New insights into nitric oxide metabolism and regulatory functions

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Nitric oxide (NO) has been intensively studied to elucidate the role of this enigmatic signaling molecule in plant development, metabolism and disease responses. Many studies using pharmacological and biochemical tools have demonstrated that NO functions in hormone responses, programmed cell death, defense gene induction and signal transduction pathways. NO originates from two sources in plants: nitrite and arginine. Recent studies using mutants and transgenic plants have confirmed these key findings and have gone further to identify (i) a new mechanism to modulate NO bioactivity involving hemoglobin, (ii) a gene involved in arginine-dependent NO synthesis, and (iii) a novel function for NO signaling in flowering. These findings continue to elucidate the expanding role of NO in plant biology.

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Roles for nitric oxide in plants

Nitric oxide (NO) serves as a signal in plants and animals. It functions in disease resistance, abiotic stress, cell death, respiration, senescence, root development, germination and hormone responses. It is an unusual signal in that it is a reactive, lipophilic and volatile free radical that can be cytotoxic. In plants, NO signaling involves cGMP, cADP ribose, Ca2+, salicylic acid and protein kinases. There is also extensive overlap and cross talk with H2O2 signaling. Excellent reviews have

Modulation of nitric oxide levels by hemoglobin

Animal studies have shown that NO and NO derivatives such as S-nitrosoglutathione regulate enzyme activity by modifying amino acid side chains of key metabolic or regulatory proteins 24, 25. For example, S-nitrosylation of a specific cysteine activates the oncoprotein GTPase p21Ras and inhibits the transcription factor NF-κB [25]. Such reactions can occur without enzyme catalysis and thus are controlled by the concentration and redox state of NO and the availability and reactivity of target

Synthesis of nitric oxide

One of the mysteries about NO has revolved around its synthesis. Eight possible enzymatic sources for NO have been proposed [5] along with non-enzymatic mechanisms [49]. It has been known for many years that nitrate reductase (NR) can produce NO and N2O from nitrite resulting in NO emission 35, 50, 51, 52. High NO emission rates correlate with high nitrite levels and NR activation, which occur during anoxia 35, 53, 54, 55. Under aerobic conditions, NO emission is low (usually less than 0.5 nmol g

Novel regulatory function for nitric oxide

A new role for NO has been identified: the control of flower timing. Internal and external signals such as photoperiod, vernalization, gibberellins and the circadian clock induce reproductive development 74, 75. While investigating the effects of NO on vegetative growth, it was found that applying the NO donor sodium nitroprusside delayed the onset of flowering [76]. A genetic screen was then performed to find NO hypersensitive mutants. Among the mutants identified were six that had mutations

Conclusion

The past two years have marked several major developments in our efforts to understand NO regulation and metabolism. The involvement of non-symbiotic hemoglobins in modulating NO levels provides a new mechanism to regulate NO bioactivity in vivo. The discovery of an enzyme involved in arginine-dependent NO synthesis establishes NOS as a mechanism for NO production in signaling and disease responses in plants. These and other findings lead to further questions about NO metabolism and signaling.

Acknowledgements

We acknowledge funding from the National Institutes of Health (grant #GM40672).

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