Karim Ben Hamed

Early effects of salt stress on the physiological and oxidative status of Cakile maritima (halophyte) and Arabidopsis thaliana (glycophyte)

Early changes in physiological and oxidative status induced by salt stress were monitored in two Brassicaceae plants differing in their tolerance to salinity, Cakile maritima (halophyte) and Arabidopsis thaliana (glycophyte). Growth response and antioxidant defense of C. maritima under 400 mM NaCl were compared with those of A. thaliana exposed to 100 mM NaCl. Salinity induced early growth reduction that is less pronounced in C. maritima than in A. thaliana. Maximum hydrogen peroxide (H₂O₂) level occurred in the leaves of both species 4 h after the onset of salt treatment. A rapid decline in H₂O₂ concentration was observed thereafter in C. maritima, whereas it remained high in A. thaliana. Correlatively, superoxide dismutase, catalase and peroxidase activities increased at 4 h of treatment in C. maritima and decreased thereafter. However, the activity of these enzymes remained higher in treated plants than that in controls, regardless of the duration of treatment, in A. thaliana. The concentrations of malondialdehyde (MDA) reached maximum values at 24 h of salt stress in both species. Again, MDA levels decreased later in C. maritima, but remained high in A. thaliana. The contents of α-tocopherol remained constant during salt stress in C. maritima and decreased during the first 24 h of salt stress and then remained low in A. thaliana. The results clearly showed that C. maritima, in contrast to A. thaliana, can rapidly evolve physiological and antioxidant mechanisms to adapt to salt and manage the oxidative stress. This may explain, at least partially, the difference in salt tolerance between halophytes and glycophytes.

Potential utilisation of halophytes for the rehabilitation and valorisation of salt-affected areas in Tunisia

In arid and semi-arid regions, irrigation water contributes to salinisation of the upper layer of the soil, where most root activity takes place. Along the path of plant domestication, many crop species have lost resistance mechanisms to various stress conditions [1], including salt stress [2]. Thus, most crop plants do not fully express their original genetic potential for growth, development and yield under salt stress, and their economic value declines as salinity levels increase [3, 4]. Improving salt resistance of crop plants is, therefore, of major concern in agricultural research. A potential genetic resource for the improvement of salt resistance in crop plants resides among wild populations of halophytes [5, 6]. These can be either domesticated into new, salt-resistant crops, or used as a source of genes to be introduced into crop species by classical breeding or molecular methods.

Salt and seawater effects on the germination of Crithmum maritimum

Crithmum maritimum (Apiaceae), also called sea fennel, is a perennial halophyte that thrives on saline environments (rocky coasts, piers and breakwaters) along the Mediterranean countries, Pacific and Atlantic coasts [1]. Several uses are known for this plant: for culinary purposes, fresh leaves and young branches are pickled in vinegar and used as condiments. Leaves have also medicinal applications, as antiscorbutic, tonic, diuretic, and vermifuge substances [2].

Growth, sodium uptake and antioxidant responses of coastal plants differing in their ecological status under increasing salinity

In the present study, we compared the response to salinity of three plants from Brittany coast with contrasted ecological status: Limonium latifolium (salt marshes), Matricaria maritima (beach tops and sand dunes) and Crambe maritima (fixed dunes). Under controlled glasshouse conditions, the growth of the three plants decreased with increasing external salinity. L. latifolium and C. maritima exhibited the highest and lowest resistance to severe salt stress (400 mM), respectively. M. maritima could be considered as an intermediate species, since it tolerated salinity up to 200 mM. The same observation could be made with sodium absorption and acuumulation in plant tissues, the most tolerant species (L. latifolium being the least Na accumulator. Hydrogen peroxide (H2O2) and malondialdehyde (MDA), commonly produced in conditions of stress, accumulated significantly in salt treated C. maritima and M. maritima while not in the tolerant L. latifolium. The latter used glutathione reductase to maintain constant H2O2 levels under salt stress while peroxidases were very low and ascorbate peroxidase did not respond to salinity stimulation. The medium tolerant halophyte M. maritima used peroxidases to protect from NaCl-induced H2O2, while the sensitive C. maritima failed to detoxify H2O2 despite a sharp increase in catalase activity. Results showed that the three coastal species differ in resistance to salinity. They also suggested that the level of plant resistance to salinity could be attributed to differing mechanisms to manage the accumulation of sodium and cope with the oxidative damages.

Diversity in the response of two potential halophytes (Batis maritima and Crithmum maritimum) to salt stress

In this study, we compared the response to NaCl of Batis maritima and Crithmum maritimum, two potential halophytes with a different range of salinity tolerance. At high NaCl concentrations (800 mM for B. maritima and 300 mM for C. maritimum), the growth of both plants was significantly reduced. A split root experiment aimed at determining whether high NaCl conditions limit growth of plants through toxic effects of excessive salt accumulation in shoots or through impairment of some essential nutrient acquisition. The split root experiment was performed with three treatments. In the first treatment (B/S), half of the roots were immersed in a basal medium (B) and the other half in the same medium supplemented with NaCl (S). In the two other treatments, the two halves of the root system were immersed either in salt-free medium (B/B) or in the basal medium containing salt (S/S). Under split-root conditions, B. maritima and C. maritimum accumulated Na in their shoots, and displayed improved growth as compared to control plants. In C. maritimum, the B/S treatment partially restored K provision to the shoots but not that of Ca, suggesting that the inhibition of K+ uptake by salt could only limit its growth under high salinity. In B. maritima (B/S plants), the concentration of K+ and Ca2+ were diluted by growth. The inhibition of K+ and Ca2+ uptake by salt did not seem to limit growth of B. maritima growth under high salinity. The growth of B. maritima and C. maritimum could be also limited by the restriction imposed by NaCl on N uptake.

