Phloem Differentiation & Hormones – HARDTKE LAB – Molecular Genetics of Root Development

Vascular differentiation and its relation to plant hormone pathways has become a major focus of our lab over the years. The importance of the vascular system for plant development cannot be overstated. Its evolution enabled plants to effectively colonize land, which has had a long-lasting impact that shaped earth history and the extant biosphere. Land plants are autotroph organisms that extract inorganic nutrients from the soil and carbon dioxide from the atmosphere to build up organic biomass, thereby converting the energy harvested by photosynthesis from sunlight. Nutrients, carbon dioxide, light and water all limit plant productivity, either because they constitute primary substrates for photosynthesis reactions, or because they are needed as co-factors or integral components of the photosynthetic machinery. Competition for these resources and their fluctuations over geological time has driven plant evolution towards the elaborate shoot and root systems characteristic of the higher plants that dominate the land biosphere today. Their vascular tissues allowed this body plan expansion, because they permit the long distance separation of the location of water and nutrient acquisition from the location of photosynthesis. At the heart of the vasculature, xylem transports water and inorganic nutrients absorbed by the root system to aboveground organs, whereas phloem distributes photosynthetic and other organic metabolites throughout the plant. The two tissues always develop in close proximity to one another, because water exchange between xylem and phloem is necessary to allow bulk transport of phloem sap, even against the water potential gradient. Xylem and phloem are continuously formed during organ formation from the stem cell niches in plant meristems, the growth apices of plant body axes. For example, in primary roots of Arabidopsis thaliana (Arabidopsis), the vascular tissues are formed from stem cells located at the root tip. Because xylem as well as phloem formation can be followed along the spatio-temporal gradient of single cell files by confocal microscopy in the Arabidopsis root, it is a particularly well-suited system to investigate this process. We mostly focus on the development of the phloem sap conduits, which are formed by connected sieve elements. Surprisingly little is known about the molecular-genetic control of sieve element development, a differentiation process that encompasses major cellular rearrangments, such as cell elongation, wall thickening and enucleation. Pertinent publications on this topic include the following:

Ana Cecilia Aliaga Fandino and Christian S. Hardtke* (2022): Auxin transport in developing protophloem: a case study in canalization. Journal of Plant Physiology, DOI:10.1016/j.jplph.2021.153594.

Moritz Graeff and Christian S. Hardtke (2021): Metaphloem development in the Arabidopsis root tip.Development, DOI:10.1242/dev.199766.

Samuel W. H. Koh, Petra Marhava, Surbhi Rana, Alina Graf, Bernard Moret, Alkistis E. L. Bassukas, Melina Zourelidou, Martina Kolb, Ulrich Z. Hammes, Claus Schwechheimer and Christian S. Hardtke (2021): Mapping and engineering of auxin-induced plasma-membrane-dissociation in BRX family proteins. The Plant Cell, DOI:10.1093/plcell/koab076.

Bernard Moret, Petra Marhava, Ana Cecilia Aliaga Fandino, Christian S. Hardtke* and Kirsten ten Tusscher* (2020): Local auxin competition explains fragmented differentiation patterns. Nature Communications, doi.org/10.1038/s41467-020-16803-7.

Moritz Graeff^†, Surbhi Rana^†, Petra Marhava, Bernard Moret and Christian S. Hardtke (2020): Local and systemic effects of brassinosteroid perception in developing phloem. Current Biology, Vol. 30: pp. 1626-1638.

Petra Marhava, Ana Cecilia Aliaga Fandino, Samuel W.H. Koh, Adriana Jelinkova, Martina Kolb, Dorina P. Janacek, Alice Breda, Pietro Cattaneo, Ulrich Z. Hammes, Jan Petrasek and Christian S. Hardtke (2020): Plasma membrane domain patterning and self-reinforcing polarity in Arabidopsis. Developmental Cell, doi.org/10.1016/j.devcel.2019.11.015.

Alice S. Breda^†, Ora Hazak^†, Patrick Schultz^†, Pauline Anne, Moritz Graeff, Rüdiger Simon and Christian S. Hardtke (2019): A cellular insulator against CLE45 peptide signaling. Current Biology, Vol. 29: pp. 2501-2508.

Petra Marhava, A. E. Lanassa Bassukas, Melina Zourelidou, Martina Kolb, Bernard Moret, Astrid Fastner, Waltraud X. Schulze, Pietro Cattaneo, Ulrich Z. Hammes, Claus Schwechheimer^* and Christian S. Hardtke^*(2018): A molecular rheostat adjusts auxin flux to promote root protophloem differentiation. Nature, Vol. 558: pp. 297-300.

Pauline Anne, Amelia Amiguet-Vercher, Benjamin Brandt, Lothar Kalmbach, Niko Geldner, Michael Hothorn and Christian S. Hardtke(2018): CLERK is a novel receptor kinase required for sensing of root-active CLE peptides in Arabidopsis. Development, Vol. 145: pp. dev.162354.

Pauline Anne and Christian S. Hardtke(2018): Phloem function and developmen — biophysics meets genetics. Current Opinion in Plant Biology, Vol. 43: pp. 22-28.

Alice S. Breda, Ora Hazak and Christian S. Hardtke (2017): Phosphosite charge rather than shootward localization determines OCTOPUS activity in root protophloem. Proceedings of the National Academy of Sciences U.S.A., Vol. 114: pp. E5721–E5730.

Ora Hazak^†, Benjamin Brandt^†, Pietro Cattaneo, Julia Santiago, Antia Rodriguez-Villalon, Michael Hothorn^* and Christian S. Hardtke^* (2017): Perception of root-active CLE peptides requires CORYNE function in the phloem vasculature. EMBO Reports, Vol. 18: pp. 1367-1381.