Promotion of Ca2+ Accumulation in Roots by Exogenous Brassinosteroids as a Key Mechanism for Their Enhancement of Plant Salt Tolerance: A Meta-Analysis and Systematic Review

Abstract

Brassinosteroids (BRs), the sixth major phytohormone, can regulate plant salt tolerance. Many studies have been conducted to investigate the effects of BRs on plant salt tolerance, generating a large amount of research data. However, a meta-analysis on regulating plant salt tolerance by BRs has not been reported. Therefore, this study conducted a meta-analysis of 132 studies to elucidate the most critical physiological mechanisms by which BRs regulate salt tolerance in plants from a higher dimension and analyze the best ways to apply BRs. The results showed that exogenous BRs significantly increased germination, plant height, root length, and biomass (total dry weight was the largest) of plants under salt stress. There was no significant difference between seed soaking and foliar spraying. However, the medium method (germination stage) and stem application (seedling stage) may be more effective in improving plant salt tolerance. BRs only inhibit germination in Solanaceae. BRs (2 μM), seed soaking for 12 h, and simultaneous treatment with salt stress had the highest germination rate. At the seedling stage, the activity of Brassinolide (C₂₈H₄₈O₆) was higher than that of Homobrassinolide (C₂₉H₅₀O₆), and post-treatment, BRs (0.02 μM) was the best solution. BRs are unsuitable for use in the germination stage when Sodium chloride is below 100 mM, and the effect is also weakest in the seedling stage. Exogenous BRs promoted photosynthesis, and antioxidant enzyme activity increased the accumulation of osmoregulatory and antioxidant substances and reduced the content of harmful substances and Na⁺, thus reducing cell damage and improving plant salt tolerance. BRs induced the most soluble protein, chlorophyll a, stomatal conductance, net photosynthetic rate, Glutathione peroxidase, and root-Ca²⁺, with BRs causing Ca²⁺ signals in roots probably constituting the most important reason for improving salt tolerance. BRs first promoted the accumulation of Ca²⁺ in roots, which increased the content of the above vital substances and enzyme activities through the Ca²⁺ signaling pathway, improving plant salt tolerance.

Keywords: brassinosteroids; meta-analysis; physiological; salt stress.

Figure 1

Flow chart of search and exclusion for meta-analysis studies.

Figure 2

Number of studies, species classification, and family classification. (A) The number of papers published each year during the period 1993–2022; (B) The percentage of all studied plants; (C) The rate of all studied plants in the different families.

Figure 3

Subgroup analysis of the effect of exogenous BRs on the seed germination rate of plants under salt stress, forest plots are combined effect values (lnR) ± 95% CI, n represents the number of independent studies. (A) Plant group and plant types, (B) family, (C) BRs, and salt levels, (D) The application method and processing time. Spray application: seeds were treated with BRs by spraying. Petri dishes: BR solution is added to Petri dishes lined with filter paper on which the seeds are laid. Culture medium: BRs are added to the culture medium (a gel containing nutrients) used for seed germination. Soak seeds: seeds are soaked using a solution of BRs. (E) Seed soaking time. Confidence intervals do not overlap with the dashed line, indicating a significant difference between the treatment and control groups. (F) The effect of BR concentration on the effect value of the germination rate in the 0–10 μM concentration range.

Figure 4

The effect of exogenous BRs on plant growth and biomass under salt stress. (A) The effect of BRs on plant height, root length, total dry weight (DW), total fresh weight (FW), shoot DW, shoot FW, root DW, and root FW under salt stress; (B) The effect of different donor types of BRs on growth and biomass under salt stress.

Figure 5

Subgroup analysis of the effect of BRs on plant growth and biomass under salt stress. (A) Subgroup analysis for different application periods. (B) Subgroup analysis for various application methods, and parameters with some studies of less than eight were removed to ensure the study’s accuracy. (C) Subgroup analysis of monocotyledons and dicotyledons.

Figure 6

Subgroup analysis of BRs and salt levels based on growth and biomass parameters. (A) Subgroup analysis of 3 BR levels, low (BRs < 0.02 μM), medium (0.02–1 μM), and high (BRs > 1 μM); (B) Subgroup analysis of 3 salt levels, low (NaCl < 100 mM), medium (100–150 mM) and high (NaCl > 150 mM).

Figure 7

Effect of exogenous BRs on photosynthesis, cell damage, and osmoregulatory substances in plants under salt stress. (A) Photosynthesis; maximal photochemical efficiency (Fv/Fm); transpiration rate (E); stomatal conductance (Gs); intercellular CO₂ concentration (Ci); net photosynthetic rate (Pn); (B) Osmoregulatory substance with indicators of oxidative stress; malondialdehyde (MDA).

Figure 8

Effect of exogenous BRs on plants’ antioxidant system and cation content under salt stress. (A) The antioxidant system, RWC: relative water content, APX: ascorbate peroxidase activity, CAT: catalase activity, GPX: glutathione peroxidase activity, GR: glutathione reductase activity, POD: peroxidase activity, POX: guaiacol peroxidase activity, SOD: superoxide dismutase activity, DHAR: dehydroascorbate reductase activity, ASA: ascorbic acid content, GSH: reduced glutathione content; (B) effect of BRs on Na⁺, Mg²⁺, K⁺, and Ca²⁺ content.

Figure 9

Model of BRs regulating salt tolerance in plants, where red arrows represent promotion. The blue arrows indicate the direction of Na⁺, K⁺, and Mg²⁺ transport. MDA: malondialdehyde, Pn: net photosynthetic rate, Gs: stomatal conductance, GR: glutathione reductase, GPX: glutathione peroxidase, POX: guaiacol peroxidase, DHAR: dehydroascorbate reductase.