Redifferentiation and lateral root formation
Inhibition of elongation
Blocked cell division in primary meristem
Inhibition of elongation
Fig. 2.2 Interaction analysis between environmental parameters, reactive oxygen species (ROS), auxins, and ethylene leading to the stress-induced morphogenic response (SIMR) phenotype (reproduced from: Potters et al. 2009). Different stresses, similar morphogenic responses: integrating a plethora of pathways. Plant, Cell and Environment 32: 158-169; permission from John Wiley and Sons data conclude that IAA concentration is also subjected to modulation under stress conditions (Jansen et al. 2001). IAA could be degraded via a peroxidative mechanism coupled to a very efficient branched-chain process in which organic peroxide is formed and/or via an oxidative mechanism, using molecular oxygen as electron sink (Savitsky et al. 1999). Additionally, stress-induced changes in auxin conjugation as well as in auxin sensitivity can alter IAA activity and hence impact on morphogenesis (Jiang et al. 2007).
Ethylene involved in the responses of several plant species to heat, drought, or ozone stress (Rao et al. 2002) . However, most data suggest ethylene minor contribution in the SIMRs process under abiotic stress conditions mainly indirectly through alteration in the auxin sensitivity of plants (Takahashi et al. 2003).
Enhanced ROS production is associated with a broad range of abiotic stresses including drought (Beis and Patakas 2010), heat stress, enhanced UV-B radiation stress (Doupis et al. 2011), heavy metal stress and anoxia. This increase in ROS production induce an upregulation of ROS scavenging systems, involving enzymatic components such as superoxide dismutase, catalase, and ascorbate peroxidase, as well as antioxidants such as ascorbate and glutathione (Noctor and Foyer 1998). Generally, this integrated system prevents oxidative damage constituting a common component of the response of plants to many distinct stresses. Despite the already known role of ROS as signaling molecules there are also increasing evidences that may also contribute in controlling developmental processes (Gapper and Dolan 2006). In particular, ROS is reported to influence cell development (e.g., xylem vessel formation; Ros-Barcelo et al. 2002), cell division (i.e., temporarily inhibiting cell cycle activity; Reichheld et al. 1999), cell elongation (Schopfer 2001), somatic embryogenesis (Pasternak et al. 2007), and adventitious root formation (Li et al. 2007). Taking into consideration that both ROS and SIMRs are common components of many distinct stresses, it could be assumed that ROS are intermediates between the stress and the development of the SIM responses. Indeed, a direct correlation between ROS and the SIM responses has been demonstrated by Pasternak et al. (2005) who found that Arabidopsis thaliana plantlets subjected to either ROS-generating compounds (e.g., paraquat), or a hydrogen peroxide derivative (Pasternak et al. 2005) had an SIMR-like pheno-type, similar to that induced by, for example, under enhanced copper concentration.
However, the above mentioned signaling pathways involving either auxins, ethylene, or ROS should not be considered functioning independently one to other. For instance, ROS may modulate auxin sensitivity, by downregulation of auxin-inducible gene expression, a process that involves changes in MAPK activity (Kovtun et al. 2000). Moreover, the auxin-induced elongation of root cells is easily mimicked using either superoxide or hydrogen peroxide (Schopfer et al.
2002) . Conversely, auxins, and cytokinins might modulate H2 O2 effects on stomatal closure, by regulating H2 O2 scavenging (cytokinins) or production (auxins) (Song et al. 2006) . Furthermore, the possible role of nitric oxide and other reactive nitrogen species (RNS) on plants morphogenesis control (Kolbert et al. 2008) as well as an interaction between nitric oxide and auxin in the control of cell division should be reconsidered (Otvos et al. 2005) . All these data confirms that the assumption that SIMRs are based solely on the three morphogenes - auxin, ROS, and ethylene - changes might be an oversimplification. It seems possible that these changes consist only a small part of a more complex mechanism involving interrelationship and interaction between a broader range of molecules whose activation depends on the kind and intensity of abiotic stress as well as on the plant species. In this frame, different stressors are likely to activate a plethora of specific sensors that operates in parallel to each other. Each sensor could regulate a complex signaling cascade, with several interactions resulting in acclimation responses to mild stress. Thus, more detailed analysis concerning interactions analysis between various environmental stress parameters and signals that control SIMRs is needed. Additionally, the specific stress-induced anatomical changes at plant organ level, that is, root, xylem, and leaf should be examined and their relevant significance and potential role to plant adaptation must be carefully evaluated. Considering that drought consist the most important and limiting environmental factor for plant production worldwide, this chapter mainly refers to the holistic approach of plant anatomical changes under limited soil water availability.
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