Mechanisms and regulation of aluminum-induced secretion of organic acid anions from plant roots

Abstract

Aluminum (Al) is the most abundant metal element in the earth’s crust. On acid soils, at pH 5.5 or lower, part of insoluble Al-containing minerals become solubilized into soil solution, with resultant highly toxic effects on plant growth and development. Nevertheless, some plants have developed Al-tolerance mechanisms that enable them to counteract this Al toxicity. One such well-documented mechanism is the Al-induced secretion of organic acid anions, including citrate, malate, and oxalate, from plant roots. Once secreted, these anions chelate external Al ions, thus protecting the secreting plant from Al toxicity. Genes encoding the citrate and malate transporters responsible for secretion have been identified and characterized, and accumulating evidence indicates that regulation of the expression of these transporter genes is critical for plant Al tolerance. In this review, we outline the recent history of research into plant Al-tolerance mechanisms, with special emphasis on the physiology of Al-induced secretion of organic acid anions from plant roots. In particular, we summarize the identification of genes encoding organic acid transporters and review current understanding of genes regulating organic acid secretion. We also discuss the possible signaling pathways regulating the expression of organic acid transporter genes.

概要

铝是地壳中最丰富的金属元素。在pH 低于5.5 的酸性土壤中,部分含铝矿物中的铝会溶解进入 土壤溶液, 严重危害植物的生长和发育。一些植 物能够进化出耐铝机理以制抵抗铝毒害。其中, 铝诱导根系分泌有机酸阴离子(包括柠檬酸、苹 果酸和草酸)是证据最确凿的机理之一。分泌到 胞外的有机酸阴离子可以通过螯合作用解除铝 毒。编码铝诱导柠檬酸和苹果酸阴离子分泌的转 运蛋白基因已被鉴定。同时, 众多证据表明这些 基因的表达调控与植物耐铝性密切相关。本文概 述了近年来植物耐铝机理, 特别是铝诱导植物根 系分泌有机酸阴离子的生理机制的研究进展。重 点总结了编码有机酸转运蛋白基因的鉴定, 以及 对这些基因表达调控的理解。本文也对调控有机 酸转运蛋白基因表达的可能的信号通路作了讨 论, 并提出了该领域的研究展望。