Robust branch patterning in moss shoots via symplasmic auxin diffusion

Uncovering the mechanisms by which developmental patterns consistently arise within species is a major goal of biology. In plants, branching patterns are key determinants of morphological diversity. Similar lateral branching modes convergently evolved in the leafy shoot of vascular plants and bryophytes, driving their independent architectural diversification. While auxin-dependent branch inhibition is shared between both lineages, long-range polar auxin transport plays a central role in branching control in vascular plants, but in bryophytes like the moss Physcomitrium patens, auxin might diffuse through plasmodesmata to regulate branch distribution. However, whether symplasmic auxin diffusion is a biophysically realistic mechanism sufficient to explain observed branch distribution patterns has yet to be assessed. Here, we address this fundamental problem by developing a physics-based, three-dimensional computational model of symplasmic auxin diffusion in the moss shoot, integrating molecular, cell, and tissue scales. Each step of model design was guided by geometry measurements and biological experiments. Our integrative approach demonstrates that branching control based solely on symplasmic diffusion for intercellular auxin movement can account for the observed branching patterns at the whole-shoot level. It also provides mechanistic interpretations of the changes in branch distribution caused by genetic perturbations affecting callose-dependent symplasmic permeability, as well as the unexpected increase in branch spacing robustness during shoot development. Altogether, our findings reveal that branching patterns arising from an auxin diffusion-based regulatory mechanism exhibit a specific developmental signature, not reported in vascular plants, but well exemplified in the moss Physcomitrium.