Mitochondrial enzymes and citrate transporter contribute to the aluminium-induced citrate secretion from soybean (Glycine max) roots

Abstract

The concentration of soluble aluminium (Al) in the soil solution increases at low pH and the prevalence of toxic Al3+ cations represent the main factor limiting plant growth on acid soils. Citrate secretion from roots is an important Al-tolerance mechanism in many species including soybean. We isolated mitochondria from the roots of an Al-resistant soybean (Glycine max L.) cv. Jiyu 70 to investigate the relationship between citrate metabolism and Al-induced citrate secretion. Spectrophotometric assays revealed that the activities of mitochondrial malate dehydrogenase and citrate synthase increased and aconitase decreased with increasing of Al concentration (0–50 µM) and duration of Al treatment (30 µM Al, 0.5–9 h). Al-induced citrate secretion was inhibited by the citrate synthase inhibitor suramin, and enhanced by the aconitase inhibitor fluorocitric acid. Mersalyl acid, an inhibitor of a citrate carrier located in mitochondria membrane, also suppressed Al-induced citrate secretion. Transcript level of the mitochondrial citrate synthase gene increased in soybean roots exposed to Al, whereas expression of aconitase showed no significant difference. Expression of Gm-AlCT, a gene showing homology to Al-activated citrate transporters was also induced after 4 h in Al treatment. The Al-dependent changes in activity and expression of these enzymes are consistent with them supporting the sustained release of citrate from soybean roots.

Salicylic acid involved in the process of aluminum induced citrate exudation in Glycine max L.

“…Furthermore, we found both citrate metabolism and citrate transporters might contribute to the Al induced citrate secretion in soybean (Xu et al 2010). Notably, Al significantly decreased the activity of ACO, but has no obvious effect on its transcription abundance.…”

“…Relative gene expression was calculated using the comparative ΔΔC T method according to Livak and Schmittgen (2001). For the normalization of gene expression, β-tubulin was used as an internal standard according to Xu et al (2010), and GeneBank accession number is GMU12286. Upstream primer 5′-GGAAGGCTTTCTTGCATTGGTA-3′ and downstream primer 5′-AGTGGCATCCTGGTACTGC-3′, the amplification size of the product was 116 bp.…”

“…Mitochondria were isolated by the conventional differential centrifugation method from the root tips (Moore and Beevers 1974) with some modifications. The specific method of mitochondria isolation is described in detail in Xu et al (2010). The activity of ACO in the extracted mitochondria was measured spectrophotometrically by following the production of cisaconitate in absorbance at 240 nm according to Villafranca and Mildvan (1971).…”

Aluminium tolerance in the tropical leguminous N2-fixing shrub Acaciella angustissima (Mill.) Britton & Rose inoculated with Sinorhizobium mexicanum

“…The root hairs are important because they excrete chelating substances that prevent Al 3+ ions from entering the plant. Root tips of soybean (Glycine max L.) release citrate, while wheat (Triticum aestivum L.) malate (Kinraide & Hagerman 2010, Xu et al 2010. Flavonoids such as catechin that protect against toxicity of the Al 3+ (Nguyen et al 2003).…”

Foreword: Understanding how plants cope with acid soils

“…While both genes are predominantly expressed in the roots, ScFRDL2 shows all the hallmarks of contributing to aluminium resistance while ScFRDL1 behaves similarly to FRD3 in Arabidopsis and OsFRDL1 in rice (Yokosho et al 2009), and is likely to be involved in long distance transport of iron to the shoots. The paper by Xu et al (2010) describes how changes in mitochondrial citrate metabolism contribute to the aluminiuminduced secretion of citrate from soybean (Glycine max L.) roots. Two final papers examine the relationship between root morphology and phosphorus acquisition in maize and soybean.…”