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, 9 (1), 19788

Silicon and Salicylic Acid Confer high-pH Stress Tolerance in Tomato Seedlings

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Silicon and Salicylic Acid Confer high-pH Stress Tolerance in Tomato Seedlings

Adil Khan et al. Sci Rep.

Abstract

Alkalinity is a known threat to crop plant growth and production, yet the role of exogenous silicon (Si) and salicylic acid (SA) application has been largely unexplored. Here, we sought to understand the beneficial impacts of Si and SA on tomato seedlings during high-pH (9.0) stress. Results showed that Si- and SA-treated plants displayed higher biomass, chlorophyll contents, relative leaf water and better root system than none-treated plants under alkaline conditions. Both Si and SA counteracted the alkaline stress-induced oxidative damage by lowering the accumulation of reactive oxygen species and lipid peroxidation. The major antioxidant defence enzyme activities were largely stimulated by Si and SA, and these treatments caused significantly increased K+ and lowered Na+ concentrations in shoot and root under stress. Moreover, Si and SA treatments modulated endogenous SA levels and dramatically decreased abscisic acid levels in both shoot and root. Additionally, key genes involved in Si uptake, SA biosynthesis, the antioxidant defence system and rhizosphere acidification were up-regulated in Si and SA treatments under alkaline conditions. These results demonstrate that Si and SA play critical roles in improving alkaline stress tolerance in tomato seedlings, by modifying the endogenous Na+ and K+ contents, regulating oxidative damage and key genes and modulating endogenous hormone levels. These findings will help to broaden our understanding regarding the physiological and molecular mechanisms associated with the alkaline soil tolerance in plants.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Effects of alkaline stress on tomato plants. (A) Shoot length. (B) Shoot diameter (C). (D) Photograph of the shoot, illustrating the typical differences among all treatments at each pH. Tomato seedlings were grown in pots. A solution at pH 6.0 and pH 9.0, respectively, was applied daily to each pot. *,**, *** and **** indicate a significant difference between treatments at a given pH where P < 0.05, 0.01 and 0.001, respectively, and “ns” indicates non-significant differences. Different letter (s) indicate a significant difference (P < 0.05) among all treatments by DMRT test.
Figure 2
Figure 2
Effects of alkaline stress on (A) root length and (B) root dry weight. (C) Photograph of the root, illustrating the typical differences between all treatments at each pH. (D) Scanning electron micrographs of the root in response to pH, and Si and SA application. Co-cortex, p-pericycle, Ex-exodermis, Xy-xylem. A solution at pH 6.0 and pH 9.0, respectively, was applied to each pot 3–4 times a week. *,**, *** and **** indicate a significant difference between treatments at a given pH where P < 0.05, 0.01 and 0.001, respectively, and “ns” indicates non-significant differences. Different letter (s) indicate a significant difference (P < 0.05) among all treatments by DMRT test.
Figure 3
Figure 3
Estimates of the (A) chlorophyll a, (B) chlorophyll b and (C) leaf relative water content in response to solution pH. *,**, *** and **** indicate a significant difference between treatments at a given pH where P < 0.05, 0.01 and 0.001, respectively, and “ns” indicates non-significant differences. Different letter (s) indicate a significant difference (P < 0.05) among all treatments by DMRT test.
Figure 4
Figure 4
Measurements of the endogenous shoot and root (A,B) salicylic acid and (C,D) abscisic acid levels in tomato plants grown under pH 6.0 and alkaline stress conditions with/without exogenously applied Si, SA and Si + SA. *,**, *** and **** indicate a significant difference between treatments at a given pH where P < 0.05, 0.01 and 0.001, respectively, and “ns” indicates non-significant differences. Different letter (s) indicate a significant difference (P < 0.05) among all treatments by DMRT test.
Figure 5
Figure 5
Effects of alkaline stress on the expression of related genes in tomato plants. (A) LHA1, (B) LHA2, (C) SA-binding protein-like 2, (D) SA methyltransferase, (E) isochorismate synthase, (F) PAL-1, (G) PAL-2, (H) LSi1, (I) catalase, (J) peroxidase, (K) ascorbate peroxidase and (L) superoxide dismutases. Total RNA was extracted from tomato plants grown under pH 6.0 and alkaline stress conditions (pH 9.0) with/without exogenously applied Si, SA and Si + SA for 5 weeks. Transcript levels were measured by real-time qPCR. Actin was used as an internal control. Error bars are calculated based on three biological replicates. *,**, *** and **** indicate a significant difference between treatments at a given pH where P < 0.05, 0.01 and 0.001, respectively, and “ns” indicates non-significant differences. Different letter (s) indicate a significant difference (P < 0.05) among all treatments by DMRT test.
Figure 6
Figure 6
Diagram of the hypothesis of how exogenously applied Si and SA alleviate alkaline stress tolerance in tomato plants. Alkaline stress reduces plant growth by enhancing ROS, reducing nutrients uptake due to high-pH and aggravating Na+ toxicity. Exogenously applied Si and SA enhance the activation of the antioxidant defence system, modulate key hormones and increase the K+ concentration, that eventually helps the tomato plants to survive alkaline stress. Furthermore, the plasma membrane H+-ATPase (as per the high expression of LHA1 and LHA2) pumps out H+ to the rhizosphere, reducing the rhizosphere pH and enhancing the nutrients uptake.

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