To understand how
auxin regulates sensitivity of abscission zone (AZ) tissues to
ethylene, we used a polymerase chain reaction-based subtractive approach to identify gene transcripts in Mirabilis jalapa AZs that changed in abundance during the time the zones became competent to abscise in response to exogenous
ethylene. Transcript expression was then examined in leaf and stem AZs over the period they became
ethylene competent following
indole-3-acetic acid (IAA) depletion either by leaf deblading, treatment with the IAA transport inhibitor
naphthylphthalamic acid, or cutting the stem above a node (
decapitation). Transcripts down-regulated by deblading/
decapitation included Mj-Aux/IAA1 and Mj-Aux/IAA2, encoding Aux/IAA
proteins, and three other transcripts showing highest identity to a
polygalacturonase inhibitor
protein, a beta-expansin, and a
beta-tubulin. Application of IAA to the cut end of petioles or stumps inhibited abscission, and prevented the decline in the levels of transcripts in both AZs. Transcripts up-regulated in the AZ following deblading/
decapitation or treatment with
naphthylphthalamic acid were isolated from plants pretreated with
1-methylcyclopropene before deblading to help select against
ethylene-induced genes. Some of the up-regulated transcripts showed identity to
proteins associated with
ethylene or stress responses, while others did not show homology to known sequences.
Sucrose infiltration of stem stumps enhanced abscission following
ethylene treatment and also enhanced the induction of some of the up-regulated genes. Our results demonstrate a correlation between acquisition of competence to respond to
ethylene in both leaf and stem AZs, and decline in abundance of
auxin regulatory gene transcripts.