Date: 19th November 2018
Title: Emerging understanding on the mechanisms and significance of autolysis during cell death of Arabidopsis tracheary elements
Location: Viikki B building, seminar room B4, Latokartanonkaari 7
Host: Anna Kärkönen
Abstract: Xylem differentiation culminates in cell death followed by complete autolysis of the cells. I will first of all summarize our current knowledge on cell morphology, cell wall chemistry and autolytic processes occurring during tracheary element (TE) and fiber cell death. Thereafter, I will focus on the Arabidopsis thaliana cysteine protease METACASPASE9 (AtMC9) that is crucial for the rapid autolysis of the tracheary elements. Suppression of AtMC9 expression in Arabidopsis tracheary element (TE) cell cultures resulted in altered cysteine protease activities during TE differentiation, increased autophagic fluxes and cell death of not only the TEs but also ectopically in the non-TEs that normally rarely die. Supression of autophagic fluxes in these cultures by transgenic means reduced the ectopic cell death in the non-TEs. These results suggest that cell death inducing signals can be derived from maturing xylem elements in a non-cell autonomous manner and that appropriate activation of the autolytic machinery as well as suppression of autophagy by AtMC9 is critical to guarantee rapid and complete degradation of the TE contents and to prevent ectopic cell death 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. Further understanding to the underlying molecular mechanism is provided by identification of a peptide that is present in the culture medium of the AtMC9 supressed TE cell cultures, and that is able to induce cell death when applied to the wild type TE cell cultures or to whole seedlings. It seems therefore plausible that this pro-death peptide needs to be maintained in the TEs by the action of AtMC9 to prevent spreading of the cell death to the neighbouring cells.
Hannele Tuominin is an associate professor at the Umeå University where she has worked since leaving the University of Helsinki in 2001.