Auxin-induced ethylene triggers abscisic acid biosynthesis and growth inhibition

The growth-inhibiting effects of indole-3-acetic acid (IAA) at high concent ration and the synthetic auxins 7-chloro-3-methyl-8-quinolinecarboxylic aci d (quinmerac), 2-methoxy-3,6-dichlorobenzoic acid (dicamba), 4-amino-3,6,6- trichloropicolinic acid (picloram), and naphthalene acetic acid, were inves tigated in cleavers (Galium aparine). When plants were root treated with 0. 5 mM IAA, shoot epinasty and inhibition of root and shoot growth developed during 24 h. Concomitantly, 1-aminocyclopropane-1-carboxylic acid (ACC) syn thase activity, and ACC and ethylene production were transiently stimulated in the shoot tissue within 2 h, followed by increases in immunoreactive ()-abscisic acid (ABA) and its precursor xanthoxal (xanthoxin) after 5 h. Af ter 24 h of treatment, levels of xanthoxal and ABA were elevated up to 2- a nd 24-fold, relative to control, respectively. In plants treated with IAA, 7-chloro-3-methyl-8-quinolinecarboxylic acid, naphthalene acetic acid, 2-me thoxy-3,6-dichlorobenzoic acid, and 4-amino-3, 6,6-trichloropicolinic acid, levels of ethylene, ACC, and ABA increased in close correlation with inhib ition of shoot growth. Aminoethoxyvinyl-glycine and cobalt ions, which inhi bit ethylene synthesis, decreased ABA accumulation and growth inhibition, w hereas the ethylene-releasing ethephon promoted ABA levels and growth inhib ition. In accordance, tomato mutants defective in ethylene perception (neve r ripe) did not produce the xanthoxal and ABA increases and growth inhibiti on induced by auxins in wild-type plants. This suggests that auxin-stimulat ed ethylene triggers ABA accumulation and the consequent growth inhibition. Reduced catabolism most probably did not contribute to ABA increase, as in dicated by immunoanalyses of ABA degradation and conjugation products in sh oot. tissue and by pulse experiments with [H-3]-ABA in cell suspensions of G. aparine. In contrast, studies using inhibitors of ABA biosynthesis (flur idone, naproxen, and tungstate), ABA-deficient tomato mutants (notabilis, f lacca, and sitiens), and quantification of xanthophylls indicate that ABA b iosynthesis is influenced, probably through stimulated cleavage of xanthoph ylls to xanthoxal in shoot tissue.