Summary: Phosphorus (P) is one of the essential nutrients for plants, and is indispensable for plant growth and development. P deficiency severely limits crop yield, and regular fertilizer applications are required to obtain high yields and to prevent soil degradation. To access P from the soil, plants have evolved high‐ and low‐affinity Pi transporters and the ability to induce root architectural changes to forage P. Also, adjustments of numerous cellular processes are triggered by the P starvation response, a tightly regulated process in plants. With the increasing demand for food as a result of a growing population, the demand for P fertilizer is steadily increasing. Given the high costs of fertilizers and in light of the fact that phosphate rock, the source of P fertilizer, is a finite natural resource, there is a need to enhance P fertilizer use efficiency in agricultural systems and to develop plants with enhanced Pi uptake and internal P‐use efficiency (PUE). In this review we will provide an overview of continuing relevant research and highlight different approaches towards developing crops with enhanced PUE. In this context, we will summarize our current understanding of root responses to low phosphorus conditions and will emphasize the importance of combining PUE with tolerance of other stresses, such as aluminum toxicity. Of the many genes associated with Pi deficiency, this review will focus on those that hold promise or are already at an advanced stage of testing (OsPSTOL1, AVP1, PHO1 and OsPHT1;6). Finally, an update is provided on the progress made exploring alternative technologies, such as phosphite fertilizer.

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.

Acid soils restrict plant production around the world. One of the major limitations to plant growth on acid soils is the prevalence of soluble aluminium (Al³⁺) ions which can inhibit root growth at micromolar concentrations. Species that show a natural resistance to Al³⁺ toxicity perform better on acid soils. Our understanding of the physiology of Al³⁺ resistance in important crop plants has increased greatly over the past 20 years, largely due to the application of genetics and molecular biology. Fourteen genes from seven different species are known to contribute to Al³⁺ tolerance and resistance and several additional candidates have been identified. Some of these genes account for genotypic variation within species and others do not. One mechanism of resistance which has now been identified in a range of species relies on the efflux of organic anions such as malate and citrate from roots. The genes controlling this trait are members of the ALMT and MATE families which encode membrane proteins that facilitate organic anion efflux across the plasma membrane. Identification of these and other resistance genes provides opportunities for enhancing the Al³⁺ resistance of plants by marker-assisted breeding and through biotechnology. Most attempts to enhance Al³⁺ resistance in plants with genetic engineering have targeted genes that are induced by Al³⁺ stress or that are likely to increase organic anion efflux. In the latter case, studies have either enhanced organic anion synthesis or increased organic anion transport across the plasma membrane. Recent developments in this area are summarized and the structure–function of the TaALMT1 protein from wheat is discussed.

Summary: The Arabidopsis AtMTP family of genes encode proteins of the cation diffusion facilitator (CDF) family, with several members having roles in metal tolerances. Four of the 11 proteins in the family form a distinct cluster on a phylogenetic tree and are closely related to ShMTP8, a CDF identified in the tropical legume Stylosanthes hamata that is implicated in the transport of Mn²⁺ into the vacuole as a tolerance mechanism. Of these four genes, AtMTP11 was the most highly expressed member of the Arabidopsis subgroup. When AtMTP11 was expressed in Saccharomyces cerevisiae, it conferred Mn²⁺ tolerance and transported Mn²⁺ by a proton‐antiport mechanism. A mutant of Arabidopsis with a disrupted AtMTP11 gene (mtp11) was found to have increased sensitivity to Mn²⁺ but not to Cu²⁺ or Zn²⁺. At a non‐toxic but sufficient Mn²⁺ supply (basal), the mutant accumulated more Mn²⁺ than the wild type, but did not show any obvious deleterious effects on growth. When grown with Mn²⁺ supplies that ranged from basal to toxic, the mutant accumulated Mn²⁺ concentrations in shoots similar to those in wild‐type plants, despite showing symptoms of Mn²⁺ toxicity. AtMTP11 fused to green fluorescent protein co‐localized with a reporter specific for pre‐vacuolar compartments. These findings provide evidence for Mn²⁺‐specific transport activity by AtMTP11, and implicate the pre‐vacuolar compartments in both Mn²⁺ tolerance and Mn²⁺ homeostasis mechanisms of Arabidopsis.