Auxin transport and the regulation of petiole abscission in cotton (Gossypium hirsutum L.): A comparison of indol-3yl-acetic acid and phenylacetic acid

Abstract

Distal applications of indol-3yl-acetic acid (IAA) to debladed cotyledonary petioles of cotton (Gossypium hirsutum L .) seedlings greatly delayed petiole abscission, but similar applications of phenylacetic acid (PAA) slightly accelerated abscission compared with untreated controls . Both compounds prevented abscission for at least 91 h when applied directly to the abscission zone at the base of the petiole . The contrasting effects of distal IAA and PAA on abscission were correlated with their polar transport behaviour -[1-14C]IAA underwent typical polar (basipetal) transport through isolated 30 mm petiole segments, but only a weak diffusive movement of [1-14 C]PAA occurred .Removal of the shoot tip substantially delayed abscission of subtending debladed cotyledonary petioles . The promotive effect of the shoot tip on petiole abscission could be replaced in decapitated shoots by applications of either IAA or PAA to the cut surface of the stem . Following the application of [l-14C]IAA or [1-14 C]PAA to the cut surface of decapitated shoots, only IAA was transported basipetally through the stem . Proximal applications of either compound stimulated the acropetal transport of ["Cl sucrose applied to a subtending intact cotyledonary leaf and caused label to accumulate at the shoot tip . However, PAA was considerably less active than IAA in this response .It is concluded that whilst the inhibition of petiole abscission by distal auxin is mediated by effects of auxin in cells of the abscission zone itself, the promotion of abscission by the shoot tip (or by proximal exogenous auxin) is a remote effect which does not require basipetal auxin transport to the abscission zone . Possible mechanisms to explain this indirect effect of proximal auxin on abscission are discussed .