Recent findings on the effect of aluminium (Al) on the functioning of legumes and their associated microsymbionts are reviewed here. Al represents 7% of solid matter in the Earth’s crust and is an important abiotic factor that alters microbial and plant functioning at very early stages. The trivalent Al (Al3+) dominates at pH < 5 in soils and becomes a constraint to legume productivity through its lethal effect on rhizobia, the host plant and their interaction. Al3+ has lethal effects on many aspects of the rhizobia/legume symbiosis, which include a decrease in root elongation and root hair formation, lowered soil rhizobial population, and suppression of nitrogen metabolism involving nitrate reduction, nitrite reduction, nitrogenase activity and the functioning of uptake of hydrogenases (Hup), ultimately impairing the N2 fixation process. At the molecular level, Al is known to suppress the expression of nodulation genes in symbiotic rhizobia, as well as the induction of genes for the formation of hexokinase, phosphodiesterase, phosphooxidase and acid/alkaline phosphatase. Al toxicity can also induce the accumulation of reactive oxygen species and callose, in addition to lipoperoxidation in the legume root elongation zone. Al tolerance in plants can be achieved through over-expression of citrate synthase gene in roots and/or the synthesis and release of organic acids that reverse Al-induced changes in proteins, as well as metabolic regulation by plant-secreted microRNAs. In contrast, Al tolerance in symbiotic rhizobia is attained via the production of exopolysaccharides, the synthesis of siderophores that reduce Al uptake, induction of efflux pumps resistant to heavy metals and the expression of metal-inducible (dmeRF) gene clusters in symbiotic Rhizobiaceae. In soils, Al toxicity is usually ameliorated through liming, organic matter supply and use of Al-tolerant species. Our current understanding of crop productivity in high Al soils suggests that a much greater future accumulation of Al is likely to occur in agricultural soils globally if crop irrigation is increased under a changing climate.

中文翻译：

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共生豆类和根瘤菌中铝毒性、耐受性和改善的性质和机制

本文综述了关于铝 (Al) 对豆类及其相关微共生体功能影响的最新发现。铝占地壳固体物质的 7%，是一种重要的非生物因素，可在非常早期阶段改变微生物和植物的功能。三价铝 (Al3+) 在 pH < 5 的土壤中占主导地位，并通过其对根瘤菌、寄主植物及其相互作用的致命影响成为豆科植物生产力的制约因素。Al3+ 对根瘤菌/豆科植物共生的许多方面具有致死作用，包括降低根伸长率和根毛形成、降低土壤根瘤菌数量，以及抑制涉及硝酸盐还原、亚硝酸盐还原、固氮酶活性和吸收功能的氮代谢氢化酶 (Hup)，最终损害 N2 固定过程。在分子水平上，已知 Al 抑制共生根瘤菌中结瘤基因的表达，以及诱导形成己糖激酶、磷酸二酯酶、磷酸氧化酶和酸/碱性磷酸酶的基因。除了豆科植物根伸长区的脂质过氧化作用外，铝毒性还可诱导活性氧和胼胝质的积累。植物对铝的耐受性可以通过根中柠檬酸合酶基因的过度表达和/或有机酸的合成和释放来实现，这些有机酸可以逆转铝诱导的蛋白质变化，以及植物分泌的 microRNA 的代谢调节。相比之下，共生根瘤菌对铝的耐受性是通过胞外多糖的产生、减少铝吸收的铁载体的合成来实现的，诱导对重金属具有抗性的外排泵和共生根瘤菌科中金属诱导 (dmeRF) 基因簇的表达。在土壤中，铝毒性通常通过石灰、有机物质供应和使用耐铝物种来改善。我们目前对高铝土壤作物生产力的理解表明，如果在气候变化的情况下增加作物灌溉，未来全球农业土壤中可能会发生更大的铝积累。