NO to drought-multifunctional role of nitric oxide in plant drought: Do we have all the answers?

• Provides an update on the role of nitric oxide under water deficit.
• the recent advances and unsolved issues in NO research in plant drought stress condition.
• Reviews the emerging molecular studies in NO-plant stress research.

Nitric oxide (NO) is a versatile gaseous signaling molecule with increasing significance in plant research due to its association with various stress responses. Although, improved drought tolerance by NO is associated greatly with its ability to reduce stomatal opening and oxidative stress, it can immensely influence other physiological processes such as photosynthesis, proline accumulation and seed germination under water deficit. NO as a free radical can directly alter proteins, enzyme activities, gene transcription, and post-translational modifications that benefit functional recovery from drought. The present drought-mitigating strategies have focused on exogenous application of NO donors for exploring the associated physiological and molecular events, transgenic and mutant studies, but are inadequate. Considering the biphasic effects of NO, a cautious deployment is necessary along with a systematic approach for deciphering positively regulated responses to avoid any cytotoxic effects. Identification of NO target molecules and in-depth analysis of its effects under realistic field drought conditions should be an upmost priority. This detailed synthesis on the role of NO offers new insights on its functions, signaling, regulation, interactions and co-existence with different drought-related events providing future directions for exploiting this molecule towards improving drought tolerance in crop plants.

Functional contributions of NO in regulating various drought stress responses in plants. NO either stimulates (normal end arrow) or inhibits (blunt end arrow) plant processes in coordination with various plant hormones under drought conditions. The dotted line denotes the pathways not studied clearly. The double-headed arrow indicates the mutual regulation of molecules. [cGMP: Cyclic guanosine monophosphate; CK: cytokinins; JA: jasmonic acid; SA: salicylic acid; IAA: indole acetic acid; ET: ethylene; H2O2: hydrogen peroxide; ONOO−: peroxynitrite; GSNO: S-nitrosoglutathione; NOS: nitric oxide synthase; NR: nitrate reductase; PAs: polyamines; ABA: abscisic acid; GA: gibberellic acid; NO: nitric oxide; MDA: malonaldehyde; CAT: catalase; APX: ascorbate peroxidase; SOD: superoxide dismutase; GR: glutathione reductase; Tyr: tyrosine].Figure optionsDownload as PowerPoint slide