Muñiz L, Minguet EG, Singh SK, Pesquet E, Vera-Sirera F, Moreau-Courtois CL, Carbonell J, Blázquez MA, Tuominen H
ACAULIS5 controls Arabidopsis xylem specification through the prevention of premature cell death
Development: 2008 135:2573-2582

Cell size and secondary cell wall patterning are crucial forthe proper functioning of xylem vessel elements in the vasculartissues of plants. Through detailed anatomical characterizationof Arabidopsis thaliana hypocotyls, we observed that mutationsin the putative spermine biosynthetic gene ACL5 severely affectedxylem specification: the xylem vessel elements of the acl5 mutantwere small and mainly of the spiral type, and the normally predominantpitted vessels as well as the xylem fibers were completely missing.The cell-specific expression of ACL5 in the early developingvessel elements, as detected by in situ hybridization and reporter geneanalyses, suggested that the observed xylem vessel defects werecaused directly by the acl5 mutation. Exogenous spermine prolongedxylem element differentiation and stimulated cell expansionand cell wall elaboration in xylogenic cell cultures of Zinniaelegans, suggesting that ACL5 prevents premature death of thedeveloping vessel elements to allow complete expansion and secondarycell wall patterning. This was further supported by our observationsthat the vessel elements of acl5 seemed to initiate the celldeath program too early and that the xylem defects associatedwith acl5 could be largely phenocopied by induction of premature,diphtheria toxin-mediated cell death in the ACL5-expressingvessel elements. We therefore provide, for the first time, mechanisticevidence for the function of ACL5 in xylem specification throughits action on the duration of xylem element differentiation.

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