Ethylene-triggered abscisic acid: A principle in plant growth regulation?

The application of auxins to sensitive plant species or their overproductio n in transgenic plants stimulates ethylene biosynthesis via induction of 1- aminocyclopropane-1-carboxylic acid (ACC) synthase. Recent studies with aux in herbicides and indole-3-acetic acid (IAA) have revealed that auxin-stimu lated ethylene triggers an increase in the biosynthesis of abscisic acid (A BA), which then functions as a second messenger, leading to growth inhibiti on and senescence. This raises the question of whether ethylene-triggered A BA is restricted to the action of auxin herbicides or whether it is a wides pread phenomenon in the normal plant growth regulation. Our own results and a reappraisal of the literature indicate that ethylene-induced ABA may, in deed, play a role in natural physiological phenomena, such as root gravirea ction and suppression of lateral bud growth in apical dominance. In additio n, it would be worthwhile to investigate whether ethylene-triggered ABA is involved in other processes which coincide with a strong stimulation of eth ylene biosynthesis, such as growth inhibition induced by cytokinins and sen escence elicited under stress conditions.