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Plant ROS Research


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Reactive Oxygen Species: Friends or Foes?

José A. Hernández Cortés (Scientist Researcher, CEBAS-CSIC, Murcia)
As we indicated in the previous chapter, the use of the molecular oxygen (O2) as final electron acceptor in the mitochondrial respiratory chain, supposed a huge benefit to the life evolution in the earth. However, the consequence of the O2 transformation into water (H2O) in the cellular metabolism is the formation of Reactive Oxygen Species (ROS), very toxic intermediates to the cellular components. O2 is a free radical, and it has two impaired electrons with the same spin quantum number. This spin restriction makes O2 to accept its electrons in univalent steps, leading to the so-called ROS, sometimes also referred as AOS (activated oxygen species) or ROI (reactive oxygen intermediaries).

Fig ROS ingles

But, who are they and what effects do they have?

Four electrons (e) and four protons (H+) are required for the full reduction of dioxygen to water, but it can be produced in single steps, giving rise to superoxide radicals (O2.-), hydrogen peroxide (H2O2) and hydroxyl radicals (.OH). All these intermediates are chemically reactive and biologically toxic. This toxicity is reflected by their short half-life for reacting with cellular components or molecules. In plant tissues, about 1-2% of oxygen consumption leads to the generation of ROS under normal conditions. So, ROS are an inevitable consequence of aerobic respiration. This percentage is higher in environmental stress conditions, being ROS involved in the cellular damages produced in such situations.

Fig reducción O2

 

Thus, most of the ROS are formed by etransfer to O2. However, others ROS, such as the singlet oxygen (1O2) is generated by the energy transfer from triplet chlorophyll to O2. As general damages, the ROS cause the inhibition of some enzymes, chlorophylls degradation, produce damages in membranes and in the DNA (can give room for unusual mutations), protein oxidation, etc.

In human beings, the ROS are implicated in different diseases such as rheumatoid arthritis, hepatitis, Alzheimer’s, Parkinson’s, muscular dystrophy, multiple sclerosis, cataracts, macular degeneration, autoimmune diseases, etc.

However,  not everything is bad. ROS have also positive effects and they are involved in other processes such as cell signaling, lignin biosynthesis, seed germination, the cell wall polysaccharide metabolism, hypersensitive response etc, and in response to stressful situations. Under normal conditions, a balance between ROS production and scavenging take place. However, if an imbalance occurs, an oxidative stress is produced.

 

More information about ROS

The O2.- is the first reduction product of the O2. It can be transformed spontaneously, or by enzyme action, in H2O2:

O2.- + O2.- + 2 H+   =   H2O2 + O2

In this reaction, an O2.-  radical is reduced (to H2O2) and the another O2.-  radical is oxidised (to O2). This process is known as dismutation, which implies that the same molecule suffers reduction and oxidation at the same reaction.

The superoxide can be protonated (to accept an H+) to form the perhydroxyl radical (HO2.-), which can cross membranes and act as a signaling molecule.

The H2O2, although it is a ROS, it is not a free radical (it has paired electrons). It also can cross membranes and it has longer half-life than other ROS. These two characteristics (longer half-life and their high permeability across membranes make it to be accepted as a second messenger) make that H2O2 can be considered as a signaling molecule. However, the true toxicity of O2.-  and H2O2 is its capacity to generate .OH in presence of transition metals (such as the Fe2+ or the Cu2+):

Fe2+ + H2O2  =  .OH + OH + Fe3+   (Fenton’s reaction)

Next, the O2.- reduces the ferrous ion (Fe3+) to produce Fe2+ and allows that the previous reaction can continue:

O2.-  +  Fe3+     =     O2  + Fe2+

The sum of both reactions is known as “Haber-Weiss reaction”:

O2.-  +  H2O2     =    .OH + OH  + O2 

 

The hydroxyl radical (.OH) is the most powerful oxidant known in the biological systems, which joined to its very low lifetime (1 ns), makes itself very toxic. It can react with any biological molecule at the same time it is formed. It causes exceptional mutations in the DNA, attacks to the membranes (membrane lipid peroxidation) and protein oxidation. An excess of .OH, ultimately, leads to the cell death.

