Regulatory Role of Silicon in Mediating Differential Stress Tolerance Responses in Two Contrasting Tomato Genotypes Under Osmotic Stress

Abstract

Previous studies have shown the role of silicon (Si) in mitigating the adverse effect of drought stress in different crop species. However, data are lacking on a comparison of drought tolerant and drought sensitive crop cultivars in response to Si nutrition. Therefore, the aim of this study was to elucidate the mechanism (s) by which two contrasting tomato genotypes respond to Si nutrition under osmotic stress condition. Two tomato lines contrasting in their response to drought stress were hydroponically grown under polyethylene glycol (PEG, 6000) and two regimes of Si (0 and 1.5 mM). Metabolite profiling was performed in two lines. Growth and relevant physiological parameters, and expression levels of selected genes were also measured. Si application resulted in improved osmotic stress tolerance in both drought tolerant line LA0147 and drought sensitive line FERUM. In the drought tolerant line, Si enhanced uptake of sulfur (S) and ammonium ( ${\text{NH}}_4^{+}$ ) which led to a significantly higher production of amino acids arginine, methionine, serine, and glycine. While in the drought sensitive line, Si significantly increased production of amino acids proline and GABA which further lowered the level of GSSG to GSH ratio and thus balanced the redox homeostasis under osmotic stress. The higher significant production of amino acids arginine, methionine, GABA, and proline enhanced production of free polyamines putrescine and spermidine and improved osmotic stress tolerance. Therefore, we conclude that Si distinctively regulated osmotic stress tolerance in two contrasting tomato genotypes by differential accumulation of relevant amino acids which eventually led to enhanced polyamine metabolism.

Keywords: amino acids; metabolites; mineral nutrition and redox state; polyamines; tomato.

FIGURE 1

Influence of Si supply on shoot fresh and dry weights, chlorophyll level, and expression pattern of drought marker gene in two contrasting tomato genotypes under osmotic stress. (A) Monitoring both root and shoot phenotype under control, OSC and OSC + Si, (B) shoot fresh weight, (C) shoot dry weight (D), chlorophyll levels and (E) relative expression of SlJUB1. Plants were grown in hydroponic culture and osmotic stress was simulated by applying polyethylene glycol (PEG 6000). Si was provided at 0.75 mM for pre-cultured plants and at 1.5 mM for osmotic stressed plants. Roots and fully expanded leaves from 21-day old plants were harvested 7 days after imposition of osmotic stress. Bars indicate means ± SE. Different letters denote significant differences according to Fischer’s LSD test (p < 0.05; n = 4).

FIGURE 2

Influence of Si supply on ${\text{NH}}_4^{+}$, S, and ${\text{SO}}_4^{2-}$ concentrations in roots and leaves of two contrasting tomato genotypes under osmotic stress. (A) ${\text{NH}}_4^{+}$ concentration in roots, (B) ${\text{NH}}_4^{+}$ concentration in leaves, (C) S concentration in roots, (D) S concentration in leaves, (E) ${\text{SO}}_4^{2-}$ concentration in roots and (F) ${\text{SO}}_4^{2-}$ concentration in leaves. Plants were grown in hydroponic culture and osmotic stress was simulated by applying polyethylene glycol (PEG 6000). Si was provided at 0.75 mM for pre-cultured plants and at 1.5 mM for osmotic stressed plants. Roots and fully expanded leaves from 21-day old plants were harvested 7 days after imposition of osmotic stress. Bars indicate means ± SE. Different letters denote significant differences according to Fischer’s LSD test (p < 0.05; n = 4).

FIGURE 3

Influence of Si supply on selected metabolite concentrations in roots of two contrasting tomato genotypes under osmotic stress. (A) Glutamic acid concentration in roots, (B) Gly concentration in roots, (C) Pro concentration in roots, and (D) GABA concentration in roots. Plants were grown in hydroponic culture and osmotic stress was simulated by applying polyethylene glycol (PEG 6000). Si was provided at 0.75 mM for pre-cultured plants and at 1.5 mM for osmotic stressed plants. Roots and fully expanded leaves from 21-day old plants were harvested 7 days after imposition of osmotic stress. Bars indicate means ± SE. Different letters denote significant differences according to Fischer’s LSD test (p < 0.05; n = 4).

