Salt tolerance evolves more frequently in C₄ grass lineages

Abstract

Salt tolerance has evolved many times in the grass family, and yet few cereal crops are salt tolerant. Why has it been so difficult to develop crops tolerant of saline soils when salt tolerance has evolved so frequently in nature? One possible explanation is that some grass lineages have traits that predispose them to developing salt tolerance and that without these background traits, salt tolerance is harder to achieve. One candidate background trait is photosynthetic pathway, which has also been remarkably labile in grasses. At least 22 independent origins of the C 4 photosynthetic pathway have been suggested to occur within the grass family. It is possible that the evolution of C 4 photosynthesis aids exploitation of saline environments, because it reduces transpiration, increases water-use efficiency and limits the uptake of toxic ions. But the observed link between the evolution of C 4 photosynthesis and salt tolerance could simply be due to biases in phylogenetic distribution of halophytes or C 4 species. Here, we use a phylogenetic analysis to investigate the association between photosynthetic pathway and salt tolerance in the grass family Poaceae. We find that salt tolerance is significantly more likely to occur in lineages with C 4 photosynthesis than in C 3 lineages. We discuss the possible links between C 4 photosynthesis and salt tolerance and consider the limitations of inferring the direction of causality of this relationship.

The evolutionary ecology of C4plants

“…Despite this phylogenetic effect, the transition to C 4 physiology was still accompanied by changes in the ecological niche. Ancestral state reconstructions show that C 4 evolution in grasses led to consistent shifts into drier and more seasonal niche space (Edwards & Smith, ), and that C 4 grasses are more likely to migrate into arid or saline habitats than their C 3 counterparts (Osborne & Freckleton, ; Bromhan & Bennett, ). These results suggest that C 4 photosynthesis facilitates adaptation to conditions of low soil water potential, probably through the continuous adaptation of stomatal conductance and plant hydraulics, and thereby allows plants to more readily access dry niche space (Edwards & Donoghue, ; Fig.…”

Photosynthetic innovation broadens the niche within a single species

“…However, these investigations are biased by differences among phylogenetic groups (Taub ), and recent interspecific comparisons accounting for phylogenetic structure have revolutionised our understanding of C 4 evolutionary ecology (reviewed in Christin & Osborne ). In particular, phylogeny‐based analyses have shown that C 4 photosynthesis evolved in groups of grasses inhabiting warm regions and facilitated shifts into drier and more saline habitats (Osborne & Freckleton ; Edwards & Smith ; Bromham & Bennett ). However, the photosynthetic transitions investigated in these analyses occurred many millions of years ago and there is often a gap of several million years between C 3 and C 4 nodes in species phylogenetic trees (Christin et al .…”

C4 photosynthesis with Kranz anatomy evolved in the Oryza coarctata Roxb

“…However, it has been suggested that C4 plants are more like to be tolerant to different abiotic stresses (Bromham and Bennett, 2014).…”