Manganese Efficiency in Barley: Identification and Characterization of the Metal Ion Transporter HvIRT1

摘要

Manganese (Mn) deficiency is an important plant nutritional disorder in many parts of the world. Barley (Hordeum vulgare) genotypes differ considerably in their ability to grow in soils with low Mn²⁺ availability. Differential genotypic Mn efficiency can be attributed to differences in Mn²⁺ uptake kinetics in the low nanomolar concentration range. However, the molecular basis for these differences has not yet been clarified. We present here the identification and characterization of the first barley gene encoding a plasma membrane-localized metal transport protein able to transport Mn²⁺. The gene is designated HvIRT1 (for IRON-REGULATED TRANSPORTER1) because it belongs to the ZIP gene family and has a high similarity to rice (Oryza sativa) OsIRT1. A novel yeast uptake assay based on inductively coupled plasma-mass spectrometry analysis of 31 different metal and metalloid ions showed that the HvIRT1 protein, in addition to Mn²⁺, also transported Fe²⁺/Fe³⁺, Zn²⁺, and Cd²⁺. Both Mn and iron deficiency induced an up-regulation of HvIRT1 in two barley genotypes differing in Mn efficiency, but the expression levels in all cases were highest (up to 40%) in the Mn-efficient genotype. The higher expression of HvIRT1 correlated with an increased Mn²⁺ uptake rate. We conclude that HvIRT1 is an important component controlling Mn²⁺ uptake in barley roots and contributes to genotypic differences in Mn²⁺ uptake kinetics.