ABC transporters are ubiquitously present in all kingdoms and mediate the transport of a large spectrum of structurally different compounds. Plants possess high numbers of ABC transporters in relation to other eukaryotes; the ABCG subfamily in particular is extensive. Earlier studies demonstrated that ABCG transporters are involved in important processes influencing plant fitness. This review summarizes the functions of ABCG transporters present in the model plant Arabidopsis thaliana. These transporters take part in diverse processes such as pathogen response, diffusion barrier formation, or phytohormone transport. Studies involving knockout mutations reported pleiotropic phenotypes of the mutants. In some cases, different physiological roles were assigned to the same protein. The actual transported substrate(s), however, still remain to be determined for the majority of ABCG transporters. Additionally, the proposed substrate spectrum of different ABCG proteins is not always reflected by sequence identities between ABCG members. Applying only reverse genetics is thereby insufficient to clearly identify the substrate(s). We therefore stress the importance of in vitro studies in addition to in vivo studies in order to (i) clarify the substrate identity; (ii) determine the transport characteristics including directionality; and (iii) identify dimerization partners of the half-size proteins, which might in turn affect substrate specificity.
Keywords: Arabidopsis thaliana; ABCG subfamily; plant ABC transporters; pleiotropy; promiscuity; sequence identity; substrate spectrum.
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