Pure biochemicals and nanomaterials as next generation biostimulants for sustainable agriculture under abiotic stress - recent advances and future scope.

Sustainable agriculture faces major challenges under abiotic stress conditions owing to extensive application of chemical fertilizers which pollute water, soil and atmosphere. Biostimulants (BSs), comprising of naturally derived complex mixtures of uncharacterized biomolecules, pure biochemicals and nanomaterials, enhance nutrient use efficiency (NUE) and trigger crop's natural defense mechanisms. While it is difficult to specify the metabolic effects of uncharacterized natural mixtures (seaweed extract, protein hydrolyzates, etc.), exogenous application of pure biochemicals and nanomaterials offers an edge as BSs since their physiological roles and mechanisms of action are decipherable. Foliar application or seed treatment of some amino acids, polyamines and biopolymers (chitosan, lipochitin oligosaccharides and thuricin 17) enable plants to overcome drought and salinity stress via activation of mechanisms for reactive oxygen species (ROS) scavenging, osmolyte regulation and chlorophyll accumulation. Interaction of nitric oxide (NO) with some vitamins and melatonin exhibits potential significance as BSs for mitigating stress by ROS scavenging and maintenance of intracellular ionic balance and membrane integrity. Near future is likely to see wide applications of nanoparticles (NPs) and nanomaterials (NMs) as BSs in view of their biphasic mode of action (bio-physical activation of membrane receptors followed by gradual release of BS into the plant cells).

Signaling mechanisms and biochemical pathways regulating pollen-stigma interaction, seed development and seedling growth in sunflower under salt stress.

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.

Salt-tolerant and -sensitive seedlings exhibit noteworthy differences in lipolytic events in response to salt stress.

Present findings hypothesize that salt-tolerant and -sensitive oilseed plants are expected to exhibit deviant patterns of growth through lipolytic events in seedling cotyledons. It reports the growth response and different lipolytic mechanisms operating during oil body (OB) mobilization in the seedling cotyledons of salt-tolerant (DRSH 1) and salt-sensitive (PSH 1962) varieties of sunflower (Helianthus annuus L.). Salt tolerance or sensitivity to 120 mM NaCl correlates with high proteolytic degradation of OB membrane proteins, particularly oleosins, whereas salt-sensitive seedling cotyledons exhibit negligible proteolytic activity, thereby retaining OB membrane integrity for a longer time. High lipoxygenase (LOX) activity and its further upregulation by salt stress are the unique features of salt-sensitive sunflower seedlings. Salt-tolerant seedling cotyledons exhibit noteworthy modulation of phospholipase-D (PLD) activity by salt stress. Salt-sensitive seedling cotyledons exhibit higher lipase activity than salt-sensitive ones and enzyme activity is downregulated by salt stress. Salt-sensitive variety exhibits higher lipid accumulation and faster lipid mobilization with seedling development than salt-tolerant variety. Accumulation of oleic and linoleic acid in the seedling cotyledons of salt-tolerant and sensitive varieties exhibits differential sensitivity to salt stress. Novel detection of hexanoic acid (6:0) is a noteworthy feature as a response to salt stress in salt-sensitive variety. These findings, thus, provide new information on long-distance salt stress sensing mechanisms at seedling stage of plant development.

Biochemical mechanisms regulating salt tolerance in sunflower.

Sunflower plants are semi-tolerant to salt stress. Calcium modulates the expression of oubain-sensitive ATPases, responsible for sodium fluxes in cells. Salt stress delays degradation of oil body (OB) membrane proteins. Serotonin and melatonin contents are elevated in response to salt stress. Melatonin can detoxify the seedlings of elevated reactive oxygen species (ROS) levels. Enhanced nitric oxide (NO) expression correlates with NaCl-induced modulation of seedling growth. Salt stress enhances S-nitrosylation of cytosolic proteins in seedling cotyledons, while in roots, denitrosylation of proteins is observed. Lipid peroxide content and glutathione peroxidase (GPX4) activity are enhanced in response to salt stress. Salt stress downregulates the activity of superoxide dismutase (SOD) and upregulates the activity of GPX4 and glutathione reductase (GR). Heme oxygenase-1 (HO-1) abundance in cells surrounding the secretory canal in seedling cotyledons is enhanced in response to salt stress. NO negatively regulates the total glutathione homeostasis and regulates polyamine and glycine betaine homeostasis in response to salt stress. An intricate biochemical crosstalk is thus observed to control salt tolerance mechanisms in sunflower.