A comparative study of salt tolerance parameters in 11 wild relatives of Arabidopsis thaliana

Abstract

Salinity is an abiotic stress that limits both yield and the expansion of agricultural crops to new areas. In the last 20 years our basic understanding of the mechanisms underlying plant tolerance and adaptation to saline environments has greatly improved owing to active development of advanced tools in molecular, genomics, and bioinformatics analyses. However, the full potential of investigative power has not been fully exploited, because the use of halophytes as model systems in plant salt tolerance research is largely neglected. The recent introduction of halophytic Arabidopsis-Relative Model Species (ARMS) has begun to compare and relate several unique genetic resources to the well-developed Arabidopsis model. In a search for candidates to begin to understand, through genetic analyses, the biological bases of salt tolerance, 11 wild relatives of Arabidopsis thaliana were compared: Barbarea verna, Capsella bursa-pastoris, Hirschfeldia incana, Lepidium densiflorum, Malcolmia triloba, Lepidium virginicum, Descurainia pinnata, Sisymbrium officinale, Thellungiella parvula, Thellungiella salsuginea (previously T. halophila), and Thlaspi arvense. Among these species, highly salt-tolerant (L. densiflorum and L. virginicum) and moderately salt-tolerant (M. triloba and H. incana) species were identified. Only T. parvula revealed a true halophytic habitus, comparable to the better studied Thellungiella salsuginea. Major differences in growth, water transport properties, and ion accumulation are observed and discussed to describe the distinctive traits and physiological responses that can now be studied genetically in salt stress research.

Fig. 1.

Effect of 150 mM NaCl stress on leaf area of different Brassicaceae species. Salt treatment started 25 d after sowing and lasted for 30 d. Leaf area was measured using a scanner and image processing software over a sample of five plants per species, with three replicates. Values are expressed as a percentage (%) of leaf area in non-salinized plants. Values are means ±SE (n=15).

Fig. 2.

Effect of 150 mM NaCl on primary root length of different Brassicaceae species. Salt treatment started 25 d after sowing and lasted 30 d. Root length was measured using a scanner and image processing software over a sample of five plants per species, with three replicates. Values are expressed as a percentage (%) of root length in non-salinized plants. Values are means ±SE (n=15).

Fig. 3.

Species-specific lethal dose for 50% of the population (LD50) under NaCl stress in different Brassicaceae species. Salt treatments ranged from 0–600 mM NaCl, started 30 d after sowing and lasted for 30 d. Survival rates were assessed at the end of the treatments over samples of 20 plants, with three replicates. Letters indicate differences at P <0.05 (n=60).

Fig. 4.

Effect of 300 mM NaCl stress on root elongation of selected Brassicaceae species. Five days after germination on regular MS medium, with plates positioned vertically, seedlings were moved to salt. The plates were then rotated 180° to visualize further growth (root bending assay). Measurements were assessed after 10 d on 10 plants per species, with three replicates. Values are expressed as a percentage (%) of growth measured in non-salinized plants. Values are means ±SE (n=30).

Fig. 5.

Effect of 150 mM NaCl on seed germination of selected Brassicaceae species. Germination rate was determined 14 d after sowing on plates containing 10 seeds per species, with three replicates. Values are expressed as a percentage (%) of germination as compared with non-salinized plates. Values are means ±SE (n=30).

Fig. 6.

Stomatal length (A) and density (B) of selected Brassicaceae species. Values refer to 20 independent measures per leaf on three leaves per species (non-salinized control plants). Letters indicate differences at P <0.05 (n=60).

Fig. 7.

Effect of salt stress on water loss in selected Brassicaceae species. Four-week-old seedlings grown under long-day conditions with cool-white fluorescent light were used for measurements of whole-plant water loss. Plants were grown singularly in 9 cm pots, which were sealed in plastic wrap and placed on electronic balances. Weight was determined every 60 min for 5 d. The experiment was repeated three times. Values are means of transpiration rates of four plants in the three independent experiments (n=12). White circle, control; black diamonds, 300 mM NaCl.

Fig. 8.

Relative water loss in 300 mM NaCl stressed plants, as compared with non-salinized plants. Four-week-old seedlings grown under long-day conditions with cool-white fluorescent light were used for measurements of whole-plant water loss. Plants were grown singularly in 9 cm pots, which were sealed in plastic wrap and placed on electronic balances. Weight was determined every 60 min for 5 d. Values are a percentage (%) of transpiration rate as compared with non-salinized plants averaged over the 5 d. Values are means ±SE (n=12).

Fig. 9.

Sodium and potassium concentrations in leaves of Arabidopsis thaliana, Thellungiella halophila, and Thellungiella parvula. NaCl treatments were applied by incremental increase of NaCl in the irrigation water until the final concentrations of 0, 100, 200, 300, and 500 mM NaCl were reached. Plants were harvested 28 d and 42 d after imposition of the final increase of the NaCl concentrations. At harvest, seedlings of treated and control plants were rinsed with deionized water and dried at 65 °C for 2 d and Na⁺ and K⁺ contents in the solutions were determined by using an atomic absorption spectrophotometer. Letters indicate differences at P <0.05 (n=2, for each of two samples four plants were ground).

Fig. 10.

Effect on growth of different concentrations of NaCl in Thellungiella parvula. NaCl was increased incrementally in the irrigation water to final concentrations of 100, 150, and 200 mM for A. thaliana and 100, 200, and 300, for T. halophila and T. parvula. Shoots and roots were harvested at day 42 of stress. FW, fresh weight. Values are means ±SE (n=5).

Fig. 11.

Representation summarizing the various parameters that have been recorded in this study. Different species are compared based on their relative ability to express pathways and phenotypes that support extremophile behaviour and success in adaptation. The species can be sorted in such a way that indicated T. parvula and T. salsuginea as the most successful species.