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

Abstract

Background 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.• Scope This article examines the hypothesis that canalization via polar auxin transport regulates vascular reconnection and patterning in the stem after wounding or grafting. We examine 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.• Conclusions 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.

What drives interspecies graft union success? Exploring the role of phylogenetic relatedness and stem anatomy

“…Until recently, it was thought that through polarization of PIN1 auxin transport proteins, plants re‐establish polar auxin transport (PAT) between the scion and rootstock across the graft junction and these cell files then undergo differentiation to reconnect the vasculature (Sawchuk and Scarpella 2013, Wang et al 2017). However, recent auxin transport studies by Wulf et al (2019) using the radiolabeled auxin [ 3 H]‐indole‐3‐acetic acid have questioned this canalization hypothesis. It was demonstrated that despite a significant disruption to PAT in interspecies grafts, no difference in the number of vascular cells reconnecting scion to rootstock could be seen (Wulf et al 2019).…”

“…However, recent auxin transport studies by Wulf et al (2019) using the radiolabeled auxin [ 3 H]‐indole‐3‐acetic acid have questioned this canalization hypothesis. It was demonstrated that despite a significant disruption to PAT in interspecies grafts, no difference in the number of vascular cells reconnecting scion to rootstock could be seen (Wulf et al 2019). Additionally, grafting experiments using auxin transport and biosynthesis mutants showed no obvious defects in the formation of new vascular connections (Melnyk et al 2015).…”

“…Additionally, grafting experiments using auxin transport and biosynthesis mutants showed no obvious defects in the formation of new vascular connections (Melnyk et al 2015). Therefore, while auxin may play a vital role in vascular development, auxin transport across the graft junction and the absolute levels of auxin do not appear to control graft success (Nanda and Melnyk 2018, Wulf et al 2019).…”

“…Plant grafting is an ancient technique, which has not only advanced horticultural and agricultural industries, but progressed scientific understanding of plant physiology and development (Goldschmidt 2014, Kyriacou et al 2017, Gaion and Carvalho 2018). Plant grafting is the joining together of two plants of the same or differing species and is made possible by the incredible plasticity of plants to heal, regenerate and reconnect vascular tissue restoring vital transport routes (Pina and Errea 2005, Ikeuchi et al 2013, Melnyk 2017a, Wulf et al 2019). This plasticity has evolved to enable plants to recover from biotic or abiotic stresses and in some plant species to enable stems and roots to fuse when in close proximity (Mudge et al 2009, Warschefsky et al 2016).…”

“…It has been speculated that graft success is attributed to either evolutionary or physiological processes (Goldschmidt 2014). However, while some recent work has focused on vascular development in homografts (grafts between the same species), there has been no systematic investigation into the driver of graft success in heterografts (grafts between species), although the key role for the canalization of auxin has recently been questioned (Wulf et al 2019). How related the two grafting partners are has long been proposed as a possible driver of interspecies graft compatibility.…”