The role of auxin in abscission of organs and tissues

Sammanfattning: Most deciduous trees drop their leaves before winter, a process which is referred to as leaf abscission. Leaf abscission is thought to be regulated by the action of auxin and ethylene. In order to test the function of auxin in leaf abscission, an experimental system in Populus was established to induce leaf shedding synchronously under controlled greenhouse conditions. Exogenous auxin and an auxin transport inhibitor delayed the abscission of dark-induced leaves and a new auxin response maximum preceded the formation of an abscission zone. The analysis of microarray results revealed that several genes encoding auxin transporters were strongly down-regulated during abscission, suggesting their involvement in the formation of the auxin maximum in the leaf axil. In ethylene-insensitive trees, leaf abscission could be delayed by the application of auxin and ethylene signaling was not required for the regulation of gene expression of auxin transporters during abscission. Thus, auxin and ethylene act partly independently of each other on leaf abscission in Populus. In order to study the effects of auxin on cell separation, isolated from its action on the development of an abscission zone, we examined root cap abscission in Arabidopsis. An auxin response gradient, spanning the root cap, was found to be established prior to the separation of the outermost root cap layer. Inhibition of polar auxin transport abolished the auxin response gradient in the root cap and disrupted abscission. Intriguingly, auxin efflux carriers of the PIN family were not expressed in the cell layer proximal to the abscising layer indicating that the outermost columella tier is disconnected from the auxin source in the quiescent center. A Populus homolog of the Arabidopsis WALLS ARE THIN1 (WAT1) was among the most strongly regulated genes during abscission. We found that WAT1 localizes to the tonoplast and facilitates auxin export from the vacuole. Whereas, WAT1-mediated auxin homeostasis is needed for secondary wall deposition, wat1 mutants do not display any phenotype related to abscission. While auxin gradients have been implicated in various growth-related processes our work provides novel data in support of a regulatory role of distinct auxin maxima and minima in organ and tissue abscission.

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