Page 265 - PC2019 Program & Proceedings

PLANT CANADA 2019

P103. Regulation of cell size in A. thaliana shoot apical meristem
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Echevin, E. ; A. Routier-Kierzkowska; P. Belska; D. Kierzkowski
Institut de Recherche en Biologie Végétale - Université de Montréal

The size and shape of individual cells can be remarkably constant for a given organ type, suggesting that
cell geometry is an important biological factor. In growing organs, such as the shoot apical meristem, a
delicate balance must be established between cell expansion and cell division to maintain cell size
homeostasis. We used time-lapse confocal data to monitor division and expansion of individual cells in
the shoot apical meristem. Next, we quantified cellular parameters and the expression of marker genes
with the 3D image analysis software MorphoGraphX. With the help of Python scripts, we could compare
cell dynamics between different regions of the meristem and across individuals. Preliminary results
already reveal some of the complex relationships between cell size, cell expansion and cell division.

Emilie Echevin (emilie.eglantine.echevin@umontreal.ca)

P104. A feedback loop modulates root apical meristem development
Halat, L.; J. Rever; M. Law; Wasteneys, G.
The University of British Columbia

Modulating root growth in response to environmental signals enables plants to compete for nutrients or
survive abiotic stress. Using cell, molecular and genetic strategies in Arabidopsis thaliana, we identified a
negative feedback loop that modulates cell proliferation in the root apical meristem. This system involves
brassinosteroid (BR) hormones suppressing expression of the microtubule-associated protein CLASP,
which results in a dramatic reorganization of microtubule arrays and a shut down of cell division. CLASP,
in turn, enhances brassinoteroid signalling by sustaining the BR receptor BRI1 at the plasma membrane
by tethering BRI1 to microtubules via its direct interaction with the retromer component sorting nexin 1
(SNX1). To test our prediction that this feedback loop is important for modulating meristem cell
proliferation, we mutated the promoter element of the CLASP gene to render it BR-insensitive
(brinCLASP). Despite constitutively higher levels of CLASP and greater abundance of BRI1 receptors in
these transgenic lines, cell cycle progression was delayed and cell proliferation and root lengths were
reduced. The brinCLASP seedlings were also maladapted to drought and cold stress, indicating that
CLASP is at the nexus of BR-mediated responses to environmental signals. Intriguingly, BLAST analysis
of CLASP and SNX1 binding site sequences shows that CLASP-SNX1 interaction is plant-specific, and
that it appears to have emerged in concert with organized root meristems in early land plants.

Geoffrey Wasteneys (geoffrey.wasteneys@ubc.ca)

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