The Role of Ethylene in the Regulation of Stem Gravitropic Curvature

Abstract

Ethylene’s involvement in a plant’s ability to orient itself to a gravitational field has been recognized for almost a century. In 1910, Neljubov identified ethylene as the active component of illuminating gas (used to heat greenhouses) that affected plant growth and caused senescence. He noted that pea epicotyls exposed to high levels of ethylene (1) became oriented horizontally, (2) showed reduced length and (3) increased in diameter. Hence, ethylene’s effect on the plants’ physiology became known as the “triple response”, which has become extensively used as an ethylene bioassay as well as a diagnostic phenotype for screening ethylene overproducing or insensitive mutants. Other early investigations indirectly demonstrated ethylene’s involvement in plant responses to gravity (reviewed by Abeles 1973). For example, horizontally placed pineapple plants flower earlier than upright plants. Since ethylene induces flowering in bromeliads, early flowering was attributed to being a side effect of the increase in ethylene production caused by horizontal reorientation of the plants. In another experiment, horizontally oriented plants that were continually rotated on a clinostat exhibited physiological responses such as downward leaf bending and growth inhibition, reactions known to be caused by increased ethylene production. Decades of research on ethylene’s involvement in the gravitropic response of plant stems have produced contradictory data concerning its effect on curvature. Plant tissues displaying different sensitivity to ethylene, and growth conditions that are likely to alter ethylene production often confound meaningful data comparison. The characterization of ethylene mutants along with genetic and molecular approaches using Arabidopsis allows for new insights into the role of ethylene in gravitropism. This chapter will present studies that characterize the regulatory role of ethylene during gravitropic curvature in different types of plant stems and inflorescence stalks, and will also discuss the current understanding of ethylene cross-talk with auxin relative to the regulation of stem gravitropism. 7 The Role of Ethylene in the Regulation of Stem Gravitropic Curvature