Molecular and Physiological Analysis of Al3+ and H+ Rhizotoxicities at Moderately Acidic Conditions    

Abstract

Al3+ and H + toxicities predicted to occur at moderately acidic conditions (pH [water] = 5-5.5) in low-Ca soils were characterized by the combined approaches of computational modeling of electrostatic interactions of ions at the root plasma membrane (PM) surface and molecular/physiological analyses in Arabidopsis (Arabidopsis thaliana). Root growth inhibition in known hypersensitive mutants was correlated with computed {Al 3+ } at the PM surface ({Al 3+ } PM ); inhibition was alleviated by increased Ca, which also reduced {Al 3+ } PM and correlated with cellular Al responses based on expression analysis of genes that are markers for Al stress. The Al-inducible Al tolerance genes ALUMINUM-ACTIVATED MALATE TRANSPORTER1 and ALUMINUM SENSITIVE3 were induced by levels of {Al 3+ } PM too low to inhibit root growth in tolerant genotypes, indicating that protective responses are triggered when {Al 3+ } PM was below levels that can initiate injury. Modeling of the H + sensitivity of the SENSITIVE TO PROTON RHIZOTOXICITY1 knockout mutant identified a Ca alleviation mechanism of H + rhizotoxicity, possibly involving stabilization of the cell wall. The phosphatidate phosphohydrolase1 (pah1) pah2 double mutant showed enhanced Al susceptibility under low-P conditions, where greater levels of negatively charged phospholipids in the PM occur, which increases {Al 3+ } PM through increased PM surface negativity compared with wild-type plants. Finally, we found that the nonalkalinizing Ca fertilizer gypsum improved the tolerance of the sensitive genotypes in moderately acidic soils. These findings fit our modeling predictions that root toxicity to Al 3+ and H + in moderately acidic soils involves interactions between both toxic ions in relation to Ca alleviation.

Under phosphate starvation conditions, Fe and Al trigger accumulation of the transcription factor STOP1 in the nucleus of Arabidopsis root cells

“…A transcriptomic study of seedlings grown in an acidic environment showed that the stop1 mutant is defective for the expression of several genes, including genes involved in cell wall composition or modeling (Sawaki et al ., ). Furthermore, physiological studies suggested that stop1 seedlings are defective in a mechanism alleviating H + toxicity, perhaps via Ca 2+ stabilization of the cell wall (Kobayashi et al ., ). It is therefore tempting to speculate that even during WT embryogenesis STOP1 is participating in the making of root cells with a cell wall able to tolerate H + a few days after germination, and in stop1 seeds the root cell walls are somehow altered, leading to reduced root growth under acidic conditions.…”

The interaction of salicylic acid and Ca2+ alleviates aluminum toxicity in soybean (Glycine max L.)

“…In Arabidopsis, Ca 2þ can reduce the concentration of Al 3þ in the plasma membrane to alleviate Al toxicity (Kobayashi et al, 2013). In wheat roots, the concentration of Al 3þ in plasma membrane of cells Fig.…”

External hyphae of Rhizophagus irregularis DAOM 197198 are less sensitive to low pH than roots in arbuscular mycorrhizae: evidence from axenic culture system

“…In most acidic soils, the major constraints of crop production include proton toxicity (low pH), Al 31 toxicity and cation deficiency (Muthukumar et al, 2014). Although proton toxicity differs from Al 31 toxicity in terms of mechanisms and symptoms (Kobayashi et al, 2013;Bose et al, 2015;Fan et al, 2015;Wehr et al, 2016), resistance to proton toxicity is regarded as the prerequisite for resistance to Al 31 toxicity (Rangel et al, 2005;Yang et al, 2005), highlighting the importance of proton toxicity resistance in adaptation to acidic soils. In addition to direct effects, low pH can reduce plant growth via indirect effects on nutrient availability, such as P. In acidic soils, soluble P is readily precipitated by Fe and Al or absorbed onto oxide surfaces, resulting in a low availability of P (Lee et al, 1990;Zhu et al, 2009).…”