Enhancing the aluminium tolerance of barley by expressing the citrate transporter genes SbMATE and FRD3

Malate and citrate efflux from root apices is a mechanism of Al³⁺ tolerance in many plant species. Citrate efflux is facilitated by members of the MATE (multidrug and toxic compound exudation) family localized to the plasma membrane of root cells. Barley (<i>Hordeum vulgare</i>) is among the most Al³⁺-sensitive cereal species but the small genotypic variation in tolerance that is present is correlated with citrate efflux via a MATE transporter named HvAACT1. This study used a biotechnological approach to increase the Al³⁺ tolerance of barley by transforming it with two MATE genes that encode citrate transporters: <i>SbMATE</i> is the major Al³⁺-tolerance gene from sorghum whereas <i>FRD3</i> is involved with Fe nutrition in <i>Arabidopsis</i>. Independent transgenic and null T3 lines were generated for both transgenes. Lines expressing <i>SbMATE</i> showed Al³⁺-activated citrate efflux from root apices and greater tolerance to Al³⁺ toxicity than nulls in hydroponic and short-term soil trials. Transgenic lines expressing <i>FRD3</i> exhibited similar phenotypes except citrate release from roots occurred constitutively. The Al³⁺ tolerance of these lines was compared with previously generated transgenic barley lines overexpressing the endogenous <i>HvAACT1</i> gene and the <i>TaALMT1</i> gene from wheat. Barley lines expressing <i>TaALMT1</i> showed significantly greater Al³⁺ tolerance than all lines expressing <i>MATE</i> genes. This study highlights the relative efficacy of different organic anion transport proteins for increasing the Al³⁺ tolerance of an important crop species.