Sodium chloride stress induces nitric oxide accumulation in root tips and oil body surface accompanying slower oleosin degradation in sunflower seedlings

Present work highlights the involvement of endogenous nitric oxide (NO) in sodium chloride (NaCl)-induced biochemical regulation of seedling growth in sunflower (Helianthus annuus L., cv. Morden). The growth response is dependent on NaCl concentration to which seedlings are exposed, they being tolerant to 40 mM NaCl and showing a reduction in extension growth at 120 mM NaCl. NaCl sensitivity of sunflower seedlings accompanies a fourfold increase in Na+/K+ ratio in roots (as compared to that in cotyledons) and rapid transport of Na+ to the cotyledons, thereby enhancing Na+/K+ ratio in cotyledons as well. A transient increase in endogenous NO content, primarily contributed by putative NOS activity in roots of 4-day-old seedlings subjected to NaCl stress and the relative reduction in Na+/K+ ratio after 4 days, indicates that NO regulates Na+ accumulation, probably by affecting the associated transporter proteins. Root tips exhibit an early and transient enhanced expression of 4,5-diaminofluorescein diacetate (DAF-2DA) positive NO signal in the presence of 120 mM NaCl. Oil bodies from 2-day-old seedling cotyledons exhibit enhanced localization of NO signal in response to 120 mM NaCl treatment, coinciding with a greater retention of the principal oil body membrane proteins, i.e. oleosins. Abolition of DAF positive fluorescence by the application of specific NO scavenger [2-phenyl-4,4,5,5-tetramethyllimidazoline-1-oxyl-3-oxide (PTIO)] authenticates the presence of endogenous NO. These novel findings provide evidence for a possible protective role of NO during proteolytic degradation of oleosins prior to/accompanying lipolysis.