Oxidative stress response in sugarcane

Resumo

Reactive Oxygen Species (ROS) are intermediates derived from molecular oxygen. High concentrations of ROS are generally considered to be dentrimental to living organisms, including plants, because they can promote lesions in biomolecules such as DNA, proteins and lipids. These lesions can impair plant cellular functions. In plants, ROS can be formed during incomplete reduction of oxygen during electron transport in mitochondria or during photossynthesis in chloroplasts. In chloroplasts, another reactive specie, singlet oxygen, is formed from the excitation of triplet oxygen through the absorption of photons by photossyntetic pigments (Halliwell and Gutteridge, 1999). The production of ROS can be increased in situations of environmental stresses such as metal pollution or antibiotic exposure. In these cases, plants can respond to these stresses increasing their antioxidant defense (Tsang et al, 1991). Plants, as other complex multicellular organisms, have complex systems to sense, transduce and respond to the environmental stress and these processes are highly regulated. Little is known about receptors, phosphatases, kinases, transcription factors and other proteins involved in the coordination of antioxidant defense in plants. The identification of these components can be facilitated by genome projects such as SUCEST. The cellular antioxidant defense system is composed of various components which can be divided as: metal chelators; low molecular weight antioxidants; antioxidant enzymes and repair systems. Metal chelators represent the first line of defense by preventing ROS formation. Low molecular weight compounds and antioxidant enzymes act as the second line of defense by protecting biomolecules from oxidation. Finally, repair systems perform their function after the ROS attack to biomolecules. Comparatively to animals, little is known about the plant antioxidant system. Phytochelatins appears to be involved in the prevention of ROS formation by metal chelation as metalothioneins do in mammals. Plants have conventional antioxidant enzymes such as catalase, glutathione peroxidase and superoxide dismutase and particular ones such as ascorbate peroxidase a hemeprotein that utilizes ascorbate to reduce hydrogen peroxide (Salin, 1987). Another important antioxidant enzyme in plants is thioredoxin peroxidase, an abundant protein located in chloroplasts (Baier and Dietz, 1997). Plants possess flavonoids and phenolic compounds among other ubiquitous low molecular weight compounds such as ascorbate and glutathone (Rice-Evans et ai, 1996). One of the objectives of this proposal is the identification of genes involved in the synthesis of components of the antioxidant system in sugar cane. Besides their deleterious properties, ROS participate in plant defense against pathogens. The defense response depends on the ability of plants to recognize the pathogen early in the infectious process. One of these early events, characteristic of the hypersensitive response (HR), is the release of ROS, known as the oxidative burst. Oxidative burst seems to play an important role in strengthening of the plant cell wall in the event of pathogen attack. In addition the HR, plants respond to necrotic pathogens by developing enhanced resistance to the same or other unrelated pathogen. This response is called systemic acquired resistance (SAR). Treatment with salicylic acid, which is known to induce SAR, also lead to accumulation of hydrogen peroxide by down-regulating the gene coding for the antioxidant enzyme catalase. Hence, hydrogen peroxide and other ROS appear to play a key defensive roIe in both HR and SAR (Wojtaszek, 1997). One of the purposes of this proposal is to identity genes which may be involved in the production or decomposition of ROS, and therefore, also involved in oxidative burst, HR and SAR. These processes are important to various fields such as agriculture, ecology and toxicology. My expertise in this field can be attested by my publications. Recently, the role of hydrogen peroxide and ROS as second messengers has been suggested in processes such as mitocondrial permeabilization, cell death and communication between mitocondria-nucleus (see my C.M. attached). Antioxidant proteins such as catalase and thioredoxin peroxidase appear to have an important role in signal transduction by modulating the steady state concentration of ROS. My experience in the field of biological oxidations was important for the annotation during the Xylella fastidiosa project, especially for the elaboration of the item 'Oxidative Stress Response", which deserved a special section in the figure 2 of the paper submitted to Nature. Therefore, based on these considerations, I believe that my contribution to data mining in the SUCEST project would be very important. (AU)