2 and C4 evolution.

Salinity Mitigates the Negative Effect of Elevated Temperatures on Photosynthesis in the C3-C4 Intermediate Species Sedobassia sedoides

The adaptation of plants to combined stresses requires unique responses capable of overcoming both the negative effects of each individual stress and their combination. Here, we studied the C3-C4 (C2) halophyte Sedobassia sedoides in response to elevated temperature (35 °C) and salinity (300 mM NaCl) as well as their combined effect. The responses we studied included changes in water-salt balance, light and dark photosynthetic reactions, the expression of photosynthetic genes, the activity of malate dehydrogenase complex enzymes, and the antioxidant system. Salt treatment led to altered water-salt balance, improved water use efficiency, and an increase in the abundance of key enzymes involved in intermediate C3-C4 photosynthesis (i.e., Rubisco and glycine decarboxylase). We also observed a possible increase in the activity of the C2 carbon-concentrating mechanism (CCM), which allowed plants to maintain high photosynthesis intensity and biomass accumulation. Elevated temperatures caused an imbalance in the dark and light reactions of photosynthesis, leading to stromal overreduction and the excessive generation of reactive oxygen species (ROS). In response, S. sedoides significantly activated a metabolic pathway for removing excess NADPH, the malate valve, which is catalyzed by NADP-MDH, without observable activation of the antioxidant system. The combined action of these two factors caused the activation of antioxidant defenses (i.e., increased activity of SOD and POX and upregulation of FDI), which led to a decrease in oxidative stress and helped restore the photosynthetic energy balance. Overall, improved PSII functioning and increased activity of PSI cyclic electron transport (CET) and C2 CCM led to an increase in the photosynthesis intensity of S. sedoides under the combined effect of salinity and elevated temperature relative to high temperature alone.