Gisekia (Gisekiaceae): phylogenetic relationships, biogeography, and ecophysiology of a poorly known C₄ lineage in the Caryophyllales

Global distribution, climatic preferences and photosynthesis-related traits of C4 eudicots and how they differ from those of C4 grasses

C₄ is one of three known photosynthetic processes of carbon fixation in flowering plants. It evolved independently more than 61 times in multiple angiosperm lineages and consists of a series of anatomical and biochemical modifications to the ancestral C3 pathway increasing plant productivity under warm and light-rich conditions. The C4 lineages of eudicots belong to seven orders and 15 families, are phylogenetically less constrained than those of monocots and entail an enormous structural and ecological diversity. Eudicot C4 lineages likely evolved the C4 syndrome along different evolutionary paths. Therefore, a better understanding of this diversity is key to understanding the evolution of this complex trait as a whole. By compiling 1207 recognised C4 eudicots species described in the literature and presenting trait data among these species, we identify global centres of species richness and of high phylogenetic diversity. Furthermore, we discuss climatic preferences in the context of plant functional traits. We identify two hotspots of C4 eudicot diversity: arid regions of Mexico/Southern United States and Australia, which show a similarly high number of different C4 eudicot genera but differ in the number of C4 lineages that evolved in situ. Further eudicot C4 hotspots with many different families and genera are in South Africa, West Africa, Patagonia, Central Asia and the Mediterranean. In general, C4 eudicots are diverse in deserts and xeric shrublands, tropical and subtropical grasslands, savannas and shrublands. We found C4 eudicots to occur in areas with less annual precipitation than C4 grasses which can be explained by frequently associated adaptations to drought stress such as among others succulence and salt tolerance. The data indicate that C4 eudicot lineages utilising the NAD-ME decarboxylating enzyme grow in drier areas than those using the NADP-ME decarboxylating enzyme indicating biochemical restrictions of the later system in higher temperatures. We conclude that in most eudicot lineages, C4 evolved in ancestrally already drought-adapted clades and enabled these to further spread in these habitats and colonise even drier areas.

Evolutionary Convergence of C4 Photosynthesis: A Case Study in the Nyctaginaceae

C4 photosynthesis evolved over 65 times, with around 24 origins in the eudicot order Caryophyllales. In the Caryophyllales family Nyctaginaceae, the C4 pathway is known in three genera of the tribe Nyctagineae: Allionia, Okenia and Boerhavia. Phylogenetically, Allionia and Boerhavia/Okenia are separated by three genera whose photosynthetic pathway is uncertain. To clarify the distribution of photosynthetic pathways in the Nyctaginaceae, we surveyed carbon isotope ratios of 159 species of the Nyctaginaceae, along with bundle sheath (BS) cell ultrastructure, leaf gas exchange, and C4 pathway biochemistry in five species from the two C4 clades and closely related C3 genera. All species in Allionia, Okenia and Boerhavia are C4, while no C4 species occur in any other genera of the family, including three that branch between Allionia and Boerhavia. This demonstrates that C4 photosynthesis evolved twice in Nyctaginaceae. Boerhavia species use the NADP-malic enzyme (NADP-ME) subtype of C4 photosynthesis, while Allionia species use the NAD-malic enzyme (NAD-ME) subtype. The BS cells of Allionia have many more mitochondria than the BS of Boerhavia. Bundle sheath mitochondria are closely associated with chloroplasts in Allionia which facilitates CO2 refixation following decarboxylation by mitochondrial NAD-ME. The close relationship between Allionia and Boerhavia could provide insights into why NADP-ME versus NAD-ME subtypes evolve, particularly when coupled to analysis of their respective genomes. As such, the group is an excellent system to dissect the organizational hierarchy of convergent versus divergent traits produced by C4 evolution, enabling us to understand when convergence is favored versus when divergent modifications can result in a common phenotype.

Retrospective analysis of biochemical limitations to photosynthesis in 49 species: C4 crops appear still adapted to pre-industrial atmospheric [CO2 ]

Leaf CO₂ uptake (A) in C₄ photosynthesis is limited by the maximum apparent rate of PEPc carboxylation (V_pmax ) at low intercellular [CO₂ ] (c_i ) with a sharp transition to a c_i -saturated rate (V_max ) due to co-limitation by ribulose-1:5-bisphosphate carboxylase/oxygenase (Rubisco) and regeneration of PEP. The response of A to c_i has been widely used to determine these two parameters. V_max and V_pmax depend on different enzymes but draw on a shared pool of leaf resources, such that resource distribution is optimized, and A maximized, when V_max and V_pmax are co-limiting. We collected published A/c_i curves in 49 C₄ species and assessed variation in photosynthetic traits between phylogenetic groups, and as a function of atmospheric [CO₂ ]. The balance of V_max -V_pmax varied among evolutionary lineages and C₄ subtypes. Operating A was strongly V_max -limited, such that re-allocation of resources from V_pmax towards V_max was predicted to improve A by 12% in C₄ crops. This would not require additional inputs but rather altered partitioning of existing leaf nutrients, resulting in increased water and nutrient-use efficiency. Optimal partitioning was achieved only in plants grown at pre-industrial atmospheric [CO₂ ], suggesting C₄ crops have not adjusted to the rapid increase in atmospheric [CO₂ ] of the past few decades.

