The Role of Auxin Ethylene and ROS

Recent studies indicate that morphogenesis is tightly linked to hormonal homeostasis, with several hormones controlling cell elongation, cell division, and reorientation of growth. Among them, auxin [indole-3-acetic acid (IAA)] and its gradients are found to be closely associated with lateral root formation and axillary branching, two key components of the SIM responses of plants exposed to abiotic stress (Potters et al. 2007).

Several mechanisms have been proposed to explain stress-induced changes in auxin metabolism with those referring to auxin transport and catabolism to be considered as the preponderant ones. In particular, various abiotic stressors are found to affect expression of auxin efflux carrier genes (Schrader et al. 2003) resulting in modification of auxin distribution throughout the plant organism (Leyser 2005; Paponov et al. 2005). An alternative interpretation suggests that the accumulation of distinct phenolic compounds in response to stress exposure (Winkel-Shirley 2002) contribute in inhibition of polar auxin transport. Indeed, Peer et al. (2004) demonstrate an inverse relation between auxin transport and flavonoid content in a series of Arabidopsis flavonoid mutants. As far as the auxin catabolism is concerned, recent

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