Abstract:
In plants, members of the B-subfamily of ABC (ATP-Binding Cassette) transporters (ABCBs) were shown to exhibit extraordinarily high substrate specificity for a few auxinic compounds. However, recent evidence that a few AtABCBs can also transport brassinosteroids has expanded the functional scope of these transporters and suggests that ABCBs act as integrators of multiple hormonal pathways rather than bring auxin-specific.

In my talk, I will present recent examples from my lab that support the idea that a single plant ABCB transporter can transport multiple, diverse substrates in vivo. To describe this phenomenon, I propose the term 'substrate multi-specificity', in contrast to the term 'multidrug resistance' (MDR) that has been used to describe yeast and mammalian ABCs. Putative advantages and regulatory mechanisms of substrate multi-specificity will be outlined.

Furthermore, I will provide an overview of the evolution of Auxin-Transporting ABCB (ATAs) and their master regulator, the FKBP42, TWISTED DWARF1 (TWD1). Our findings suggest that auxin-transporting ABCBs were the first auxin transporters but that regulation by FKBPs, such as TWISTED DWARF1, came much later in evolution. In summary, our data suggest that the ABCB-FKBP module has co-evolved to meet the demands of substrate multi-specificity.

Bio:
Markus Geisler began his academic career with a PhD at the Heinrich-Heine University Düsseldorf (Germany), where he developed a strong foundation in membrane transport processes. His doctoral work on a cyanobacterial calcium ATPases established his long-term interest in transport.

He continued his research as a postdoctoral fellow at the University of Copenhagen in the group of Mickey Palmgren, expanding his expertise in biochemical and cell biological approaches to plant transport systems. During this period, he deepened his focus on calcium pumps.

He subsequently joined the lab of Enrico Martinoia at the University of Zurich, where he established an independent research profile centered on ATP-binding cassette (ABC) transporters inviolved in auxin transport, and the regulation of plant development. His work significantly contributed to understanding the directionality and mechanistic regulation of ABCG and ABCB transporters in Arabidopsis, combining structural, biochemical, and in vivo approaches.

He is currently based at the University of Fribourg, where his research integrates molecular transport biology with cellular biology in respect to hormone-mediated growth regulation. His group’s work bridges biochemistry with structural biology, plant physiology, and evolutionary perspectives on plant hormone transport.