Keywords Extreme-halophyte ∙ signal compound ∙ salt tolerance ∙ succulent ∙ antioxidant system ∙ osmoprotectant
Introduction
Salt stress, one of the abiotic stresses has become a common problem for crops all over the world, especially in recent years. Salinity can cause osmotic and ionic imbalance, functional and structural protein damage and membrane injury in plants. However, plants can promote some tolerance mechanism linked with signal transduction, reactive oxygen species (ROS) production, induction of antioxidants, synthesizing of osmoprotectants, controlling ion absorption, and expression of salt responsive genes and transcription factor to cope with salinity (Gupta and Huang 2014; Mishra and Tanna 2017). In addition, plants adapted to live in salinized areas may exhibit physiological and morphological changes such as leaf shedding and succulent structures (Mishra and Tanna 2017).
Signal compounds, a member of signaling networks, participate in the growth and developmental responses of plants to environmental factors. One of them, ascorbic acid (ASC) reported in recent years, may trigger cytosolic calcium elevation, contribute to the propagation of ROS/Ca2+ signals in plant tissue and thus ascorbate may be linked to calcium signaling. Ascorbate, a major antioxidant in plants, hypothetically acts as an extracellular signaling and regulatory molecule having high importance for a number of physiological functions (Makavitskaya et al. 2018). The role of glutathione (GSH) as a key-redox signaling component was also established in activation of various defense mechanisms against unfavorable stress factors. In this signaling pathway, GSH can interact with other established signaling molecules including ROS (Ghanta and Chattopadhyay 2011). As the same way, some osmoprotectants such as proline and soluble sugar play signal role in abiotic stress tolerance of maize seedlings (Hayat et al. 2012; Altuntaş et al. 2019). On the other hand, osmoprotectants accumulated in plants during water deficiency and salinity help plants to avoid ion toxicity and maintain water uptake under the stress conditions without preventing the normal metabolic processes. Like organic solutes (soluble sugars, proline, betaine, glycerol, and other low molecular weight metabolites), inorganic ions (Na+, K+, Ca2+, Mg2+ and Cl-) also have a osmoprotectant property and play a key role in osmotic adjustment to maintain water uptake (Zhou and Yu 2009; Chen and Jiang 2010). As seen, signaling roles of some organic osmoprotectants and calcium, an inorganic osmoprotectant were well known in plants. Although, putative second messenger function of magnesium has been currently reported in an animal cell (Stangherlin and O’Neill 2018), no the signaling roles of magnesium and other inorganic ions in responses of plants to abiotic stresses have been clarified yet.
The osmotic regulation by accumulation of inorganic anions and cations in the vacuole is the most important feature of succulent halophytes (Shabala and Mackay 2011). It was reported that detrimental effects of increased Na+ and Cl- contents were prevented by enhanced uptake of ions such as Ca2+, Mg2+ and K+ in extreme halophyte two grasses (Mangalassery et al. 2017). Conversely, Ahmad et al. (2013) noted that Ca2+, Mg2+ and K+ contents declined while Na+ concentration was increasing in shoot of Salicornia persica. They suggested that succulent halophytic Salicornia persica may use sodium ion for acceleration of water uptake under water shortage, to regulate sodium concentration in cellular spaces.  On the other hand, in plant cells, ensuring potassium hemostasis in a salinity environment is a key factor in detecting capability of salt tolerance. Moreover, the plants maintaining cytosolic K+/Na+ ratio can have salt tolerance capacity (Liang et al. 2018). In the light of these records, the contribution of variations in the mineral ion levels to the salinity tolerance of halophytes exposed to high salt concentrations is not well understood.
High salt concentrations in the living environment can cause hyperosmotic stress as well as ion imbalance in plants. A result of these primary effects, oxidative stress can occur as a secondary effect (Gupta and Huang 2014). However, the induction of antioxidant defense system consisting of enzymatic and non-enzymatic components maintains balance of reactive oxygen species, which include hydrogen peroxide (H2O2), superoxide radical (O2-), hydroxyl free radicals (•OH) and singlet oxygen (1O2) within the cell. Enzymatic antioxidants such as superoxide dismutase (SOD) (EC 1.15.1.1), catalase (CAT) (EC 1.11.1.6), guaiacol peroxidase (GPOD) (EC. 1.11.1.7), ascorbate peroxidase (APX) (EC 1.11.1.11), glutathione reductase (GR) (EC 1.6.4.2) and non-enzymatic antioxidants such as ascorbate, glutathione, carotenoids and polyphenols scavenge different types of ROS (Gupta and Huang 2014). Ascorbate is considered as the most popular and powerful ROS detoxifying compound because of its electron donor ability in a number of enzymatic and non-enzymatic reactions (Gupta and Huang 2014). Well managed oxidative stress significantly contributed to the tolerance capacity of salinized plants by keeping cellular ROS levels in balance (Gupta and Huang 2014). In this context, salt tolerance capacity of plant species is closely related to maintaining the effectiveness of the antioxidant system.
Some studies have been conducted on the salinity tolerance mechanism of non-succulent or succulent halophytes. For instance, Shabala and Mackay (2011) recorded that succulent structures were associated with an increase in mesophyll cell size and the relative size of their vacuoles, a decrease in surface area per tissue volume, and high water content per unit surface area. Zeng et al. (2018) investigated some antioxidant enzyme activities in succulent halophyte Carpobrotus rosii suggested that salt stress resulted in significant increases in activities of major antioxidant enzymes, such as APX, CAT and GR and decrease in SOD activity in the mesophyll tissue. Likewise, non-succulent extreme-halophyte Thellungiella parvula, a halophytic relative of Arabidopsis (Arabidopsis thaliana), could regulate ion hemostasis by providing osmotic adjustment and alleviate hazardous effects of excess salinity by inducing antioxidant system (Uzilday et al. 2015). However, more research should be performed on signal compounds to explain the salt tolerance mechanism of succulent-extreme halophytes.
Scorzonera hieraciifolia is an endemic halophyte species that is mainly distributed in salt habitats in Irano-Turanian regions. It has been currently reported that S. hieraciifolia is a medicinal plant having in vitro antioxidant and anti-inflammatory activity (Sarı et al. 2019). We have observed fleshy shoots in S. hieraciifolia in the habitat. Understanding of the salt tolerance mechanism of S. hieraciifolia, a medicinal succulent extreme-halophyte, may provide a contribution to cultivation of medicinal plants in salinized areas.
The aim of the present study is to elucidate the signaling compounds involved in responses of the succulent extreme-halophytes to salt stress, including osmotic regulation and induction of antioxidant system. Firstly, extreme halophyte, Thellungiella parvula (syn. Eutrema parvulum), closely related to Arabidopsis (Uzilday et al. 2015), is used in the present research to determine whether S. hieraciifolia is an extreme-halophyte. Based on the measurement of some stress parameters indicating stress damage such as relative water content, lipid peroxidation and H2O2 content by comparing to Thellungiella parvula under different salinity conditions (0, 150, 300, 450 and 600 mM), it was determined that S. hieraciifolia was an extreme-halophyte. We hypothesized that extraordinary salt tolerance of succulent extreme-halophytes can be provided by combined induction of multiple signal compounds especially calcium, magnesium, osmoprotectants, reactive oxygen species and antioxidant substances. We also tested the hypothesis that induction of antioxidant system can make strong contributions to salt stress tolerance of S. hieraciifolia.