C₄ anatomy can evolve via a single developmental change

Abstract

C4 photosynthesis is a complex trait that boosts productivity in warm environments. Paradoxically, it evolved independently in numerous plant lineages, despite requiring specialised leaf anatomy. The anatomical modifications underlying C4 evolution have previously been evaluated through interspecific comparisons, which capture numerous changes besides those needed for C4 functionality. Here, we quantify the anatomical changes accompanying the transition between non‐C4 and C4 phenotypes by sampling widely across the continuum of leaf anatomical traits in the grass Alloteropsis semialata. Within this species, the only trait that is shared among and specific to C4 individuals is an increase in vein density, driven specifically by minor vein development that yields multiple secondary effects facilitating C4 function. For species with the necessary anatomical preconditions, developmental proliferation of veins can therefore be sufficient to produce a functional C4 leaf anatomy, creating an evolutionary entry point to complex C4 syndromes that can become more specialised.

Key changes in gene expression identified for different stages of C4 evolution in Alloteropsis semialata

“…This shift indicates a strengthened connection between the C 3 and C 4 cycles and a decreased leakiness, so that less atmospheric CO 2 is directly fixed by the Calvin–Benson cycle (Monson et al , 1988; von Caemmerer, 1992). Within A. semialata , this might have been mediated by the reduced distance between veins in the C 4 A. semialata (Lundgren et al , 2016, 2019; Dunning et al , 2017) and/or biochemical alterations. The up-regulation of relatively few genes (0.06%) coincided with the phenotypic transitions, and only one of these encoded an enzyme with a known C 4 function, namely PPDK.…”

“…The C 4 populations of A. semialata have decreased CO 2 compensation points, increased carboxylation efficiencies, and shifts in carbon isotopes compared with the C 3 populations that confirm their photosynthetic type (Lundgren et al , 2016). The C 4 leaves are characterized by increased vein density, phosphoenolpyruvate carboxylase (PEPC) protein abundance, and transcript abundance of genes encoding some C 4 enzymes compared with the C 3 types (Lundgren et al , 2016, 2019; Dunning et al , 2017). The C 3 +C 4 A. semialata also show elevated leaf levels of PEPC protein and genes for some C 4 enzymes, and increased concentration of chloroplasts in bundle sheaths in comparison with the C 3 populations, but no increase in vein density (Lundgren et al , 2016; Dunning et al , 2017).…”

Why is C4 photosynthesis so rare in trees?

“…Since C 4 photosynthesis was first observed >50 years ago ( Kortschak et al , 1965 ; Hatch and Slack, 1966 ), numerous studies have attempted to elucidate exactly which modifications are typically required to assemble the components of C 4 physiology ( Box 2 ). To travel the path of C 4 evolution, an ancestral C 3 progenitor arrived in an environment selective for a C 4 benefit ( Ehleringer et al , 1997 ; Sage, 2001 a ; Sage et al , 2018 ), and, starting from an initial set of genetic and anatomical pre-adaptations ( Monson, 2003 ; Christin et al ., 2013 , 2015 ; Griffiths et al , 2013 ), evolved developmental and genetic modifications ( Stata et al , 2016 ; Moreno-Villena et al , 2018 ; Dunning et al , 2019 a ; Lundgren et al , 2019 ), navigated energetic constraints ( Bellasio and Lundgren, 2016 ), and underwent progressive optimization ( Rondeau et al , 2005 ; Christin et al , 2009 )—or, in some cases, potentially ‘cheated’ this lengthy final step via horizontal gene transfer ( Christin et al ., 2012 a , b ; Dunning et al , 2019 b ). Given that some version of this path has been repeatedly travelled nearly 70 times by diverse plant lineages spanning a wide range of lifeforms and ecological niches ( Sage et al , 2011 a ), it seems unusual that only a single group of true trees (i.e.…”

“…To facilitate the C 4 cycle, C 4 plants have higher bundle sheath to mesophyll area ratios ( Hattersley, 1984 ), often via increased density of vascular bundles ( Lundgren et al , 2019 ). In addition, the connectivity of the mesophyll and bundle sheath cells is enhanced in C 4 plants by an increased density of plasmodesmata at the cell interface, which allows for the increased flux of metabolites that is required for a functional C 4 cycle ( Danila et al , 2016 ).…”

“…C 4 anatomy can evolve via a single developmental change: an increase in vein density. Lundgren et al (2019) demonstrated that an increase in vein density driven by proliferation of minor veins is sufficient, given necessary anatomical pre-conditions, to produce a functional C 4 leaf anatomy and create an evolutionary entry to more complex C 4 syndromes in A. semialata . Christin et al (2013) present the necessary anatomical pre-conditions (or ‘enablers’) in grasses as a high (>15%) proportion of bundle sheath tissue (combination of a short distance between bundle sheaths and large bundle sheath cells), which, when combined with environmental changes, facilitated the emergence of the C 4 trait.…”