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DEVELOPMENT AND HORMONE ACTION

Molecular Changes Occurring during Acquisition of Abscission Competence following Auxin Depletion in Mirabilis jalapa^1,[W]

Department of Postharvest Science of Fresh Produce, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel (S.M., V.H.); New Zealand Institute for Crop & Food Research Limited, Private Bag 11600, Palmerston North, New Zealand (D.A.H.); and Department of Plant Sciences, University of California, Davis, California 95616 (J.-C.C., M.S.R.)

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 -expansin, and a -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.

² These authors contributed equally to the paper.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Shimon Meir (shimonm{at}volcani.agri.gov.il).

^[W] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.079277.

^* Corresponding author; e-mail shimonm{at}volcani.agri.gov.il; fax 972–3–9683878.

Received February 14, 2006; returned for revision June 2, 2006; accepted June 5, 2006.

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