7 - Evolutionary origin of the ethylene biosynthesis pathway in angiosperms

Abstract

This chapter discusses the evolutionary origin of the angiosperm ethylene biosynthesis pathway. Responsiveness to ethylene appears to be present throughout the plant kingdom. Evolution of ethylene responsiveness was driven by the natural connection between ethylene generation and plant stress. When cells are damaged, oxidative breakdown of cell constituents—particularly membrane fatty acids—generates ethylene, albeit in amounts that are orders of magnitude lower than those generated physiologically. As land plants evolved, they acquired increasingly complex physiological and biochemical responses to the stresses to which they were exposed. Among the biochemical responses were betaine synthesis, for the reduction of water potential during drought, and lignifications for the limit predator and pathogen attack. The synthesis of both betaine and lignin depends upon the availability of S-adenosylmethionine (SAM). This is an abundant metabolite in all organisms and plays a key role in transmethylation reactions. It is derived from methionine by the action of SAM synthetase. SAM is also the precursor of 1-aminocyclopropane-l-carboxylic acid (ACC) that arises by the action of ACC synthase on SAM. ACC is detectable in representatives of all major groups of land plants. In the earliest land plants, cell damage arising from stress leads to the formation of ethylene by chemical breakdown of cell constituents. In ferns and allied non-seed plants, stress leads to the increased expression of SAM synthetase for lignin and betaine synthesis, enhanced ACC synthase activity, and the accumulation of ACC. Ethylene is produced from ACC with the help of the enzyme ACC oxidase.