Auxin transport synchronizes the pattern of cell division in a tobacco cell line

The open morphogenesis of plants requires coordination of
patterning by intercellular signals. The tobacco (Nicotiana
tabacum cv Virginia Bright Italia) cell line VBI-0 provides a
simple model system to study the role of intercellular
communication in patterning. In this cell line, singular cells
divide axially to produce linear cell files of distinct
polarity. The trigger for this axial division is exogenous
auxin. When frequency distributions of files are constructed
over the number of cells per file during the exponential phase
of the culture, even numbers are found to be frequent, whereas
files consisting of uneven numbers of cells are rare. We can
simulate these distributions with a mathematical model derived
from nonlinear dynamics, which describes a chain of
cell-division oscillators where elementary oscillators are
coupled unidirectionally and where the number of oscillators is
not conserved. The model predicts several nonintuitive
properties of our experimental system. For instance, files
consisting of six cells are more frequent than expected from a
strictly binary division system. More centrally, the model
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predicts a polar transport of the coordinating signal. We
therefore tested the patterns obtained after treatment with
1-N-naphthylphthalamic acid, an inhibitor of auxin efflux
carriers. Using low concentrations of 1-Nnaphthylphthalamic
acid that leave cell division and axiality of division
unaltered, we observe that the frequencies of files with even
and uneven cell numbers are equalized. Our findings are
discussed in the context of auxin transport as synchronizing
signal in cell patterning.