Why Aren't More Plant Breeders and Geneticists Interested in Xylem Traits?.

Plants in all habitats need to maintain an adequate supply of water to the sites of photosynthesis and evaporation within their leaves. Although the efficiency and safety (i.e., safe from embolism) of this water supply have been long appreciated, the importance of these traits in conferring fitness across the world’s angiosperm species are now coming to light, mainly through the connectivity of large datasets. However, despite evident relationships between xylem traits and aridity, few breeding and transgenic studies have focused on xylem traits. Here, we present our present knowledge of xylem-climate relationships. We then ask, on the basis of these relationships, which traits are likely to confer drought resistance in crop plants?

Climate explained meaningful proportions of variation in xylem traits across the globe. In arid habitats, natural selection has favored strategies that increase the embolism resistance of xylem, investment in xylem tissue per unit leaf area, as well as the density of xylem tissue. Mean annual precipitation and soil water potential explained 11 – 19% and 14 – 54% of the variation in hydraulic traits across a global spread of species. Tradeoffs between xylem efficiency and xylem safety, as well as the often assumed tradeoff between xylem efficiency and mechanical safety were not supported by the analyses.

The evolutionary alignment between aridity and xylem traits suggests that xylem transformation was key to angiosperm success in arid habitats. As such, it is likely that these same traits could confer drought resistance in crop species.  We suggest that research directed towards finding the genes coding for these traits, as well as their post-transcriptional regulation should be a research priorities for agriculture.