FIGURE 4

Influence of Si supply on selected amino acids concentration in leaves of two contrasting tomato genotypes under osmotic stress. (A) Arg concentration in leaves, (B) Met concentration in leaves, (C) Ser concentration in leaves, (D) Gly concentration in leaves, (E) Ala concentration in leaves, (F) Pro concentration in leaves, (G) Thr concentration in leaves, and (H) GABA concentration in leaves. Plants were grown in hydroponic culture and osmotic stress was simulated by applying polyethylene glycol (PEG 6000). Si was provided at 0.75 mM for pre-cultured plants and at 1.5 mM for osmotic stressed plants. Roots and fully expanded leaves from 21-day old plants were harvested 7 days after imposition of osmotic stress. Bars indicate means ± SE. Different letters denote significant differences according to Fischer’s LSD test (p < 0.05; n = 4). The units of Arg and Met are expressed in ng mg^-1 FW.

FIGURE 5

Influence of Si supply on glutathione concentrations in roots and leaves of two contrasting tomato genotypes under osmotic stress. (A) GSH concentration in roots, (B) GSH concentration in leaves, (C) GSSG concentration in roots, (D) GSSG concentration in leaves (E) GSSG to GSH ratio in roots and (F) GSSG to GSH ratio in leaves. Plants were grown in hydroponic culture and osmotic stress was simulated by applying polyethylene glycol (PEG 6000). Si was provided at 0.75 mM for pre-cultured plants and at 1.5 mM for osmotic stressed plants. Roots and fully expanded leaves from 21-day old plants were harvested 7 days after imposition of osmotic stress. Bars indicate means ± SE. Different letters denote significant differences according to Fischer’s LSD test (p < 0.05; n = 4).

FIGURE 6

Influence of Si supply on polyamines concentrations in roots and leaves of two contrasting tomato genotypes under osmotic stress. (A) Put concentration in roots, (B) Put concentration in leaves, (C) spermidine concentration in roots, (D) spermidine concentration in leaves, (E) spermine concentration in roots, and (F) spermine concentration in leaves. Plants were grown in hydroponic culture and osmotic stress was simulated by applying polyethylene glycol (PEG 6000). Si was provided at 0.75 mM for pre-cultured plants and at 1.5 mM for osmotic stressed plants. Roots and fully expanded leaves from 21-day old plants were harvested 7 days after imposition of osmotic stress. Bars indicate means ± SE. Different letters denote significant differences according to Fischer’s LSD test (p < 0.05; n = 4).

FIGURE 7

Influence of Si supply on expression levels of the genes involved in arginine, methionine, and GABA synthesis pathway in leaves of two contrasting tomato genotypes under osmotic stress. (A) relative expression of SlASS, (B) relative expression of SlASL, (C) relative expression of SlMS, (D) relative expression of SlGAD1, and (E) relative expression of SlGAD3. Plants were grown in hydroponic culture and osmotic stress was simulated by applying polyethylene glycol (PEG 6000). Si was provided at 0.75 mM for pre-cultured plants and at 1.5 mM for osmotic stressed plants. Roots and fully expanded leaves from 21-day old plants were harvested 7 days after imposition of osmotic stress. Bars indicate means ± SE. Different letters denote significant differences according to Fischer’s LSD test (p < 0.05; n = 4).

FIGURE 8

Schematic model demonstrating the distinct regulatory role of Si in mediating differential stress tolerance responses in two contrasting tomato lines exposed to osmotic stress. Under osmotic stress, the two tomato genotypes LA0147 (drought tolerant) and FERUM (drought sensitive) showed differential stress tolerance responses facilitated by supplemental Si. In LA0147, under osmotic stress, application of Si mediated increased uptake and translocation of ${\text{NH}}_4^{+}$ and S, which led to consequent alterations in the accumulation of different amino acids like arginine, methionine, serine, and glycine and regulation of genes involved in Arg and Met synthesis. An upsurge of these amino acids further resulted in augmentation of polyamines putrescine and spermidine which maintained shoot growth and other stress tolerance responses. On the other hand, Si supply provided to the osmotic stressed plants of sensitive line FERUM showed a completely different pattern in accumulation of specific amino acids proline and GABA, which are known to provide better stress tolerance in plants by balancing the redox homeostasis with evident decrease in GSSG/GSH ratio. The higher accumulation of GABA was caused by Si induced expression of SlGAD1 and SlGAD3, which are involved in GABA synthesis. A simultaneous increase in the levels specifically of the free polyamine Put was also observed in FERUM which subsequently promoted an improved osmotic stress tolerance as suggested by increased shoot growth and higher chlorophyll content (Red arrows = increase/upregulation, blue arrows = decrease/downregulation).