Advances in understanding the functions of plant ABCG transporters

Biomembranes serve as barriers that separate organelles and cells. Despite the divisions, a significant number of signals and substrates must be transported across biomembranes during plant metabolism and responses to various abiotic and biotic stresses. In plants, transmembrane transportations are mostly facilitated by two main types of transporters: Carrier proteins and channel proteins. Among these transporters, the ATP binding cassette (ABC) transporter family, a large and multifunctional group, plays a crucial role in mediating the transmembrane transportation of diverse substances. The ABC transporter family can be further grouped into eight subfamilies: ABCA to ABCG and ABCI. Notably, the ABCG subfamily stands out as the largest, and displays heightened higher functional diversity in the ABC transporter family. In plants, only two types of ABCG transporters have been identified: The full-size pleiotropic drug resistance (PDR) transporters and the half-size white-brown complex (WBC) transporters. PDR transporters are equipped with two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs), and can independently carry out substrate transportation. Conversely, WBC transporters possess only a single NBD and TMD, requiring the formation of homodimers or heterodimers for substrate transportation. There is a greater abundance of ABCG transporters as compared to other eukaryotes, suggesting their diverse range of important roles in plant biology. Indeed, many studies have shown that plant ABCG transporters play key roles in many aspects of plant development, including stomata opening, hormone transportation (e.g., cytokinin, abscisic acid, jasmonic acid, and strigolactones), sporopollenin transport, cuticle synthesis precursor exportation, and transportation of secondary metabolites such as tonquinol, ginsenosides, ss-caryophyllene, and capsaicin. Furthermore, plant ABCG transporters are widely involved in plant responses to abiotic stresses such as heavy metal (e.g. aluminum, cadmium, copper, and lanthanum) resistance by facilitating efflux of heavy metals, drought resistance, osmotic stress, and high-temperature resistance through abscisic acid transportation and formation of thicker cuticle. They are also known to regulate abscisic acid transportation, cuticle thickness, as well as kanamycin resistance through antibiotic sequestration into the vacuole of plant cell. Plant ABCG transporters are known to play roles in biotic resistance against pathogens and pests by regulating jasmonic acid, salicylic acid, and anti-pathogen metabolites transportation and accumulation. They play roles in allelopathy by mediating chemical compound secretion into the environment. This review presents a comprehensive discussion of recent progress in the structures, classifications, physiological functions, and roles of plant ABCG transporters. Current research advances highlight the crucial roles of ABCG transporters not only in plant growth and development but also in plant resistance to diverse abiotic and biotic stresses. We conclude by proposing five important research trends for future studies on ABCG transporters and highlighting the potential use of plant molecular breeding technology to generate new plant germplasm with enhanced characteristics by regulating the expression of ABCG transporter genes.