Seminar Laure Mancini (ENS Lyon)

Establishment of phyllotaxy from a single cell in the moss Physcomitrium
Patens

The shape of living organisms follows robust architectural plans that
determine the main axis and the relative position of each organ. In
plants, organs such as leaves
arrange themselves around the stem with a mathematical regularity,
forming large-scale patterns like spirals. This organization is called
phyllotaxis and emerges thanks to the activity of stem cells located at
the apical meristem, which corresponds to the tip of the leafy shoot.

Depending on the group of plant, the apical meristem can be multi- or
unicellular. In the case of the moss Physcomitrium Patens, this
structure is composed of a single stem cell, the apical cell (AC), which
is responsible for producing all the leaves. As a result, the position
of each new leaf strongly depends on the division pattern of the AC.
However, the mechanisms that control the positioning of the apical cell
division plane to pattern moss phyllotaxis remain to be discovered.

As an entry point to the problem, we studied the role of auxin signaling
in this context because, the hormone auxin is known to be instrumental
in establishing phyllotactic patterns in multicellular meristems. We
asked if this hormone can also act at the scale of a unique stem cell,
to insure a robust phyllotactic pattern. To do so, we used a combination
of quantitative time-lapse imaging, genetic tools and mutants as well as
cellular shape analysis and modeling. With these approaches, we found
that auxin is involved in the robustness of the spiral phyllotaxy of the
moss and that it affects cellular properties such as growth anisotropy,
that can impact the global architecture of the moss leafy shoot.