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:
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.
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.
Yeon Hee Kang, Alice S. Breda and Christian S. Hardtke (2017): Brassinosteroid signaling directs formative cell divisions and protophloem differentiation in Arabidopsis root meristems. Development, Vol. 144: pp. 272-280.
David Pacheco-Villalobos, Sara M. Díaz-Moreno, Alja van der Schuren, Takayuki Tamaki, Yeon Hee Kang, Bojan Gujas, Ondrej Novak, Nina Jaspert, Zhenni Li, Sebastian Wolf, Claudia Oecking, Karin Ljung, Vincent Bulone, and Christian S. Hardtke (2016): The Effects of High Steady State Auxin Levels on Root Cell Elongation in Brachypodium. The Plant Cell, Vo. 28: pp. 1009-1024.
Antia Rodriguez-Villalon, Bojan Gujas, Ringo van Wijk, Teun Munnik and Christian S. Hardtke (2015): Primary root protophloem differentiation requires balanced phosphatidylinositol-4,5-biphosphate levels and systemically affects root branching. Development, doi: 10.1242/dev.118364.
Antia Rodriguez-Villalon, Bojan Gujas, Yeon Hee Kang, Alice Breda, Pietro Cattaneo, Stephen Depuydt and Christian S. Hardtke (2014): A Molecular Genetic Framework For Protophloem Formation. Proceedings of the National Academy of Sciences U.S.A., Vol. 111: pp. 11551-11556.
Stephen Depuydt, Antia Rodriguez-Villalon, Luca Santuari, Céline Wyser-Rmili, Laura Ragni and Christian S. Hardtke (2013): Suppression of protophloem differentiation and root meristem growth in Arabidopsis by CLE45 requires the receptor-like kinase BAM3. Proceedings of the National Academy of Sciences U.S.A., Vol. 110: pp. 7074-7079.
David Pacheco-Villalobos, Martial Sankar, Karin Ljung and Christian S. Hardtke (2013): Disturbed local auxin homeostasis enhances cellular anisotropy and reveals alternative wiring of auxin-ethylene crosstalk in Brachypodium distachyon seminal roots. PLoS Genetics, doi:10.1371/journal.pgen.1003564.
Luca Santuari, Emanuele Scacchi, Antia Rodriguez-Villalon, Paula Salinas, Esther M.N. Dohmann, Geraldine Brunoud, Teva Vernoux, Richard S. Smith and Christian S. Hardtke (2011): Positional information by differential endocytosis splits auxin response to drive Arabidopsis root meristem growth. Current Biology, Vol. 21: pp. 1918-1923.
Emanuele Scacchi, Paula Salinas, Bojan Gujas, Luca Santuari, Naden Krogan, Laura Ragni, Thomas Berleth and Christian S. Hardtke (2010): Spatio-temporal sequence of cross-regulatory events in root meristem growth. Proceedings of the National Academy of Sciences U.S.A., Vol. 107: pp. 22734-22739.
Emanuele Scacchi, Karen S. Osmont, Julien Beuchat, Paula Salinas, Marisa Navarrete-Gómez, Marina Trigueros, Cristina Ferrándiz and Christian S. Hardtke (2009): Dynamic, auxin-responsive plasma membrane to nucleus movement of Arabidopsis BRX. Development, Vol. 136: pp. 2059-2067.
Richard Sibout, Poornima Sukuma, Chamari Hettiarachchi, Magnus Holm, Gloria K. Muday and Christian S. Hardtke (2006): Opposite root growth phenotypes of hy5 vs. hy5 hyh mutants correlate with increased constitutive auxin signaling. PLoS Genetics, Vol. 2: pp. 1898-1911, e202.