Christian Hardtke

  • Apr 8, 2019

Date & Location: April 8, 2019, at 4p; Room 101 Biochemistry Building

Subject: The phloem nexus of the plexus

Host: Brad Day

About the Speaker

University: University of Illinois

Abstract: Angiosperms, the extant plants that dominate terrestrial ecosystems, are characterized by extensive vascular xylem and phloem networks, which permit nutrient distribution as well as systemic coordination of physiology and development. Molecular mechanisms of vascular differentiation and their relation to plant hormone pathways are the major research focus of my lab. In this context, the development of phloem sieve elements is particularly interesting, because during their differentiation, sieve elements reduce some organelles and lose others (notably the nucleus) as they interconnect via sieve plates to form continuous sieve tubes. Thus, sieve elements represent a unique cell type that retains complex functionality in the absence of a nucleus.

A major question in our research is how the onset of the peculiar sieve element differentiation process is controlled at the molecular level? The Arabidopsis root is particularly well-suited to investigate this process, because vascular tissues are continuously formed from stem cells at its tip and phloem formation can be followed along the spatio-temporal gradient of single cell files. Over the last years we have built up an extensive molecular genetic network that governs the differentiation of sieve element precursors. This network comprises angiosperm-specific positive regulators, which drive sieve element differentiation through tipping a delicate quantitative balance, opposing negative regulators of the process. Both auxin transport regulation and brassinosteroid signaling are key aspects of the positive regulatory output, whereas CLE peptide signaling is central to the negative regulatory output.

I will present data that illustrate how these pathways intersect to guide sieve element differentiation. Our most recent results reveal crosstalk between a non-canonical brassinosteroid signaling output and CLE peptide signaling, which is crucial to permit non-cell autonomous action of a phloem-derived signal both in root and shoot organs. The data suggest that via this signal, phloem acts as an organizer tissue in plant organ formation.