While signaling pathways involving plant hormones have been studied for a long time, it has only recently been recognized that a common factor interacts or cross-talks with a multitude of other signaling pathways. This common and central factor is the cellular redox homeostasis (or balance between oxidants and antioxidants) that is affected by both internal and environmental events. A common response to a host of different types of adverse environmental conditions is an increased internal production of potentially destructive oxidants (ROS), resulting in oxidative stress. These ROS arise through interaction of oxygen with the electron-transport chains of both respiration in the mitochondria and photosynthesis in the chloroplast, as well as during plant defense against invaders. The downregulation of photosynthesis in response to stress mentioned above has a role in minimizing the generation of these ROS. In addition, ROS are formed by the light-absorbing, photosynthetic pigments whenever more light is absorbed than can be used through photosynthesis.
On the one hand, generation of such potentially harmful products serves as a signal to upregulate certain metabolic pathways that increase the chances of survival. On the other hand, if allowed to accumulate unchecked, these ROS have the potential to (1) cause direct damage to vital biomolecules, such as components of membranes, proteins, and DNA/RNA; and (2) trigger signaling pathways that can ultimately lead to the demise of the organism via, for example, programmed cell death. Therefore, acclimation to all forms of environmental stress involves enhancement of defenses against ROS.
These defenses detoxify ROS and/or restore oxidized biomolecules. Due to the common enhancement of ROS production as a result of many kinds of environmental disturbances, all organisms possess vitally important redox-sensitive signaling pathways that inform the organism of environmental change and orchestrate adjustments, defenses, life-and-death decisions (involving the control of cell division and cell death), and other crucial decisions affecting resource allocation, reproduction, and senescence. This central role of cellular redox homeostasis in gene regulation is common to all organisms, including microbes, plants, and animals.
This area is a new and fascinating field of biology, and much remains to be explored. What is clear is that there is a continuum from the ability to sensitively detect environmental change via redox sensing and signaling all the way to destruction by excess ROS. Furthermore, highly complex interactions exist between multiple signaling networks, and different cell compartments as well as different tissues and organs are in communication and redox balance. Current research is revealing interactions, or cross talk, between redox signals and previously characterized major signaling and regulatory pathways involving hormones, photoreceptors, and a range of other messengers. Finally, and as stated above, the most central aspects of life, including growth, development, reproduction, defense, and death, are controlled by these redox-modulated signaling networks as the organism's eyes to the world in their nature as global environmental sensors.
While under moderate conditions ROS production is balanced via detoxification by antioxidants and other defenses, survival under the harshest conditions depends on the ability to go to more extreme measures and actually shut down the metabolic processes that generate ROS. Some of these metabolic processes, such as respiration and photosynthesis, are vital for the support of an organism's normal activity. Paradoxically, these essential metabolic processes are also the primary internal sources of ROS production. Both plants and animals that have the capacity to survive severe environmental conditions are capable of entering a metabolically inactive, but highly stress resistant state.
See also: Adaptation; Alpine Forest; Autotrophs; Deserts; Ecophysiology; Environmental Tolerance; Global Warming Potential and the Net Carbon Balance; Life Forms, Plants; Limiting Factors and Liebig's Principle; Organismal Ecophysiology; Physiological Ecology; Plant Growth Models; Plant Physiology; Respiration; Salinity; Temperate Forest; Temperature Regulation; Tolerance Range; Transport over Membranes; Tree Growth; Water Availability.
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