Extremophytes, or eXtreme plants, or plant extremophiles, are plants evolutionarily adapted to thrive in environments characterized by a high degree of abiotic stress. The list of extreme stresses includes, for example:
- very high or very low temperatures;
- high irradiance or high UV;
- extreme soil or water pH;
- high salinity and/or alkalinity;
- soil nutrient limitations – especially P;
- heavy metal toxicities and other challenging chemistries;
- very low, or very high levels of soil water (severe drought and flooding)
Clearly, the greater the number of these stresses that occur simultaneously, the greater the overall stress, and the more eXtreme the plants.
There is a very large number of plants that could, potentially, be classified as extreme. For example, roughly 2% of all angiosperms are considered to be halophytes, adapted to high salinity. Perhaps every native species in Western Australia, especially in the arid/semi-arid areas, classifies due to tolerance of drought, fire, salinity, temperature and seriously low soil P. The highly diverse flora of the world’s desert regions is another large group. All epiphytes, all alpine and arctic plants, everything that grows on mine spoils… the total number of extremophytes is definitely large. Basically, however, extremophytes are found in plant communities with low diversity due to environmental limitations… areas in which there would be few if any humans in the absence of serious technological assistance.
Note, however, that the ecological restriction of a species to a particular environment does not mean that it would necessarily be unable to complete its life cycle under less extreme conditions, were (for example) competition from other plants to be prevented. Halophytes, for example, can generally be cultivated in botanical gardens without adding salt even if they might be overgrown with “weeds” in the absence of human care.
Extremophytes have long been studied in the realm of “natural history” or with ecological objectives in mind. Certainly, this is the only level at which the huge diversity can be comprehended. But even then, the number of possible solutions to the problems of extreme stress seems to be dwarfed by the number of actual ones.
Fortunately, the evolution of eXtreme plants from less stress-adapted progenitors has left traces in their genomes that are reflected in their form and function. These have been studied for at least 50 years at the morphological, physiological and biochemical levels. In the last 10-15 years, there has been a rapid expansion of considerations at the molecular level. By necessity, as focus has narrowed, both the number of species targeted and the range of environmental/experimental conditions addressed has decreased.
If we are to understand (and manipulate) the responses of plants to any degree of stress, let alone to extreme stress, we will need a concerted and collaborative effort on the part of researchers addressing all levels of complexity and organization. In putting together this web site, it is our hope to facilitate that effort.