Understanding Ameliorating Effects of Boron on Adaptation to Salt Stress in Arabidopsis

Abstract

When faced with salinity stress, plants typically exhibit a slowdown in their growth patterns. Boron (B) is an essential micronutrient for plants that are known to play a critical role in controlling cell wall properties. In this study, we used the model plant Arabidopsis thaliana Col-0 and relevant mutants to explore how the difference in B availability may modulate plant responses to salt stress. There was a visible root growth suppression of Col-0 with the increased salt levels in the absence of B while this growth reduction was remarkably alleviated by B supply. Pharmacological experiments revealed that orthovanadate (a known blocker of H⁺-ATPase) inhibited root growth at no B condition, but had no effect in the presence of 30 μM B. Salinity stress resulted in a massive K⁺ loss from mature zones of A. thaliana roots; this efflux was attenuated in the presence of B. Supplemental B also increased the magnitude of net H⁺ pumping by plant roots. Boron availability was also essential for root halotropism. Interestingly, the aha2Δ57 mutant with active H⁺-ATPase protein exhibited the same halotropism response as Col-0 while the aha2-4 mutant had a stronger halotropism response (larger bending angle) compared with that of Col-0. Overall, the ameliorative effect of B on the A. thaliana growth under salt stress is based on the H⁺-ATPase stimulation and a subsequent K⁺ retention, involving auxin- and ROS-pathways.

Keywords: Arabidopsis; H+; K+; NaCl; boron; halotropism.

Figure 1

Effect of NaCl on phenotype (A) and root length (B) of A. thaliana Col-0 plants. Data are mean ± SE (n = 8–12). One-way ANOVA followed by Tukey’s post-hoc test was conducted for statistical analysis. Data labeled with different low-case letters are significantly different at p < 0.05.

Figure 2

Effect of boron on phenotype (A), root length (B), and root angle (C) of A. thaliana Col-0 plants grown in the presence of 100 mM NaCl. Data are mean ± SE (n = 8–12). One-way ANOVA followed by Tukey’s post-hoc test was conducted for statistical analysis. Data labeled with different low-case letters are significantly different at p < 0.05.

Figure 3

Effect of inhibitors on the relative root length of A. thaliana Col-0 plants in the presence of different boron and salt treatments. Data are mean ± SE (n = 8–12). One-way ANOVA followed by Tukey’s post-hoc test was conducted for statistical analysis. Data labeled with different low-case letters are significantly different at p < 0.05.

Figure 4

Effect of salt on phenotype (A,C) and root angle (B,D) of A. thaliana plants. Data are mean ± SE (n = 8–12). A t-test was conducted for statistical analysis. Data is significant at **** p < 0.0001, and ns = not significant at p < 0.05.

Figure 5

Transient net H⁺ (A) and K⁺ (B) flux measured mature root zone of A. thaliana Col-0 plants in response to acute 100 mM NaCl treatment. Mean ± SE (n = 6–8). The sign convention is efflux negative.

Figure 6

Suggested model of B-mediated amelioration of salinity stress tolerance. Central to this process is B-mediated activation of H⁺-ATPase with the consequences for (i) cytosolic K⁺ retention; (ii) Na⁺ exclusion from metabolically active compartments; and (iii) stress avoidance via halotrppism. The causal role of B in regulating plant redox balance (oxidative stress component) was not discussed in detail in this work but explicitly addressed in our previous studies [60]. It is also evident from the reported negative effects of DPI (a known inhibitor of NADPH oxidase) on root growth shown in Figure 3.