Increased sensitivity to salt stress in tocopherol-deficient Arabidopsis mutants growing in a hydroponic system

Recent studies suggest that tocopherols could play physiological roles in salt tolerance but the mechanisms are still unknown. In this study, we analyzed changes in growth, mineral and oxidative status in vte1 and vte4 Arabidopsis thaliana mutants exposed to salt stress. vte1 and vte4 mutants lack α-tocopherol, but only the vte1 mutant is additionally deficient in γ-tocopherol. Results showed that a deficiency in vitamin E leads to reduced growth and increased oxidative stress in hydroponically-grown plants. This effect was observed at early stages, not only in rosettes but also in roots. The vte1 mutant was more sensitive to salt-induced oxidative stress than the wild type and the vte4 mutant. Salt sensitivity was associated with (i) high contents of Na+, (ii) reduced efficiency of PSII photochemistry (Fv/Fm ratio) and (iii) more pronounced oxidative stress as indicated by increased hydrogen peroxide and malondialdeyde levels. The vte 4 mutant, which accumulates γ- instead of α-tocopherol showed an intermediate sensitivity to salt stress between the wild type and the vte1 mutant. Contents of abscisic acid, jasmonic acid and the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid were higher in the vte1 mutant than the vte4 mutant and wild type. It is concluded that vitamin E-deficient plants show an increased sensitivity to salt stress both in rosettes and roots, therefore indicating the positive role of tocopherols in stress tolerance, not only by minimizing oxidative stress, but also controlling Na+/K+ homeostasis and hormonal balance.

Comparison of NaCl-induced programmed cell death in the obligate halophyte Cakile maritima and the glycophyte Arabidospis thaliana

Salinity represents one of the most important constraints that adversely affect plants growth and productivity. In this study, we aimed at determining possible differences between salt tolerant and salt sensitive species in early salt stress response. To this purpose, we subjected suspension-cultured cells from the halophyte Cakile maritima and the glycophyte Arabidopsis thaliana, two Brassicaceae, to salt stress and compared their behavior. In both species we could observe a time and dose dependent programmed cell death requiring an active metabolism, a dysfunction of mitochondria and caspase-like activation although C. maritima cells appeared less sensitive than A. thaliana cells. This capacity to mitigate salt stress could be due to a higher ascorbate pool that could allow C. maritima reducing the oxidative stress generated in response to NaCl. It further appeared that a higher number of C. maritima cultured cells when compared to A. thaliana could efficiently manage the Na(+) accumulation into the cytoplasm through non selective cation channels allowing also reducing the ROS generation and the subsequent cell death.

Phenolic content and antioxidant activity in two contrasting Medicago ciliaris lines cultivated under salt stress

The objective of this study was to determine more indepth physiological and antioxidant responses in two Medicago ciliaris lines (a salt-tolerant line TNC 1.8 and a salt-sensitive line TNC 11.9) with contrasting responses to 100 mM NaCl. Under salt stress, both lines showed a decrease in total biomass and in the growth rate for roots, but TNC 1.8 was less affected by salt than TNC 11.9 in that it maintained leaf growth even in the presence of added salt. In both lines, salt stress mainly affected micronutrient status (Fe, Mn, Cu and Zn) rather than K nutrition, but the tolerant line TNC 1.8 accumulated more Na in leaves and less in roots compared with TNC 11.9. Salt stress decreased total soluble sugars (TSS) in all organs of the sensitive line TNC 11.9, whereas TSS was only reduced in roots of the tolerant line. The salt-induced drop in growth was linked to an increase in lipid peroxidation in roots of both lines and in leaves of the sensitive line. The salt-tolerant line TNC 1.8 was more efficient at managing salt-induced oxidative damage in leaves and to a lesser extent in roots than the salt-sensitive line TNC 11.9, by preserving higher phenolic compound and superoxide dismutase levels in both organs.

Kinetics of the antioxidant response to salinity in Crithmum maritimum

Salinity limits the production of approximately 40% of the world’s agricultural land [1]. In order to overcome the decline of cultivated areas and the high demands for food and energy, a particular interest was accorded to the salty lands. To regreen these areas, two strategies have been developed: i) the genetic manipulation of common crop species for increased salt tolerance, and ii) the utilization of naturally salt-tolerant species (halophytes) [2].

Insights into the Ecology and the Salt Tolerance of the Halophyte Cakile maritima Using Multidisciplinary Approaches

We review information on the halophyte Cakile maritima, member of the Brassicaceae family and native of coastal ecosystems. The first investigations focused on the ecology of the species and demonstrated latitudinal range, dispersal and environmental adaptation that make Cakile an ideal model system in which to study phenological adaptation to climate. Cakile maritima can be used as a model plant to learn about plant reactions that are not observed or experimentally reproduced in traditional glycophytic models. Ion homeostasis, osmotic adjustment, and antioxidant protection are the most studied mechanisms of salt tolerance in this species. By contrasting stress responses of C. maritima to those of stress-sensitive models such as Arabidopsis, we are convinced that differences between both plant types lie mainly in the mechanisms that control how stress signals are perceived, transduced and how adaptive processes are controlled within the plant. The establishment of cell suspension culture in C. maritima will help to evaluate salt tolerance at the cellular level. Preliminary results showed that cells exhibited similar growth and ion response to those of halophytes.