PLASTIDS AND GRAVITROPIC SENSING

Data and theories about the identity of the mass that acts in gravitro pic sensing are reviewed. Gravity sensing may have evolved several tim es in plants and algae in processes such as gravitropism of organs and tip-growing cells, gravimorphism, gravitaxis, and the regulation of c ytoplasmic streaming in internodal cells of Chara. In the latter and i n gravitaxis, the mass of the entire cell may function in sensing. But gravitropic sensing appears to rely upon the mass of amyloplasts that sediment since (i) the location of cells with sedimentation is highly regulated, (ii) such cells contain other morphological specialization s favoring sedimentation, (iii) sedimentation always correlates with g ravitropic competence in wild-type plants, (iv) magnetophoretic moveme nt of rootcap amyloplasts mimics gravitropism, and (v) starchless and intermediate starch mutants show reduced gravitropic sensitivity. The simplest interpretation of these data is that gravitropic sensing is p lastid-based.