Transient silencing of an expansin HvEXPA1 inhibits root cell elongation and reduces Al accumulation in root cell wall of Tibetan wild barley

Acid soil and the associated aluminum (Al) toxicity are one of major abiotic stresses, costing global agriculture significant production loss in major crops such as barley. Expansins are known as cell wall loosening proteins that regulate the loosening process of plant cell wall. However, the functional relationship between expansins and Al stress in barley was still poorly understood. In this study, we functionally characterized an Al inducible expansin gene, HvEXPA1, using a range of molecular and physiological approaches. There was a significant genotypic difference in Al-induced gene expression of HvEXPA1, where both Al-tolerant genotypes of Tibetan wild barley XZ16 and cv. Dayton showed significant upregulation but not in Al-sensitive wild barley XZ61. In XZ16, a significant upregulation of HvEXPA1 is exclusively induced by Al in low pH (4.3) at root tips, but not by low pH (4.3), normal apoplastic pH (5.8) or other metals ions such as Cr or La. Subcellular localization analysis indicated that HvEXPA1 is located in the plasma membrane. Bioinformatic analysis indicated that HvEXPA1 carried 3 domains and conserved 3D structure among seven genera. Barley stripe mosaic virus-induced gene silencing (BSMV-VIGS) of HvEXPA1 led to a significant decrease in root length and root dry weight under both of control and 200 mu M Al treatments, but the inhibition in root cell length only recorded in control condition. Interestingly, reduced Al concentration via silencing of HvEXPA1 was recorded only in root cell wall but not in cell sap. Our results indicate that HvEXPA1 is an Al-inducible expansin gene, which participates in the root cell elongation and probably influences the Al content through regulating root cell wall loosening in barley.