Boron Supply Enhances Aluminum Tolerance in Root Border Cells of Pea (Pisum sativum) by Interacting with Cell Wall Pectins

Abstract

Aluminum (Al) toxicity is the primary factor limiting crop growth in acidic soils. Boron (B) alleviates Al toxicity in plants, which is mainly considered to be due to the formation of Rhamnogalacturonan II-B (RGII-B) complexes, which helps to stabilize the cytoskeleton. It is unclear yet whether this is due to the increasing of net negative charges and/or further mechanisms. Kinetics of Al accumulation and adsorption were investigated using entire cells, cell wall and pectin of root border cells (RBCs) of pea (Pisum sativum), to reveal the mechanism of B in interacting with alkali-soluble and chelator-soluble pectin for an increased Al tolerance in RBCs. The results show that B could rescue RBCs from Al-induced cell death by accumulating more Al in the cell wall, predominately in alkali-soluble pectin. Boron also promotes Al3+ adsorption and inhibits Al3+ desorption from alkali-soluble pectin. Thus, more Al3+ is immobilized within the alkali-soluble pectin fraction and less in the chelator-soluble pectin, rendering Al3+ less mobile. Boron induces an increase of RG-II (KDO,2-keto-3-deoxyoctonic acid) content for forming more borate-RGII complexes, and the decrease of pectin methyl-esterification, thus creates more negative charges to immobilize Al3+ in cell wall pectin. The study provides evidence that abundant B supply enhances the immobilization of Al in alkali-soluble pectin, thus most likely reducing the entry of Al3+ into the symplast from the surroundings.

Boron alters carboxyl group binding capacity and Al transport pathway to relieve Al toxicity

“…2e), especially in WSP and ASP (Table S1), but was not statistically significant in Al content in HC1 and HC2, except for +Al B50 treatment (Fig. 2f-g), suggesting that B addition on the ability of pectin binding Al is more obvious than other components, and more specific action in ASP, which in line with the conclusion that ASP is the predominant target of Al accumulation in CW (Li et al , 2017). A large amount of pectin, HC1, and HC2 were observed under +Al B0.1 treatment (Fig.…”

“…Similar conclusions were drawn in Arabidopsis research that the increase of Al binding in HC1 component inhibited root elongation (Zhu et al , 2012). Meanwhile, Al 3+ binding in pectin can also inhibit plant growth (Li et al 2017; Yan et al , 2018a), interestingly, different B levels supply decreased Al content in pectin gradiently (Fig. 2e), especially in WSP and ASP (Table S1), but was not statistically significant in Al content in HC1 and HC2, except for +Al B50 treatment (Fig.…”

“…S4), which consequently decreased the carboxyl groups in pectin, thus excluded CW bound Al (Yan et al , 2018a). In addition, the role of B in promoting the demethylation of pectin is related to the surface pH (Li et al , 2017), PME activity in roots was positively correlated with apoplast pH and intracellular H + level (Denès et al , 2000; Jolie et al , 2009). The PM H + -ATPase is a proton pump, and its hydrolysis is accompanied by transmembrane H + transport (Volkmar & Hu, 1998).…”

“…Boron supply to Al-containing hydroponic nutrient media increases root elongation in higher plants such as Citrus grandis (Zhou et al . 2015), pea (Li et al . 2017), rape seedlings (Yan et al .…”

Boron supply alleviates Al-induced inhibition of root elongation and physiological characteristics in rapeseed (Brassica napusL.)

“…Hematoxylin results in the present study confirmed that roots were heavily stained with dense blue color under + Al-B treatment due to Al-hematoxylin complex as shown in Figure 2(c). It has been documented that Bdeficiency increases the concentration of un-methylated pectin in the cell wall of roots, creating higher negative charges thus producing more favorable condition for Al binding (Stass et al 2007;Li et al 2017). Moreover, the role of B has been reported in the formation of the primary cell wall by cross-linking with pectic polypectic polysaccharide RG-II (rhamnogalacturonan II).…”