Although we have summarized the recent proteome studies on stress responses for single stress factors, it is evident that under many natural conditions the plants have to respond to a combination of stresses. Consistent with biochemical and molecular data, proteome approaches also indicate an overlap in the cellular responses to different stresses, such as detoxification of ROS by enzyme components as a ubiquitous element of defense. It should also be mentioned that due to limitations in current techniques, proteome analysis is likely to be restricted to the more abundant proteins of central metabolic pathways, implying that specific aspects of cellular responses characteristic to certain stress factors might not yet be detected with current methodology. However, a survey of published data indicates that in many experimental systems, fundamental reprogramming of central metabolic routes can be monitored. Whereas changes in the abundance of proteins representing house-keeping functions are covered by current proteome methods, analysis of signaling events in response to abiotic stresses is less evident. Targeted analysis of changes in the phos-phoproteome of rice seedlings upon treatment with stresses or hormones revealed phosphorylation of enzymes, but also identified Ca2+-signaling processes . NO signaling was identified as a major route in the cellular responses after pathogen attack, but is also part of physiological processes such as stomatal closure, growth, and development. Investigation of S -nitrosylation in Arabidopsis identified a range of candidates representing metabolic as well as signaling proteins . Clearly, more of such studies are needed to reveal common and specific cellular responses to abiotic stresses.
In a comparative analysis to define the cellular mechanisms operating in plant trichomes, the soluble proteins of whole leaves and isolated trichomes from tobacco were subjected to 2-DGE . Proteins related to stress defense responses (cytosolic Cu-ZnSOD, glutathione peroxidase, PR-2, PRQ, and an a. thaliana stress-inducible protein) were strongly represented among the products specifically enriched in tobacco trichomes. The specificity of expression of these proteins in the trichomes was confirmed by protein gel blotting and enzyme assays, indicating that tobacco trichomes constitute a valuable system to study defense response networks on the proteome level.
When investigating rice seedlings, the expression of SOD was a common response to cold, drought, salt, and ABA treatment while the expression of three proteins with unknown function was enhanced by salt and ABA stresses .
In a study combining determination of physiological parameters of pea leaves— such as photosynthetic and respiratory rates as well as lipid peroxidation product accumulation—with BN and 2-DGE separation of isolated mitochondrial proteins, the differential impact of environmental stresses on the pea mitochondrial proteome was investigated. Data suggested that herbicide treatment exerted a more severe stress on the mitochondria than did chilling and drought. Differential induction of HSPs and specific changes in other proteins suggested divergence of cellular responses upon the various stresses .
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