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What is tropospheric ozone?


Tropospheric or ground-level ozone is a secondary air pollutant that results from the photochemical reactions of Nitrogen oxides (NOx), Methane, and Volatile Organic Compounds (VOCs) which are largely present in the environment due to anthropogenic emissions. Approximately 10% of the tropospheric ozone comes from the episodic stratospheric influx. In the upper troposphere, ozone is a major greenhouse gas contributing to global warming. In the lower troposphere, ozone act as a strong oxidant and is harmful to human health, the ecosystem, and agricultural crops. According to the World Health Organization (WHO) air quality guidelines for ozone, the 8-hour mean ozone concentration exceeding 100 µg/m3 can lead to significant health effects. Tropospheric ozone is known to be highly phytotoxic causing a significant reduction in agricultural productivity worldwide.

How tropospheric ozone affect plants?

The phytotoxic effects of ozone depend upon the exposure pattern and the amount of ozone diffusing into the leaves. Leaves are the primary route of ozone uptake controlled by stomatal conductance and boundary layer resistance. Boundary layer resistance is the function of leaf morphology, orientation, and wind speed. Ozone after passing through leaf intercellular spaces then dissolves into the apoplast and reacts with cellular components to produce Reactive Oxygen Species (ROS) such as hydroxyl, peroxyl, and superoxide radicals. The ROS quenching capacity of apoplast is the first line of defense against ozone damage. Following exposure to higher levels of ozone amplify ROS production which causes alterations in the physical and chemical properties of plasma membranes and trigger a wide array of signal cascades. The signal activates ethylene, salicylic acid, and jasmonic acid signaling pathways and changes the global gene expression for defense against the oxidative stress caused by ozone. Thus chronic ozone exposure induces visible foliar injury symptoms, decreases photosynthesis and plant biomass, and causes early senescence.

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Fig 1. Ozone dependent signaling pathways

What is the visible foliar injury?

Ozone exposure results in visible foliar injury symptoms on the leaves in increasing order of severity as stippling, chlorosis, flecks, and necrosis. It is characterized by the occurrence of interveinal spots of red, brown, purple, or black pigmentation on the upper leaf surface. It can be either uniformly distributed over the leaf surface or concentrated in certain areas. The color, density, distribution of the foliar injury depends on plant species, duration and nature of ozone exposure. Foliar injury from acute ozone exposure is evident between hours/days of exposure. Foliar injury from chronic ozone exposure becomes evident over weeks or months. Assessment of visible foliar injury symptoms can be a strong diagnostic tool for plant ozone sensitivity. The criteria for identification of foliar injury symptoms can be summarised as:

                               Fig. 2: Flowchart for identification of foliar ozone injury symptoms on leaves (Source: Novak et al., 2003)

What is OSRGD ver. 1.1?

Ozone Stress Responsive Gene Database (OSRGD ver. 1.1) is a specialized database which integrates ozone stress-specific gene dataset of different plant species. This comprehensive biocuration will offer researchers a better biological insight into plant response to ozone stress.

What is the purpose of using this database?

Researchers working on ozone stress can easily trace a gene expressed or inactivated in a plant species in response to ozone stress exposure by searching a keyword, gene name, gene function. In the public domain, no other existing plant stress databases cover the genes regulated specifically in response to ozone stress. The results can be further interpreted and candidate genes can be chosen for the experiments.

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