The role of auxin in abscission of organs and tissues

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