Mechanical Aspects of Gravity-Controlled Growth, Development and Morphogenesis

Abstract

Plants are “addicted” to gravity. Gravity is the most constant physical force and acts on all organisms in the same way. Plants need this vectorial physical force for their roots to grow down into soil to search for water and mineral nutrients, and for their shoots to grow up to allow optimal exposure of their leaves to light. The plant body form and plant morphogenesis depend on gravity, which acts as a reference force for plant development. Gravity-controlled morphogenesis of plants seems to rely on multiple gravity-sensing mechanisms. Despite more than 100 years of study, plant gravisensing is still as mysterious as it was in 1900, when, for the first time, the statolith theory was proposed by Bohumil Nemec and Gottlieb Haberlandt. Some 10 years after its initial acceptance, this theory was put aside for some 50 years. In the 1960s, it was resuscitated and presently dominates our scientific thinking. Nevertheless, how gravity is perceived and what the gravity transduction pathways are remain enigmatic. In this chapter, we first discuss the physical properties of the cytoplasm. Then, we provide an overview the starch-based amyloplasts and some other possible statolith candidates for sedimentation to the physical bottom. We list several unconventional processes and structures, both at the subcellular and at the supracellular level, which emerge to play a role in plant gravisensing. Finally, we point out surprising differences in sensing of gravity between roots and shoots. Although there are several common themes, such as amyloplast sedimentation, relevance of endomembranes and endocytic vesicle recycling, as well as of the actin cytoskeleton, and the polar transport of auxin, there are very profound differences between root and shoot sensing of gravity. Obviously, plant gravisensing will keep plant scientists busy in the future.