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Sunflower ( Helianthus annuus L.) is one of the major oilseed crops cultivated world over for its high-quality oil rich in linoleic acid. It also has established applications in pharmaceutical and biotechnological industries, mainly through recombinant production of unique oil body (OB) membrane proteins-oleosins, which are used for producing a wide variety of vaccines, food products, cosmetics and nutraceuticals. The present review provides a critical analysis of the progress made in advancing our knowledge in sunflower biology, ranging from mechanisms of pollen-stigma interaction, seed development, physiology of seed germination and seedling growth under salt stress, and finally understanding the signaling routes associated with various biochemical pathways regulating seedling growth. Role of nitric oxide (NO) triggered post-translational modifications (PTMs), discovered in the recent past, have paved way for future research directions leading to further understanding of sunflower developmental physiology. Novel protocols recently developed to monitor temporal and spatial distributions of various biochemicals involved in above-stated developmental events in sunflower, will go a long way for similar applications in plant biology in future.

Nitric oxide (NO) produced in plant cells has the unique ability to interact with various other biomolecules, thereby facilitating its own as well as their signaling and associated actions at their sites of biosynthesis and at other sites via transcellular long distance transport of the molecular complexes. Melatonin (Mel) is one such biomolecule produced in plant cells which has fascinated plant biologists with regard to its molecular crosstalk with other molecules to serve its roles as a growth regulator. Present work reports the synthesis of N-nitrosomelatonin (NOMela) and its preferential uptake by Arabidopsis seedlings roots and long distance transport to the leaves through vascular strands. Equimolar (250 μM) concentrations of NOMela and S-nitrosoglutathione (GSNO) in aqueous solutions bring about 52.8% more release of NO from NOMela than from GSNO. Following confocal laser scanning microscopic (CLSM) imaging, Pearson's correlation coefficient analysis of the Scatter gram of endogenously taken up NOMela demonstrates significant NO signal in roots emanating from mitochondria. NOMela (250 μM) taken up by Arabidopsis seedling roots also proved more efficient as a NO transporter from primary root to leaves than 250 μM of GSNO. These novel observations on NOMela thus hold promise to decipher its crucial role as a NO carrier and reservoir in plant cells, and also as a facilitator of melatonin action in plant development., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Present work focuses on tissue and concentration-dependent effect of nitric oxide (NO) on the modulation of cytosolic peroxidase (POD; EC 1.11.1.7) activity in 2-day old etiolated sunflower (Helianthus annuus L.) seedlings. Exogenously supplied NO (in the form of sodium nitroprusside [SNP] or diethylenetriamine NONOate [DETA]; 125 to 500 μM) results in noteworthy enhancement in seedling growth in a concentration dependent manner irrespective of salt-stress and differentially affects POD activity in 2-day old seedling cotyledons. Elevated NO availability leads to an increase in the specific activity of POD in a concentration-dependent manner within 48 hrs as a rapid signaling response. Purification of POD protein using immunoprecipitation technique has shown that cotyledons derived from salt stressed seedlings exhibit a higher extent of tyrosine nitration of POD as compared to the control seedlings. Out of the four tyrosine residues found in the amino acid sequence of POD, the one at position 100 has been predicted to undergo nitration. Thus, a probable NO-POD crosstalk is evident in sunflower seedling cotyledons accompanying salt stress.

Nitric oxide (NO) and various reactive nitrogen species produced in cells in normal growth conditions, and their enhanced production under stress conditions are responsible for a variety of biochemical aberrations. The present findings demonstrate that sunflower seedling roots exhibit high sensitivity to salt stress in terms of nitrite accumulation. A significant reduction in S-nitrosoglutathione reductase (GSNOR) activity is evident in response to salt stress. Restoration of GSNOR activity with dithioerythritol shows that the enzyme is reversibly inhibited under conditions of 120 mM NaCl. Salt stress-mediated S-nitrosylation of cytosolic proteins was analyzed in roots and cotyledons using biotin-switch assay. LC-MS/MS analysis revealed opposite patterns of S-nitrosylation in seedling cotyledons and roots. Salt stress enhances S-nitrosylation of proteins in cotyledons, whereas roots exhibit denitrosylation of proteins. Highest number of proteins having undergone S-nitrosylation belonged to the category of carbohydrate metabolism followed by other metabolic proteins. Of the total 61 proteins observed to be regulated by S-nitrosylation, 17 are unique to cotyledons, 4 are unique to roots whereas 40 are common to both. Eighteen S-nitrosylated proteins are being reported for the first time in plant systems, including pectinesterase, phospholipase d-alpha and calmodulin. Further physiological analysis of glyceraldehyde-3-phosphate dehydrogenase and monodehydroascorbate reductase showed that salt stress leads to a reversible inhibition of both these enzymes in cotyledons. However, seedling roots exhibit enhanced enzyme activity under salinity stress. These observations implicate the role of S-nitrosylation and denitrosylation in NO signaling thereby regulating various enzyme activities under salinity stress in sunflower seedlings., (© 2017 Scandinavian Plant Physiology Society.)

Sunflower seedlings subjected to 120 mM NaCl stress exhibit high total peroxidase activity, differential expression of its isoforms and accumulation of lipid hydroperoxides. This coincides with high specific activity of phospholipid hydroperoxide glutathione peroxidase (PHGPX) in the 10,000g supernatant from the homogenates of 2-6 d old seedling cotyledons. An upregulation of PHGPX activity by NaCl is evident from Western blot analysis. Confocal laser scanning microscopic (CLSM) analysis of sections of cotyledons incubated with anti-GPX4 (PHGPX) antibody highlights an enhanced cytosolic accumulation of PHGPX, particularly around the secretory canals. Present work, thus, highlights sensing of NaCl stress in sunflower seedlings in relation with lipid hydroperoxide accumulation and its scavenging through an upregulation of PHGPX activity in the cotyledons.