Evolution of drought resistance strategies following the introduction of white clover (Trifolium repens L.).

BACKGROUND AND AIMS

Success during colonization is likely to depend on growing quickly and tolerating novel and stressful environmental conditions. However, rapid growth, stress avoidance and stress tolerance are generally considered divergent physiological strategies.

METHODS

We evaluated how white clover (Trifolium repens) has evolved to a divergent water regime following introduction to North America. We conducted RNA sequencing within a dry-down experiment using accessions from low- and high-latitude populations from native and introduced ranges and assessed variation in dehydration avoidance (ability to avoid wilting) and dehydration tolerance (ability to survive wilting).

KEY RESULTS

Introduced populations were better at avoiding dehydration, but poorer at tolerating dehydration than native populations. There was a strong negative correlation between avoidance and tolerance traits, and expression of most drought-associated genes exhibited similar trade-offs. Candidate genes with expression strongly associated with dehydration avoidance were linked to stress signalling, closing stomata and producing osmoprotectants. However, genes with expression linked to dehydration tolerance were associated with avoiding excessive production of reactive oxygen species and toxic bioproducts of stress responses. Several candidate genes showed differential expression patterns between native and introduced ranges and could underlie differences in drought resistance syndromes between ranges.

CONCLUSIONS

These results suggest that there has been strong selection following introduction for dehydration avoidance at the cost of surviving dehydration. More broadly, trade-offs between dehydration avoidance and tolerance responses are likely to exist at both the genetic and phenotypic scales that will influence evolutionary responses and potentially limit the global spectrum of plant form and function.