A Defective Vacuolar Proton Pump Enhances Aluminum Tolerance by Reducing Vacuole Sequestration of Organic Acids

Abstract

Plants cope with aluminum (Al) toxicity by secreting organic acids (OAs) into the apoplastic space, which is driven by proton (H 1) pumps. Here, we show that mutation of vacuolar H 1-translocating adenosine triphosphatase (H 1-ATPase) subunit a2 (VHA-a2) and VHA-a3 of the vacuolar H 1-ATPase enhances Al resistance in Arabidopsis (Arabidopsis thaliana). vha-a2 vha-a3 mutant plants displayed less Al sensitivity with less Al accumulation in roots compared to wild-type plants when grown under excessive Al 31. Interestingly, in response to Al 31 exposure, plants showed decreased vacuolar H 1 pump activity and reduced expression of VHA-a2 and VHA-a3, which were accompanied by increased plasma membrane H 1 pump (PM H 1-ATPase) activity. Genetic analysis of plants with altered PM H 1-ATPase activity established a correlation between Al-induced increase in PM H 1-ATPase activity and enhanced Al resistance in vha-a2 vha-a3 plants. We determined that external OAs, such as malate and citrate whose secretion is driven by PM H 1-ATPase, increased with PM H 1-ATPase activity upon Al stress. On the other hand, elevated secretion of malate and citrate in vha-a2 vha-a3 root exudates appeared to be independent of OAs metabolism and tolerance of phosphate starvation but was likely related to impaired vacuolar sequestration. These results suggest that coordination of vacuolar H 1-ATPase and PM H 1-ATPase dictates the distribution of OAs into either the vacuolar lumen or the apoplastic space that, in turn, determines Al tolerance capacity in plants.

An ICln homolog contributes to osmotic and low‐nitrate tolerance by enhancing nitrate accumulation in Arabidopsis

“…Seedlings grown on the indicated medium were collected for the measurement of cation contents by inductively coupled plasma mass spectrometry (ICP‐MS), NO 3 − content by the salicylic acid nitration procedure (Cataldo, Maroon, Schrader, & Youngs, 1975) and Cl − content by the silver titration method (Chen, Zhang, Zhao, Korpelainen, & Li, 2010). Internal malate and citrate contents were detected as described by F. Zhang et al (2019). Briefly, 2 g of leaves was ground at 4°C with 10 ml of cold water, and the homogenate was filtered through four layers of cheesecloth and centrifuged at 4,950 g for 10 min.…”

Recent Advances in Understanding Mechanisms of Plant Tolerance and Response to Aluminum Toxicity

“…The transmembrane transport of OAs depends on the driving force formed by the proton electrochemical gradient on both sides of the PM, which provides continuous power for organic acid secretion [81,82]. Al stress increases the activity of PM H + -ATPase and inhibits the expression of VHA-a2, VHA-a3, and vacuolar proton pump activity, and therefore increases the proton gradient on both sides of the PM and promotes OAs secretion by AtALMT1 and AtMATE [83]. Al stress reduces the entry of OAs into vacuoles by inhibiting vacuolar proton pump activity and activating the PM transport system, which secretes large amounts of intracellular OAs to the apoplast.…”

“…Al stress reduces the entry of OAs into vacuoles by inhibiting vacuolar proton pump activity and activating the PM transport system, which secretes large amounts of intracellular OAs to the apoplast. While OAs secretion is blocked, vacuolar proton pump activity is activated, indicating that intracellular Al tolerance may be an alternative [83]. Additionally, it is known that OAs secretion depends on the maintenance of internal concentrations of OAs.…”

The role of solute transporters in aluminum toxicity and tolerance

“…The vacuolar H + ‐ATPases (V‐ATPases) also regulate Al toxicity. The mutation in VHA‐a2 and VHA‐a3 subunits of the Arabidopsis V‐ATPase enhance Al resistance (Zhang et al 2019a). The vha‐a2 vha‐a3 mutants exhibited reduced sensitivity to toxic Al concentrations and accumulated less Al in their roots (Zhang et al 2019a).…”

“…The mutation in VHA‐a2 and VHA‐a3 subunits of the Arabidopsis V‐ATPase enhance Al resistance (Zhang et al 2019a). The vha‐a2 vha‐a3 mutants exhibited reduced sensitivity to toxic Al concentrations and accumulated less Al in their roots (Zhang et al 2019a). The expression of V‐ATPases is repressed upon Al treatment, whereas there is a relative increased expression of plasma membrane H + ‐ATPase (PM H + ‐ATPase) in vha‐a2 vha‐a3 .…”