Research interests PD Dr. Ulrich Hammes

The transport of hormones and metabolites across cellular membranes are crucial for the physiology of cells, tissues and organs of plants and animals. A multitude of transport proteins is devoted to the transport of hormones, metabolites, macro- and micronutrients and other substrates.

Despite the similarities of the underlying biophysical principles among the transport processes in plants and animals, there are also fundamental differences. For example, plants have a much more negative membrane potential across the plasma membrane and consequently a much higher driving force for the transport of charge substrates. Further, symport or antiport processes across plant plasmamembrane are coupled to protons – not to sodium like in animals.

The Hammes lab is interested in understanding the transport mechanism, the biophysical and biochemical properties, and kinetics of hormone and metabolite transporters from plants. Our main focus are transporters catalyzing the transport of the phytohormone auxin or of amino acids across the plasmamembrane or endogenous membranes. We use these results to understand the molecular mechanism of physiological processes, e.g., growth responses to environmental stimuli (light, gravity) or the distribution of nutrients, primarily amino acids, in the plant body.

To address these goals, we use a combination of flux studies using Xenopus leavis oocytes as heterologous expression system, electrophysiology (two electrode voltage clamp, solid supported membrane-based electrophysiology) and plant growth assays and phenotyping.

Our objective is to use these insights to eventually be able to improve plants to be more resilient to stresses and to increase plant yield quantity and quality.

Previous results

• We identified the UmamiT family of amino acid exporters and characterized several members of this family to demonstrate their role in the amino acid supply of seeds and amino acid cycling (Müller et al., 2015, Tegeder and Hammes 2018, The et al., 2023). 
• In collaboration with the Schwechheimer lab, we demonstrated that the PIN-FORMED auxin exporters are activated by phosphorylation of their cytoplasmic loop domain by several AGCVIII kinases (Zourelidou et al., 2014, Weller et al., 2017, Marhava et al., 2018).
• Together with the Pedersen group in Aarhus, we determined the first structure and performed a detailed characterization of the transport mechanism of PIN8, a member if the PIN family of auxin transporters (Ung et al., 2022).

Research goals

• We want to understand how amino acid importers and exporters act together to distribute amino acids between shoots and roots and ultimately seeds to ensure the adequate supply of all tissues with reduced nitrogen during plant growth and development.
• We want to characterize auxin transporters that catalyze the import of auxin into cells or across endogenous membrane to understand how the compartmentalization of auxin impacts the cytosolic levels of auxin that ultimately is the decisive criterion for the recognition of the hormone by its receptor that subsequently controls auxin responses on the transcriptional level.
• We want to understand of substrate recognition works in PINs and other auxin transporters and identify amino acids that are critical for substrate recognition in order to generate plants that show different responses to auxin herbicides and perhaps lead to a new generation of novel, environmentally safe herbicides.
• We want to understand how the individual domains of the PIN auxin exporters work together to bring about the individual properties of the transporters to gain deeper insight into the molecular mechanism of polar auxin transport.