Evolutionary Origins of Drought Tolerance in Spermatophytes

It is commonly known that drought stress is a major constraint limiting crop production.
Drought stress and associated drought tolerance mechanisms are therefore under
intense investigation with the view to future production of drought tolerant crops.
With an ever-growing population and variable climate, novel approaches need to
be considered to sustainably feed future generations. In this context, definitions of
drought tolerance are highly variable, which poses a major challenge for the systematic
assessment of this trait across the plant kingdom. Furthermore, drought tolerance is
a polygenic trait and understanding the evolution of this complex trait may inform us
about patterns of gene gain and loss in relation to diverse drought adaptations. We
look at the transition of plants from water to land, and the role of drought tolerance in
enabling this transition, before discussing the first drought tolerant plant and common
drought responses amongst vascular plants.We reviewed the distribution of a combined
“drought tolerance” trait in very broad terms to encompass different experimental
systems and definitions used in the current literature and assigned a binary trait
“tolerance vs. sensitivity” in 178 extant plant species. By simplifying drought responses
of plants into this “binary” trait we were able to explore the evolution of drought tolerance
across the wider plant kingdom, compared to previous studies. We show how this
binary “drought tolerance/sensitivity” trait has evolved and discuss how incorporating
this information into an evolutionary genomics framework could provide insights into the
molecular mechanisms underlying extreme drought adaptations.