Engineering high-level aluminum tolerance in barley with the ALMT1 gene

Abstract

Acidity is a serious limitation to plant production on many of the world's agricultural soils. Toxic aluminium (Al) cations solubilized by the acidity rapidly inhibit root growth and limit subsequent uptake of water and nutrients. Recent work has shown that the ALMT1 gene of wheat (Triticum aestivum) encodes a malate transporter that is associated with malate efflux and Al tolerance. We generated transgenic barley (Hordeum vulgare) plants expressing ALMT1 and assessed their ability to exude malate and withstand Al stress. ALMT1 expression in barley conferred an Al-activated efflux of malate with properties similar to those of Al-tolerant wheat. The transgenic barley showed a high level of Al tolerance when grown in both hydroponic culture and on acid soils. These findings provide additional evidence that ALMT1 is a major Al-tolerance gene and demonstrate its ability to confer effective tolerance to acid soils through a transgenic approach in an important crop species.A cid soils cover some 40% of the Earth's arable land and represent a major limitation to plant production (1). The main constraint to plant growth on these soils is the aluminum (Al) that is solubilized by the acidity into the toxic Al 3ϩ cation. Al toxicity is manifest by inhibition of root growth resulting in poor uptake of water and nutrients (2). Plant production on acid soils can be maintained by neutralizing the acidity with lime (CaCO 3 ) and through the use of Al-tolerant plant species. Lime can take decades to correct acidity at depth, and many important crop and pasture species lack sufficient Al tolerance within their germplasm to allow effective breeding for this character. Genetic engineering provides an opportunity to enhance the Al tolerance of sensitive species through the overexpression of endogenous genes or by the expression of foreign genes. Toward this end, the Al tolerance of canola (Brassica napus) (3), Arabidopsis thaliana (4), tobacco (Nicotiana tabacum) (5), and alfalfa (Medicago sativum) (6) have been reported to be enhanced by increasing organic acid biosynthesis through overexpression of citrate synthase or malate dehydrogenase genes derived from plants or bacteria. Other strategies have sought to increase Al tolerance by overexpression of genes associated with stress responses (7-9). In some cases, manipulation of organic acid biosynthesis has led to increased secretion of organic acids from roots, and the increased Al tolerance was attributed to the ability of organic acids to chelate and detoxify Al 3ϩ . However, the increases in Al tolerance have at best been modest or, as in the case of a Psuedomonas aeuriginosa citrate synthase gene, not easily reproducible (10). Whereas an enhanced ability to secrete organic acids from roots might need to be linked to the biosynthesis of organic acids, the transport of these molecules to the external medium appears to be a rate-limiting step (11).The Al-activated efflux of organic acid anions from roots is now a well established mechanism that is proposed to be used by a range of...

AtALMT1 , which encodes a malate transporter, is identified as one of several genes critical for aluminum tolerance in Arabidopsis

“…These genes will need to be evaluated in the search for the gene or genes responsible for the Al tolerance QTL and their possible direct interaction with AtALMT1. Given the utility of wheat ALMT1 for crop improvement purposes using transgenic barley (21), developing a deeper understanding of the regulation of ALMT1-related proteins should provide additional opportunities for increasing Al tolerance in economically important plant species. 3 , and micronutrients as previously described.…”

Stress Response Versus Stress Tolerance: A Transcriptome Analysis of Two Rice Lines Contrasting in Tolerance to Phosphorus Deficiency

“…In any case, increased synthesis of LMWOAs is not necessarily indicative of a higher LMWOA exudation since the limiting step is not the plantinternal concentration of LMWOAs (synthesis) but their rate of release (Delhaize et al 2001). This release is mediated by specific efflux transporters, and their level of activity was shown to be directly linked to malate or citratemediated tolerance to excess aluminum (Delhaize et al 2004;Magalhaes et al 2007). However, putative efflux transporters were not among the root-specific genes differentially expressed between genotypes, corroborating earlier results that neither genotype exuded citrate or malate at high enough concentrations to have a significant effect on P solubilization (Wissuwa 2005).…”

The BnALMT1 Protein that is an Aluminum-Activated Malate Transporter is Localized in the Plasma Membrane

“…16,17 Heterologous expression of TaALMT1 in cultured-tobacco cells, Xenopus oocytes, and intact rice and barley plants conferred an Al-activated malate efflux and increased the Al-tolerance of tobacco suspension cells and intact barley plants. 16,18 Recently AtALMT1, one of the homologues of TaALMT1 in Arabidopsis, has been implicated in an Al-resistance mechanism which also relies on malate release. 19 In an effort to elucidate the molecular mechanism underlying the Al-induced malate efflux in rape we have cloned two homologues of the TaALMT1 gene, designated BnALMT1 and BnALMT2.…”