In the next chapter, we will talk about the defence mechanisms which plants have designed to cope with ROS.

 

Bibliography

Halliwell B, Gutteridge JMC (2003) Free Radicals in Biology and Medicine. Third Edition, Oxford University Press Inc, New York, ISBN 0 19 850044 0.

 

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Origin of the oxygen in the Earth’s atmosphere: A necessity to live, a threat to the living organisms.

José A. Hernández Cortés (Scientist Researcher, CEBAS-CSIC, Murcia)

Molecular oxygen (O2) appeared in our atmosphere about 2500 billion of years ago. The most accepted theory for the appearance of the O2 in our environment is the theory of the oxygen chemical evolution. Lazcano-Araujo (1989) defends this theory and suggests the atom fusion of hydrogen (H) at high temperature to make new atoms – two atoms of H plus 2 neutrons would produce a helium atom (He). Two atoms of He would generate a beryllium atom (Be). The carbon (one of the pillar element of the life) would be originated as He plus Be. Subsequently, the oxygen would be created by reaction of He and C. However, the O2 would soon appear since most of the atoms were associated to other elements (H2O, SO2, NO2, joined to metals, etc…).

The internal activity of the earth also contributed to release compounds that contain oxygen. For example, the actions of the volcanoes would release CO2, SO2, water vapour, etc…Lately, the action of the ultraviolet radiation would produce the photo-degradation of these molecules enriching the atmosphere in O2.

In order to explain the contribution of the cyanobacteria in the O2 appearance, it is necessary to start from a situation where water was already present, in which primitive microorganism would live, both anaerobic autotrophs and/or heterotrophs and photosynthetic. In this situation, the primitive cyanobacteria would release O2 by the photolysis of the water molecules (H2O) and by employing the protons (H+) and electrons (e-) to generate energy used in the biosynthesis of carbohydrates.

CO2 + H2O+ Energy of the Light = CH2O + O2

 

These three mechanisms would cooperate in the contribution of O2 to the atmosphere, which would change its reductive condition to oxidizing.
Once the atmosphere was enriched in O2, it was originated the ozone layer (O3) by the action of the ultraviolet radiation on the O2 present in the highest layers of the atmosphere. This fact would become a milestone in the evolution of species, since ozone would provide protection against the ultraviolet radiation to the existing microorganisms. This would allow a future colonization of the earth.

In this sense, the appearance of the O2 supposed the extinction of existing organisms and the appearance of new ways of life. The primitive organisms lived in an atmosphere without oxygen (reductive) or with too little available oxygen. In this way, as the content of O2 increased in the atmosphere, many of these primitive microorganisms would die. The current anaerobic or facultative anaerobic microorganisms -can live as in both the presence and absence of oxygen- are presumably descendants of those primitive microorganisms which have adapted to live in free O2 environments or with a very low concentration of O2.

In other microorganisms, the use of the O2 allowed a great energy generation and represented a huge benefit for their evolution. The consequence of the O2 use in the energy production was the generation of the so-called Reactive Oxygen Species (ROS). It must be noted that oxygen, vital for our life, is also a lethal and corrosive gas that moreover, can generate toxic intermediates. Thus, the aerobic microorganisms -those that live in presence of O2- have developed mechanisms, both physical and biochemical, to protect themselves from toxic effects of the O2 and from the ROS.

All matters relating to ROS and to protective mechanisms against these toxic molecules will be dealt with in a new chapter.
References: (1) Lazcano-Araujo A. (1989) El origen de la vida. Evolución química y evolución biológica. 3ra. edición, Editorial Trillas, México DF. (2) Folsome E. (1989) Origen de la vida, Editorial Reverté, Barcelona. (3) Halliwell B., Gutteridge J.M.C. (2003) Free Radicals in Biology and Medicine. Third Edition, Oxford University Press Inc, New York.