Profile
Settings
Refer your friends
Sign out
Nitric oxide is one of the signalling molecules involved in the growth and development of plants. A gaseous molecule dominantly participates in the signalling process either alone or in association with other phytohormones like Auxin. It is one of the proven bioactive molecules found in all living organisms, especially plants. It is also involved in the formation of root nodules in leguminous plants, promoting the process of seed germination, senescence of flowers, de-etiolation, and inhibits hypocotyl elongation.
The synthesis of Nitric oxide in plants occurs under stress conditions such as pathogen infection in the root, drought, Fe-deficient conditions, etc. It can be synthesized in plants by various mechanisms, one of the major sources is the action of the enzyme nitrate reductase. It involves the process of the reduction of nitrite to NO through different enzymatic or non-enzymatic mechanisms.
It may also be classified under reactive oxygen species. As a result, it may interact with superoxide anions to synthesize peroxynitrite or with H2S to produce nitrosothiols. This peroxynitrite and nitrosothiols are responsible for the initiation of NO donors.
Dual behaviour of NO donors
In-plant physiology, NO governs the numerous functions through which plants germinate, grow, development of root nodules, as well as senescence of flowers. During this whole mechanism of NO, it behaves differently, or we can say that the dual behaviour of NO is observed.
First, Nitric oxide behaves as a signalling molecule that promotes growth and development in plants. It associates with auxin signalling to promote the ubiquitinylation of inhibitors of Auxin function (AUX/IAA) and thereby increases the auxin-dependent gene expression. Overall, it enhances the rate of Auxin signalling in plants.
Second, Nitric oxide is classified as the precursor of reactive oxygen species in plants. These compounds then react with several superoxide anions to form peroxynitrite and nitrosothiols S-nitrosylation.
Leaf expansion in plants
Leaf expansion is an example of growth in plants. It is the process through which the cells of plant leaves occupy space through the influx of energy, carbon, and water. Due to the presence of water molecules, plant cells experience a turgor pressure which initiates the leaf to expand.
In plants, Nitric oxide is responsible for the early closure of stomata that prevents water loss in warm and humid conditions. This step gives plants a sufficient amount of water to perform their metabolic activity. But when the conditions are cold with low temperature, stomata open, movement of chloroplast should change, the rate of photosynthesis increases, and the leaf expands. The process of leaf expansion is indirectly related to the presence of nitric oxide in plants.
Functions of Nitric oxide
As we discussed above, Nitric oxide is a signalling molecule that actively takes part in the growth and development of plants. It is also responsible for the process of seed germination along with phytohormone gibberellins. It stimulates the gibberellins signalling to promote seed germination.
Apart from seed germination and the growth of plants, NO is also involved in the de-etiolation mechanism. De-etiolation means the stimulation or presence of pigments that are required for photosynthesis. It promotes the process of de-etiolation, which increases the photosynthesis rate.
It helps the plants to resist stress conditions. It involves the formation of root nodules in leguminous plants through the action of nitrogen-fixing bacteria called rhizobia entering the cells of a host plant.
Pea seedling
A seedling that grows in red light displays a de-etiolated growth pattern. If Nitric oxide promotes the mechanism of de-etiolation, it inhibits hypocotyl. Because de-etiolation involves green expanded cotyledons, short hypocotyl, and no apical hook. Hence, Nitric oxide inhibits hypocotyl elongation.
All the given three functions of nitric oxide depend on light-inducible reactions in plants. It depicts that nitric oxide relies on light to perform its functions and stimulate the pattern of growth and morphogenesis in plants.
Conclusion
Nitric oxide is a gaseous molecule that belongs to reactive oxygen species. It is involved in many physiological processes in plants like growth and development, de-etiolation, inhibition of hypocotyl, root nodule formation, etc. It is a molecule that possesses dual behaviour. One as a signalling molecule and the other as a reactive oxygen species. It also [promotes the process of leaf expansion in plants. It associates with auxin signalling to promote the ubiquitinylation of inhibitors of Auxin function (AUX/IAA) and thereby increases the auxin-dependent gene expression. It interacts with phytohormones, calcium channels, reactive oxygen species, and another post-translational modification that governs each step of the plant life cycle.
