Online Friday Seminar – Flux and polarity in the plant vascular system

This week’s Friday Seminar speaker is Professor Christian Hardtke from the University of Lausanne.

The Friday Seminar series will be run on a virtual platform for the foreseeable future.

Email to request access to the recording.


Higher animals and plants are two evolutionary outcomes of complex multicellularity.

An important driver of cellular specialisation and body plan expansion in both phyla was the evolution of vascular distribution networks.

Most prominently, the cardiovascular network of mammals distributes oxygen and nutrients throughout the body, driven by the pumping action of the heart. By comparison, the vascular transport systems of the dominant group of higher plants, the angiosperms, are fundamentally different. For instance, transport of photosynthates in the phloem network is governed by dynamic source-sink relations and driven by a pressure differential that builds up through locally controlled cellular osmolarity.

The conducting phloem channels are the so-called sieve tubes, which not only transport nutrients but also developmental signals such as the phytohormone auxin.

Sieve tubes are formed from interconnected individual sieve elements that are meticulously aligned in cell files as they develop. Sieve elements represent a cellular between-life-and-death state, because they lack numerous organelles, including the nucleus, and are nurtured by their neighbouring, so-called companion cells.

The peculiar sieve element differentiation process can be observed in growth apices. For example, in the root meristem of Arabidopsis thaliana seedlings their development from stem cell into mature sieve element is laid out in a spatiotemporal gradient.

Christian’s lab has characterised a molecular network that guides this differentiation process through the interplay between controlled transcellular auxin transport across developing sieve element cell files and autocrine receptor kinase signaling pathways that respond to small peptide ligands.

Key players in this network constitute a molecular rheostat that finetunes transcellular auxin flux and maintains its pronounced cellular polarity through a self-reinforcing mechanism.

Their research suggests that this module is intimately connected to the evolution of phloem, which enabled plants to effectively colonise land and thereby had a major impact on the extant biosphere.

Professor Christian Hardtke biography

Christian S. Hardtke obtained a PhD in Developmental Biology from the Ludwig-Maximilians University of Munich in 1997, for his work on plant embryogenesis.

After a year as Postdoctoral Associate at the University of Toronto, he moved to Yale University as a Human Frontier Science Program fellow to study photomorphogenesis.

Eventually he joined McGill University as Assistant Professor to start his own lab in 2001. He was appointed Associate Professor at the University of Lausanne in 2004, where he became Full Professor in 2010 and directed the Department of Plant Molecular Biology from 2009 to 2017.

His research revolves around the molecular genetic control of plant development, with a focus on quantitative aspects of plant growth and morphology. He is particularly interested in mechanisms of vascular tissue differentiation and their relation to root system architecture.

His lab mostly investigates the dicotyledon model system Arabidopsis thaliana, but also the monocotyledon model, Brachypodium distachyon.

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