A sorghum pathogen-inducible gene predicted to encode a simple extracellular leucine-rich repeat (LRR) protein SbLRR2 was previously isolated. LRR was the only domain identified in SbLRR2 and its homologous sequences. Phylogenetic analysis revealed that they are distinct from the simple extracellular LRR proteins reported previously. Agrobacterium-mediated transient expression in tobacco leaf cells demonstrated that the SbLRR2-EYFP (enhanced yellow fluorescent protein) fusion protein was targeted to the extracellular space. Transgenic analysis of SbLRR2 revealed its role in enhancing lead [Pb(II)] tolerance in Arabidopsis. Consequently, SbLRR2-overexpressing lines were found to show alleviated Pb(II)-induced root inhibition, lower levels of Pb(II) accumulation and enhanced transcription of AtPDR12 which encodes a plasma membrane ATP-bind cassette (ABC)-type transporter formerly shown to contribute to Pb(II) detoxification. However, all the Pb(II) tolerance responses were abolished when SbLRR2 was overexpressed in an atpdr12 T-DNA insertion line. The extracellular localization of SbLRR2 was also shown to be essential for the Pb(II) phenotypes and AtPDR12 up-regulation. Taken together, SbLRR2 appears to mediate Pb(II) tolerance through the elevation of AtPDR12 expression in transgenic Arabidopsis, thus activating a glutathione-independent mechanism for detoxification. Further investigations revealed the Pb(II)-induced transcriptional activation of SbLRR2 and several highly conserved AtPDR12 homologs in sorghum seedlings, suggesting the possibility of a common molecular mechanism for Pb(II) tolerance in diverse plant species.
Keywords: ABC transporter; Arabidopsis; Lead tolerance; Simple extracellular LRR protein; Sorghum.