Editing Reactive oxygen species (section)

===Exogenous sources===
The formation of ROS can be stimulated by a variety of agents such as pollutants, [[heavy metals]],<ref name="Nachiappan" /> [[tobacco]], smoke, drugs, [[xenobiotics]], [[microplastics]], or radiation. In plants, in addition to the action of dry [[abiotic factor]]s, high temperature, interaction with other living beings can influence the production of ROS.{{citation needed|date=September 2024}}

Ionizing radiation can generate damaging intermediates through the interaction with water, a process termed [[radiolysis]]. Since water comprises 55–60% of the human body, the probability of radiolysis is quite high under the presence of ionizing radiation. In the process, water loses an electron and becomes highly reactive. Then through a three-step chain reaction, water is sequentially converted to [[hydroxyl radical]] (<sup>•</sup>OH), [[hydrogen peroxide]] (H<sub>2</sub>O<sub>2</sub>), [[superoxide radical]] (<sup>•</sup>{{chem|O|2|−}}), and ultimately [[oxygen]] (O<sub>2</sub>).{{citation needed|date=September 2024}}

The [[hydroxyl radical]] is extremely reactive and immediately removes electrons from any molecule in its path, turning that molecule into a free radical and thus propagating a chain reaction. However, [[hydrogen peroxide]] is actually more damaging to DNA than the hydroxyl radical, since the lower reactivity of hydrogen peroxide provides enough time for the molecule to travel into the nucleus of the cell, subsequently reacting with macromolecules such as DNA.{{citation needed|date=February 2019}}

In plants, the production of ROS occurs during events of abiotic stress that lead to a reduction or interruption of metabolic activity. For example, the increase in temperature, drought are factors that limit the availability of CO<sub>2</sub> due to [[stomata]]l closure, increasing the production of ROS, such as O<sub>2</sub>·- and <sup>1</sup>O<sub>2</sub> in chloroplasts.<ref>{{Cite journal |vauthors=Baniulis D, Hasan SS, Stofleth JT, Cramer WA |date=December 2013 |title=Mechanism of enhanced superoxide production in the cytochrome b(6)f complex of oxygenic photosynthesis |journal=Biochemistry |volume=52 |issue=50 |pages=8975–8983 |doi=10.1021/bi4013534 |pmc=4037229 |pmid=24298890}}</ref><ref name="auto1">{{Cite journal |vauthors=Kleine T, Leister D |date=August 2016 |title=Retrograde signaling: Organelles go networking |journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics |volume=1857 |issue=8 |pages=1313–1325 |doi=10.1016/j.bbabio.2016.03.017 |pmid=26997501 |doi-access=free}}</ref> The production of <sup>1</sup>O<sub>2</sub> in chloroplasts can cause reprogramming of the expression of nucleus genes leading to [[chlorosis]] and [[apoptosis|programmed cell death]].<ref name="auto1" />
In cases of biotic stress, the generation of ROS occurs quickly and weakly initially and then becomes more solid and lasting.<ref>{{Cite journal |vauthors=Grant JJ, Loake GJ |date=September 2000 |title=Role of reactive oxygen intermediates and cognate redox signaling in disease resistance |journal=Plant Physiology |volume=124 |issue=1 |pages=21–29 |doi=10.1104/pp.124.1.21 |pmc=1539275 |pmid=10982418 |doi-access=free}}</ref> The first phase of ROS accumulation is associated with plant infection and is probably independent of the synthesis of new ROS-generating [[enzyme]]s. However, the second phase of ROS accumulation is associated only with infection by non-virulent pathogens and is an induced response dependent on increased [[mRNA]] transcription encoding enzymes.