Heat‐tolerant Flowering Plants of Active Geothermal Areas in Yellowstone National Park

If you can't stand the heat . . .

Although many thermo-tolerant micro-organisms have been described, habitats such as hot springs are a very hostile environment for more complex life-forms; it is thus not surprising that only a few vascular plants grow in geothermal areas. Previous studies suggest that soil temperature is the major limiting factor for plants in these habitats. In a fascinating study carried out in the Yellowstone National Park, USA, Stout and Al-Niemi (Montana State University, pp. 259-267) have found only nine angiosperm species, four of which are grasses, that can grow in soils with a mean temperature of above 40 degrees C at 10 cm depth. Both perennial and annual lifestyles are represented and several of the species are actually confined to hot soils; the dominant grass is Dicanthelium lanuginosum (formerly Panicum thermale). The mean temperature of 40 degrees C hides the extremes that these plants survive: rhizosphere temperatures as high as 65 degrees C have been recorded and it is not uncommon for temperatures to remain above 50 degrees C for extended periods. How do these plants survive? The authors have started to tackle this question by surveying the distribution of small heat-shock proteins (sHSPs) and of one larger HSP (HSP101). Taking D. lanuginosum as an example, sHSPs are expressed in roots at soil temperatures above 35 degrees C but are not present in leaves until the soil temperature reaches 45-50 degrees C. (Leaf temperatures are generally 15-25 degrees C lower than rhizosphere temperatures.) The expression of sHSPs is also induced by transferring laboratory-grown plants to 40 degrees C for 2 h. HSP101 is present under all conditions in both roots and leaves but the amount of protein increases with increasing temperature. This is an exciting start to understanding the extreme thermo-tolerance of this fascinating group of plants: it is particularly encouraging to see molecular biology being applied to wild plants in the field.