[Seminar] Molecular control of xylem cell death and impact on plant performance

Seminar

Title: Molecular control of xylem cell death and impact on plant performance

Speaker: Hannele Tuominen

Affiliation: Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, Sweden

Abstract:

The last stage of xylem development is programmed death of the cells, followed by complete autolysis of the cell contents. Two cell types predominate in the xylem, the vessel elements and the fibres, and both of these cell types display programmed cell death (PCD), but in a very different manner [1]. However, the underlying molecular mechanism is poorly understood. Transcriptomic analyses in the woody tissues of Populus trees resulted in identification of a METACASPASE9 (MC9) as a putative new regulator of xylem cell death, and we could indeed show on the basis of a detailed electron microscopic analyses that the Arabidopsis MC9 was crucial for the rapid autolysis of the xylem vessel elements [2]. Localisation of the AtMC9 protein in cytoplasmic aggregates of late maturing vessel elements and the effect of AtMC9 on both stability and activities of papain-like cysteine proteases supported this conclusion. Additional insight was obtained in Arabidopsis tracheary element (TE) cell cultures, where suppression of AtMC9 resulted in altered cysteine protease activities during TE differentiation, increased autophagic fluxes and cell death of not only the TEs but also the non-TEs that in normal cultures rarely die [3]. These results suggest that PCD inducing signals can be derived from maturing xylem elements in a non-cell autonomous manner and that appropriate activation of the autolytic machinery by AtMC9 is critical to guarantee rapid and complete degradation of the vessel cell contents and to prevent ectopic PCD during xylem maturation. Evidence is provided that this control mechanism involves a cascade of cysteine proteases in an analogous manner to the animal caspase cascades, and that the mechanism is active also in connection to another developmental context.

From the biotechnological point of view, the xylem elements should stay alive as long as possible, since extending the lifetime of the xylem elements is expected to result in thicker cell walls and therefore higher biomass yields of wood. I will present finally two different large scale approaches involving transgenic Populus trees and association mapping in Populus tremula to identify the effect of xylem cell death on important properties of wood, such as wood density.

[1] Escamez S and Tuominen H (2014) Programs of cell death and autolysis in tracheary elements: When a suicidal cell arranges its own corpse removal. J. Exp. Bot. 65, 1313-21.

[2] Bollhöner B, Zhang B, Stael S, Denancé N, Overmyer K, Goffner D, van Breusegem F and Tuominen H (2013) Post mortem function of AtMC9 in xylem vessel elements.  New Phytologist 200, 498-510. 

[3] Escamez S, André D, Zhang B, Bollhöner B, Pesquet E, Tuominen H (2016) METACASPASE9 modulates autophagy to confine cell death to the target cells during Arabidopsis vascular xylem differentiation. Biol. Open 5, 122-9.