Gravistimulated asymmetries in the outer epidermal cell walls of graviresponding coleoptiles

Gravitropic plant growth is due to gravistimulated asymmetric extension rates of the affected flanks of the graviresponding organ. Differential growth of the upper and lower flanks (UFs, LFs) of graviresponding plant organs may in principle be achieved by various biochemical and/or biophysical asymmetries. The gravistimulated mechanism(s) by which the different growth rates are determined is still unresolved, as is the mechanism of IAA-regulated growth. The purpose of this brief review is to summarize and critically evaluate data concerning gravistimulated asymmetries, especially with respect to the interface of the plasma membranes and of the extension-restricting, load-bearing epidermal cell walls (Masuda and Yamamoto 1972 Physiol Plant 27: 109-115). In addition, recent results obtained by the author in experiments with rye coleoptiles will be presented and discussed in the context of a tentative model of gravistimulated wall asymmetries temporarily causing differential growth.