Identification of a bacterial-type ATP-binding cassette transporter implicated in aluminum tolerance in sweet sorghum (Sorghum bicolor L.)

Aluminum (Al) toxicity in acidic soils severely reduces crop production worldwide. Sorghum (Sorghum bicolor L.) is an important agricultural crop widely grown in tropical and subtropical regions, where Al toxicity is prevalent. ATP-binding cassette (ABC) transporters play key roles in the development of plants and include the member sensitive to aluminum rhizotoxicity 1 (STAR1), which is reported to be associated with Al tolerance in a few plant species. However, a STAR1 homolog has not been characterized in sorghum with respect to Al tolerance. Here, we identified and characterized a SbSTAR1 gene in sweet sorghum encoding the nucleotide-binding domain of a bacterial-type ABC transporter. The transcriptional expression of SbSTAR1 is induced by Al in a time- and dosage-dependent manner in root, especially in root tip, which is the key site of Al toxicity in plants. The typical Al-associated transcription factor SbSTOP1 showed transcriptional regulation of SbSTAR1. SbSTAR1 was present at both the cytoplasm and nuclei. Overexpression of SbSTAR1 significantly enhanced the Al tolerance of transgenic plants, which possibly via regulating the hemicellulose content in root cell wall. This study provides the first ABC protein in sorghum implicated in Al tolerance, suggesting the existence of a SbSTAR1-mediated Al tolerance mechanism in sorghum.