ABCG transporters in phytohormone dynamics: mechanisms, challenges, and future perspectives

Phytohormones are low-concentration signaling molecules synthesized under controlled conditions. They can exert their effects beyond their site of synthesis to modulate a wide range of developmental processes and responses to environmental stimuli, thereby highlighting the necessity of their transport. ATP-binding cassette (ABC) transporters are ubiquitous in bacteria, animals, and plants, facilitating the transport of a wide array of substrates. Among them, ABCG transporters constitute the largest subfamily in plants and exhibit the broadest spectrum of hormone substrates. Given their prevalence and crucial role in facilitating the transport of multiple phytohormones, this review specifically spotlights ABCG transporters. These ABCG transporters hold a pivotal position in orchestrating both intercellular and long-distance transport of plant hormones. They are instrumental in the movement of various hormones, such as abscisic acid, strigolactones, cytokinins, jasmonic acid, and auxin precursors. Here, we provide a comprehensive summary of all currently characterized ABCG transporters known to mediate phytohormone transport, covering their transport mechanisms, tissue-specific expression, functional redundancy, and subcellular compartmentalization. Additionally, this review delves into pivotal challenges confronting the field, including the identification of substrates, functional analysis in non-model species, and methodological approaches for probing long-distance transport.