Abiotic Stress Induced Morphogenic Responses in Plants

As far as the morphological adaptations in response to abiotic stress is concerned, recent studies revealed the development of distinct morphological responses in plants (stress-induced morphogenesis responses - SIMRs) irrespective the abiotic stresses applied. In fact, different stressors was found to induce similar morphogenic responses which mainly comprised of an inhibition of elongation, localized stimulation of cell division, and complex changes in cell differentiation (Fig. 2.1). This is completely surprising

Control root

Root showing SIMRs under heavy metal stress or phosphate deficiency

Redifferentiation of pericycle and formation of lateral roots

Inhibition of elongation and consequent formation of , root hairs near apex

Inhibition of elongation and consequent formation of , root hairs near apex

Blocked cell division in primary meristem

Control shoot

Shoot showing SIMR

under UV-B or mechanical stress

Control shoot

Shoot showing SIMR

under UV-B or mechanical stress

Decreased leaf area and increased leaf thickness

Inhibition of elongation

Increased formation of lateral shoots

Fig. 2.1 Schematic presentation of the effects of abiotic stresses on plant morphology. (a) Overview of root stress-induced morphogenic responses, including an inhibition of root elongation, blocked cell division in the primary meristem, and increased formation of lateral roots. (b) Overview of shoot stress-induced morphogenic

Decreased leaf area and increased leaf thickness

Inhibition of elongation

Increased formation of lateral shoots a b

Fig. 2.1 Schematic presentation of the effects of abiotic stresses on plant morphology. (a) Overview of root stress-induced morphogenic responses, including an inhibition of root elongation, blocked cell division in the primary meristem, and increased formation of lateral roots. (b) Overview of shoot stress-induced morphogenic responses, consisting the inhibition of shoot elongation, increased formation of lateral shoots, and increased leaf thickness (reproduced from: Potters et al. 2007). Stress-induced morphogenic responses: growing out of trouble? Trends in Plant Science 12: 98-105; permission from Elsevier taking into consideration the differences in types of stress and plants species (Potters et al. 2007, 2009), as well as in stress perception mechanisms, target tissues, and effects on cellular metabolism. For instance, increased concentration of cadmium in plants which affects calcium equilibrium (Perfus-Barbeoch et al. 2002), drought which affects cell wall elasticity and cell redox balances (Sgherri et al. 2007), and elevated UV-B which impacts on DNA transcription and replication, all induce similar morphogenesis responses which contribute to plant adaptation. These common plant morphogenic responses under abiotic stress imply the possible existence of a distinct mechanism operating at either the cell or the organism level. This mechanism should be responsible for sensing, perception, and trans-duction of the environmental stresses signals leading to specific changes in plants morphology. Till now there are several theories concerning the structure and functioning of the above mentioned mechanism. The first one (reviewed by Prusinkie-wicz and Rolland-Lagan 2006) suggests that morphogenesis is controlled by organ and organism-wide gradients of signaling molecules, the so-called "morphogenes." Abiotic stressors that caused morphogenesis are translated into specific patterns of "morphogene" distribution at the organism, organ, or tissue scale, leading to cellular regulation of the processes that are linked to single cells. This theory was expanded recently by incorporating the cellular signaling network (Rauch and Millonas 2004). An alternative interpretation considered morphogenesis as a cellular process that is controlled by the rate of division and expansion of individual cells (Sugimoto-Shirasu and Roberts 2003). Taking into consideration that both cell division and elongation are the major cellular processes by which a plant expands, any morphogenetic process should be based on these two processes. Thus, changes in plant morphology in response to different abiotic stressors are the ultimate consequence of changes in the division and/or expansion rate of the cells. Recently an integrated approach of morphogenesis, whereby growth is being controlled at both the cellular and the organismal levels, has also been proposed (Beemster et al. 2003) . Following this approach, the growth and organs formation responses under different abiotic stress are coordinated via growth substance gradients and signaling molecules; all operating at the organ and organismal levels. This theory clearly suggests the occurrence of certain cellular and organismal components that control SIMR phenotype. Whatever interpretation is true the similarities in the morphogenic responses induced by distinct stresses, implies common processes at least at molecular level. At this level, increasing data tend to confirm that stress-induced changes in absolute values as well as interactions between the three morphogenes - auxin, Reactive Oxygen Species (ROS), and ethylene - mainly mediate and control SIM responses in plants (Fig. 2.2).

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