Propyl gallate promotes salt stress tolerance in green microalga Dunaliella salina by reducing free radical oxidants and enhancing β-carotene production

Abstract

The objective of present work was to study the role of n-propyl gallate (PG), a synthetic antioxidant, in antioxidative responses and salinity tolerance in Dunaliella salina. Algal cultures containing three level of salinity (1, 2 and 3 M NaCl) were treated with two level of 0 and 1 mM of PG for 48 h. 3 M NaCl-grown cells exhibited a minor increase in cell density in comparison to other salt treatments. PG treatment had no effect on cell growth under different salinities. However, the chlorophyll and b-carotene contents significantly increased in PG-incubated cells. Protein concentration clearly reduced in PG-incubated cells grown at 1 and 2 M NaCl compared with those of PG-free, whereas, no significant change influenced by PG was obtained at 3 M NaCl. Ascorbate peroxidase assay showed a minor increase at 3 M NaCl compared with 1 and 2 M NaCl-grown cells. Catalase activity decreased concurrently with salt concentrations, while superoxide dismutase activity pronouncedly increased in response to 2 M NaCl accompanied by a statistically equal increase at 1 and 3 M NaCl. However, the activity of all three enzymes significantly decreased in all PG-incubated algae compared with PG-free ones. Along with increase in total and reduced ascorbate in response to salinity and PG treatments, oxidized ascorbate content was significantly decreased.Hydrogen peroxide and malonyldialdehyde accumulation increased concomitantly with salinity. However, a large decrease in these metabolites occurred in response to PG added into the algal cultures. The results suggest that antioxidant enzymes are not pivotal in improving salinity tolerance and only have short-term adjustment effects to protect against salt stress in D. salina. These data also provide the first direct evidence that PG pretreatment ameliorates salinity stress by electron donation to free radical oxidants, and by inducing b-carotene, chlorophyll, and ascorbic acid biosynthesis, rather than activation of antioxidant enzymes.

Metabolic responses and β-carotene production by the unicellular green alga Dunaliella salina exposed to leaf extracts

“…Thus, it is probably that the leaf extracts-induced β-carotene concentration increase could be brought about maintenance of Chl, in addition to its biosynthesis stimulation under leaf extracts-induced stress. It is in agreement with our previous results on propyl gallateinduced β-carotene under salt stress (Einali & Shariati 2012;Einali & Valizadeh 2015). Based on the results, the highest concentrations of both Chl and β-carotene were observed in cells exposed to 1 % eucalyptus and 2 % elderberry (Figs.…”

“…In addition, increase of Chl b biosynthesis can regulate the expression of some special thylakoid membrane proteins, which raise the antenna complex size and likewise, the electron transport rates (Tanaka et al 2001;Biswal et al 2012). Such massive increase in Chl b content has also been observed in some studies including our pervious report on D. salina treated with propyl gallate, a synthetic antioxidant (Einali & Valizadeh 2015), and another study on D. salina grown over a salinity gradient (Mishra et al 2008). Total Chl concentration was pronouncedly increased as observed for Chl a content (Fig.…”

“…The suspension was sonicated twice for 30 s with 2 min interval. To remove pigments, 10 mg charcoal was added to the homogenate and then centrifuged (12,000×g for 10 min at 4 °C) (Einali & Valizadeh 2015). The resultant supernatant was used to estimate total soluble proteins using bovine serum albumin (BSA) as the standard (Bradford 1976).…”

“…It is known that Dunaliella accumulate large amount of β-carotene while cell division is retard. Thus, adverse environmental conditions, such as high salinity, low temperature, high irradiance, or nutrient deficiency increase the accumulation of β-carotene in this alga (BenAmotz et al 1982;Ben-Amotz & Avron 1983;Ben-Amotz 1996;Einali & Shariati 2012;Einali & Valizadeh 2015). Due to that, Dunaliella has been widely considered as an interesting model for studying β-carotene biosynthesis.…”

“…Dunaliella salina is the best known commercial source of natural β-carotene among all organisms in the world (Borowitzka 1995). This alga was shown to accumulate β-carotene to more than 10 % of its dry weight when grown under growth-limiting conditions such as high irradiance, high salinity, high temperature, and limiting nutrients (Ben-Amotz et al 1982;Ben-Amotz & Avron 1983;Cowan et al 1992;Salguero et al 2003;Raja et al 2007;Einali & Valizadeh 2015). β-carotene accumulates as droplets in intrachloroplastic lipoidal globules and plays a role in protecting chlorophyll against photo-damage (Ben-Amotz & Avron 1983;Ben-Amotz et al 1988).…”