Evolution of salt tolerance in angiosperms: A phylogenetic approach

Salt tolerance in plants involves a number of physiological, anatomical, and morphological traits that mitigate the effects of osmotic and ionic stress of salinity. Despite the complexity of adaptations to environmental salinity, salt tolerance has arisen independently in many different lineages of flowering plants. In this chapter, we discuss phylogenetic perspectives on the study of salt tolerance. Although few angiosperm species are halophytes, salt tolerance appears to be evolutionarily labile, with relatively numerous instances of independent evolutionary origins. Salt tolerance evolves more often in some lineages than others, which may reflect the fact that it builds upon enabling pre-existing traits in those lineages that facilitate the evolution of tolerance to a wider range of environmental stresses. We describe phylogenetic comparative analyses that explore the evolutionary association between salt tolerance and other related ecophysiological strategies: C4 photosynthesis, heavy metal tolerance, and alkalitolerance. However, we discuss the limitations in identifying causality and direction of causality of these associations. We conclude that phylogenetic comparative studies can help us to understand the evolutionary dynamics of salt tolerance in angiosperms and potentially aid identification of shared mechanisms underlying tolerance to several environmental stresses. Despite outstanding challenges in conducting these studies, these comparative approaches have the potential to reveal evolutionary associations between different tolerances to environmental stress.