The Relationship Between Auxin and Nitric Oxide

Auxin was the first hormone that was discovered in plants. It promotes the growth of plants by activating cell division directly or indirectly along with other phytohormones like cytokinin and gibberellin. Apart from Auxin, Nitric oxide also plays a pivotal function in the growth and development of the plant. Auxin is synthesized by the amino acid tryptophan which contains an indole ring, hence auxin may also be known as Indole-3-Acetic acid (IAA). On the other hand, nitric oxide is a gaseous molecule that is directly involved in the signaling pathway alone or in association with Auxin. 

Auxin is stored in the cytosol as well as in the chloroplast. The transport of auxin in plants occurs from end to end which means from apical end to basal end. This type of transport is known as polar transport. The transport of auxin in plants requires energy and gravity independence. Nitric oxide is also known to mediate the process of growth and development in plants. Apart from this, it is also involved in the formation of root nodules in leguminous plants, inhibiting the process of seed germination, senescence of flowers, etc. 

IAA treatment

The treatment or artificial addition of Auxin is given to those plants that are found to be shorter as compared to their normal length or sometimes they suffer from various diseases and disorders that affect the production of auxin naturally. When the treatment of IAA is done, the efflux of Auxin is mediated by PIN proteins, while the influx of the auxin is mediated by H⁺/IAA^– cotransporters. 

This transport of auxin is mediated by the pH of cytosol which is higher than the pH of the outer environment. Some toxins or inhibitors which block the PIN proteins are known as auxin transport inhibitors. These inhibitors inhibit the auxin efflux. PIN proteins are only activated when the concentration of auxin falls. During such conditions, these proteins are re-vesicles and perform their key role of Auxin transport. 

Nitric oxide (NO) synthesis

Nitric oxide in plants is produced under stress conditions such as pathogen infection in the root, drought, etc. It can be synthesized in plants by various mechanisms, one of the major sources is the action of the enzyme nitrate reductase. NO may be classified under reactive oxygen species. As a result of this, it may interact with superoxide anions to synthesize peroxynitrite or with H₂S to produce nitrosothiols. 

When NO is synthesized in the plant cells, it may react with other downstream proteins or enzymes which consist of Iron (Fe), leghemoglobin, guanylyl cyclase (GC), etc. These proteins further lead to other signaling and try to eliminate this NO from the plant cells. 

GC catalyzed synthesis

It is one of the processes which carries the protein Guanylyl cyclase (GC). The downstream signaling of this protein may be affected by Nitric oxide which is synthesized during any stress response. In response to NO, guanylyl cyclase carries another signaling mechanism to eliminate the level of reactive oxygen species from the plant cell. 

During this process, a feedback loop or mechanism is seen which indicates the inhibition of the activator component. 

Signaling transduction of Auxin in response to NO

In absence of IAA, Auxin response factor (ARF) forms a heterodimer with AUX/IAA which is an inhibitor of auxin function. IAA and NO are the plant growth regulators that harmonise numerous plant physiological reactions. This effect of signaling transduction was observed in the model plant Arabidopsis thaliana where F-box proteins Transport Inhibitor Response 1 (TIR1) and AFB (auxin signalling F-box) are the auxin receptors that mediate the degradation of inhibitor of Auxin function (AUX/IAA). 

In presence of Auxin ARF-AUX/IAA heterodimer undergoes ubiquitinylation and ARF forms homodimer which binds to the DNA which has the Auxin Response element (ARE) synthesizes H+ ion ATPases (expansins) as well as synthesizes AUX/IAA for the feedback inhibition. The presence of NO increases the auxin-dependent gene expression by enhancing the interaction between TIR1-AUX/IAA. While the depletion of NO in the roots of various plants blocks the degradation of AUX/IAA. 

The major role of NO in auxin signaling is that it promotes the ubiquitinylation of inhibitors of Auxin function (AUX/IAA) and thereby increases the auxin-dependent gene expression. 

Conclusion

Auxin is a phytohormone that promotes the growth and development of plants by activating the process of cell division. Polar transport of auxin is observed in the plants. The transport of Auxin is mediated by PIN proteins. PIN proteins are only activated when the concentration of auxin falls. NO is synthesized in response to stress conditions. After synthesis, NO may interact with other downstream protein molecules like Iron (Fe), leghemoglobin, guanylyl cyclase (GC), etc. In response to NO, guanylyl cyclase carries another signaling mechanism to eliminate the level of reactive oxygen species from the plant cell. The presence of NO increases the auxin-dependent gene expression by enhancing the interaction between TIR1-AUX/IAA. S-nitrosylation also governs NO-mediated auxin signalling by acting at Auxin receptor TIR1  mediates the degradation of inhibitors of Auxin function (AUX/IAA).