Some like it hot: the physiological ecology of C4 plant evolution

The evolution of C₄ photosynthesis requires an intermediate phase where photorespiratory glycine produced in the mesophyll cells must flow to the vascular sheath cells for metabolism by glycine decarboxylase. This glycine flux concentrates photorespired CO₂ within the sheath cells, allowing it to be efficiently refixed by sheath Rubisco. A modest C₄ biochemical cycle is then upregulated, possibly to support the refixation of photorespired ammonia in sheath cells, with subsequent increases in C₄ metabolism providing incremental benefits until an optimized C₄ pathway is established. 'Why' C₄ photosynthesis evolved is largely explained by ancestral C₃ species exploiting photorespiratory CO₂ to improve carbon gain and thus enhance fitness. While photorespiration depresses C₃ performance, it produces a resource (photorespired CO₂) that can be exploited to build an evolutionary bridge to C₄ photosynthesis. 'Where' C₄ evolved is indicated by the habitat of species branching near C₃-to-C₄ transitions on phylogenetic trees. Consistent with the photorespiratory bridge hypothesis, transitional species show that the large majority of > 60 C₄ lineages arose in hot, dry, and/or saline regions where photorespiratory potential is high. 'When' C₄ evolved has been clarified by molecular clock analyses using phylogenetic data, coupled with isotopic signatures from fossils. Nearly all C₄ lineages arose after 25 Ma when atmospheric CO₂ levels had fallen to near current values. This reduction in CO₂, coupled with persistent high temperature at low-to-mid-latitudes, met a precondition where photorespiration was elevated, thus facilitating the evolutionary selection pressure that led to C₄ photosynthesis.

Why are there no C4 forests?

C₄ photosynthesis is absent from the arborescent life form, with the exception of seven Hawaiian Euphorbia species and a few desert shrubs that become arborescent with age. As a consequence, wherever C₃ trees can establish, their height advantage enables them to outcompete low stature C₄ vegetation. Had C₄ photosynthesis been able to evolve in an arborescent life form, forest cover (by C₄ trees) could have been much more extensive than today, with significant consequences for the biosphere. Here, we address why there are so few C₄ trees. Physiological explanations associated with low light performance of C₄ photosynthesis are not supported, because C₄ shade-tolerant species exhibit similar performance as shade-tolerant C₃ species in terms of quantum yield, steady-state photosynthetic and use of sunflecks. Hence, hypothetical C₄ trees could occur in the regeneration niche of forests. Constraints associated with the evolutionary history of the C₄ lineages are more plausible. Most C₄ species are grasses and sedges, which lack meristems needed for arborescence, while most C₄ eudicots are highly specialized for harsh (arid, saline, hot) or disturbed habitats where arborescence may be maladapted. Most C₄ eudicot clades are also young, and have not had sufficient time to radiate beyond the extreme environments where C₄ evolution is favored. In the case of the Hawaiian Euphorbia species, they belong to one of the oldest and most diverse C₄ lineages, which primed this group to evolve arborescence in a low-competition environment that appeared on the remote Hawaiian Islands.

From proto-Kranz to C4 Kranz: building the bridge to C4 photosynthesis

In this review, we examine how the specialized "Kranz" anatomy of C4 photosynthesis evolved from C3 ancestors. Kranz anatomy refers to the wreath-like structural traits that compartmentalize the biochemistry of C4 photosynthesis and enables the concentration of CO2 around Rubisco. A simplified version of Kranz anatomy is also present in the species that utilize C2 photosynthesis, where a photorespiratory glycine shuttle concentrates CO2 into an inner bundle-sheath-like compartment surrounding the vascular tissue. C2 Kranz is considered to be an intermediate stage in the evolutionary development of C4 Kranz, based on the intermediate branching position of C2 species in 14 evolutionary lineages of C4 photosynthesis. In the best-supported model of C4 evolution, Kranz anatomy in C2 species evolved from C3 ancestors with enlarged bundle sheath cells and high vein density. Four independent lineages have been identified where C3 sister species of C2 plants exhibit an increase in organelle numbers in the bundle sheath and enlarged bundle sheath cells. Notably, in all of these species, there is a pronounced shift of mitochondria to the inner bundle sheath wall, forming an incipient version of the C2 type of Kranz anatomy. This incipient version of C2 Kranz anatomy is termed proto-Kranz, and is proposed to scavenge photorespiratory CO2. By doing so, it may provide fitness benefits in hot environments, and thus represent a critical first stage of the evolution of both the C2 and C4 forms of Kranz anatomy.