Auxin transport and stem vascular reconnection – has our thinking become canalized?

The presence of a polar auxin transport stream has long been correlated with the differentiation and patterning of vascular cells across vascular plants. As our understanding of auxin transport and vascular development has grown, so too has evidence for the correlation between these processes. However, a clear understanding of the cellular and molecular mechanisms driving this correlation has not been elucidated.

Vascular and polar auxin transport re-establishment after wounding or complete severing of the stem.
Vascular and polar auxin transport re-establishment after wounding or complete severing of the stem. (A) Vascular reconnection after wounding of the stem (left-hand side) or complete severing of the stem (right-hand side). (B) Polar auxin transport re-establishment over time after wounding, through polarization of PIN1 auxin transport proteins. Dark blue cells contain high concentrations of auxin, light green cells contain low concentrations of auxin, callus is shown in yellow, and polarized PIN1 proteins are shown in red

Wulf et al. examine the hypothesis that canalization via polar auxin transport regulates vascular reconnection and patterning in the stem after wounding or grafting. The authors investigate the evidence for the causal nature of the relationship and the suggested role that other hormones may play. Data are presented indicating that in grafted plants the degree of auxin transport may not always correlate with vascular reconnection. Furthermore, data on grafting success using plants with a range of hormone-related mutations indicate that these hormones may not be critical for vascular reconnection.

In the past, excellent work examining elements of auxin synthesis, transport and response in relation to vascular development has been carried out. However, new experimental approaches are required to test more directly the hypothesis that auxin transport regulates stem vascular reconnection after wounding or grafting. This could include studies on the timing of the re-establishment of auxin transport and vascular reconnection after grafting and the influence of auxin transport mutants and inhibitors on these processes using live imaging.

Further reading

Wulf, K. E., Reid, J. B., & Foo, E. (2018). Auxin transport and stem vascular reconnection – has our thinking become canalized? Annals of Botany. https://doi.org/10.1093/aob/mcy180