Abstract
ATP-binding cassette (ABC) transporters represent a large family in plants, but the functions of most of these transporters are unknown. Here we report a gene, AtSTAR1, only encoding an ATP-binding domain of a bacterial-type ABC transporter in Arabidopsis (Arabidopsis thaliana). AtSTAR1 is an ortholog of rice (Oryza sativa) OsSTAR1, which has been implicated in aluminum (Al) tolerance. Knockout of AtSTAR1 resulted in increased sensitivity to Al and earlier flowering. Unlike OsSTAR1, AtSTAR1 was expressed in both the roots and shoots and its expression was not induced by Al or other stresses. Investigation of tissue-specific localization of AtSTAR1 through beta-glucuronidase fusion revealed that AtSTAR1 was predominantly expressed at outer cell layers of root tips and developing leaves, whose localization is also different from those of OsSTAR1. However, introduction of OsSTAR1 into atstar1 mutant rescued the sensitivity of atstar1 to Al, indicating that AtSTAR1 has a similar function as OsSTAR1. Furthermore, we found that AtSTAR1 may interact with ALS3, a transmembrane-binding domain in Arabidopsis to form a complex because introduction of OsSTAR1, a functional substitute of AtSTAR1, into als3 mutant resulted in the loss of OsSTAR1 protein. All these findings indicate that AtSTAR1 is involved in the basic detoxification of Al in Arabidopsis.
Highlights
ATP-binding cassette (ABC) transporters represent a large family in plants, but the functions of most of these transporters are unknown
We found that AtSTAR1 is required for Al tolerance in Arabidopsis, but the expression patterns and localization are different from those of OsSTAR1
Similar to OsSTAR1, AtSTAR1 is characterized by conserved domains of a nucleotide-binding domain (NBD) of an ABC transporter, such as Walker A, Q loop, ABC signature, Walker B, and H motif (Fig. 1B)
Summary
A BLAST search using OsSTAR1 protein sequence resulted in identification of a unique homolog, AtSTAR1 (At1g67940), in the Arabidopsis genome. When the wild-type Arabidopsis and atstar knockout line were grown in the absence of Al, their root growth was similar (Fig. 2B). In the presence of 2 mM Cd or 0.1 mM La, the root growth inhibition caused by these metals was similar between the wild type and atstar (Fig. 3B), the difference in Al tolerance was significantly evident These results indicate that the sensitivity of atstar to Al was highly specific. Al tolerance test showed that introduction of OsSTAR1 into atstar rescued the Al sensitivity of the knockout line, being a similar tolerance level to the wild type (Fig. 6B). These results indicate that OsSTAR1 and AtSTAR1 have similar function in conferring Al tolerance. This result indicates that ALS3 might be required for the expression of OsSTAR1 protein in Arabidopsis, indirectly implicating the interaction between AtSTAR1 (OsSTAR1) and ALS3
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