Defence, Symbiosis and ABCG Transporters

Abstract

Plant genomes encode more than 100 ATP-binding cassette (ABC) transporters, a number far exceeding those of other organisms. The membrane-bound ABC transporters belonging to the G subfamily (ABCGs) can be categorised by their distinctive topology and taxa distribution. ABCGs form the largest known subfamily of ABC proteins, with 43 and 50 members in Arabidopsis and rice, respectively. Collected experimental data have revealed the great functional diversity of these proteins. The substrates known to be transferred by ABCGs, usually through the plasma membrane, include surface lipids, plant hormones and secondary metabolites. Therefore, ABCGs are recognised as being important for plant development as well as interactions with the environment. Historically, certain members of the ABCG subfamily were considered as proteins that evolved to be involved in pathogenic processes or biotic stress responses. However, recent discoveries have demonstrated that the function of ABCGs in plants extends beyond simply the secretion of anti-microbial molecules. Equally important as defence against invaders are interactions of plants with microorganisms that are beneficial to both partners. Such beneficial interactions include (1) symbiotic associations with fungi of the phylum Glomeromycota, also known as arbuscular mycorrhizae (AM) and (2) legume-rhizobia symbiosis (LRS). We have only just begun to discover that plant ABC transporters are important modulators of symbioses, but how they participate in these processes is unclear. Here, we provide basic information regarding the members of the G subfamily of ABC transporters and position them in the context of the defence reactions and symbiotic associations of plants, with special emphasis on legumes.