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James_2002 - news and views motifs have a general function...

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motifs have a general function in protein recognition 7 , indicating that LRR receptors are only indirectly involved in recognizing carbohydrate microbial factors (such as the Nod factors). Primary recognition could be performed by secreted extracellular molec- ules, lectins for instance, which are recog- nized by the LRR receptors after binding of microbial signals 13 . The evolutionary relatedness of the plant and animal receptors shown in Fig. 1 is sur- prising if one considers that the last common ancestors of higher plants and animals sepa- rated more than 1.8 billion years ago. In ani- mals, the relatives of the plant receptors are also involved in recognizing microbes, mean- ing that similar carbohydrate-recognition mechanisms could underlie the animal innate immune system. Carbohydrate recog- nition in the human immune system is still poorly understood, so unravelling microbe- recognition mechanisms in plants should also provide lessons for medical science. S ome mountain ranges form along the boundaries of colliding tectonic plates, and the classic account of their rise and fall runs as follows. In these ‘collisional belts’, the Earth’s crust is squeezed, thickened and uplifted by the forces that drive the plates together, and is also thickened by the addi- tion of magmas from the underlying mantle. Where powerful mountain-building forces are at work today, as in the central Andes and Tibet, the mass of the mountains is gravita- tionally supported by a thick ‘root’ of buoy- ant low-density rock beneath them. When active uplift and crustal thickening cease, erosion begins the inexorable process of reducing once lofty mountains to low-lying plains. As the mountains are eroded, gravi- tational balance is maintained by continued uplift of the buoyant crustal root to compensate for mass loss at the surface. As Fischer reports on page 933 of this issue 1 , however, this classic balancing act seems not to apply to many old and deeply eroded collisional belts. In these places, dis- proportionately thick crustal roots have often survived for hundreds of millions — even billions — of years, and Fischer provides an explanation for why that should be so (Fig.1). She has examined collisional mountain belts of all ages worldwide. She finds that the ratio of elevation to crustal-root thickness decreases systematically from about 0.1–0.2 for the youngest mountain belts (root thick- ness 5–10 times greater than surface relief) to essentially zero for mountain belts several hundred million years old (still appreciable root thickness, but minimal surface relief; see Fig. 1a on page 933). This is perplexing because, given the mass balance between crustal roots and surface load in young mountain ranges, it would seem that removal of the mass at the surface should involve proportional loss of the buoyant root. Old mountains eroded to flat-lying plains should have no remaining roots.
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