Plants possess an extraordinary ability to adapt and regenerate, a property known as developmental plasticity. One of the best examples of this phenomenon lies in the xylem pole pericycle (XPP) cells of the Arabidopsis thaliana root. These cells exhibit two distinct developmental fates under normal conditions: they can either form lateral roots (LRs), contributing to the root system architecture, or differentiate into cambium, which drives secondary growth to support the plant body. Under culture conditions, XPP cells can even be reprogrammed to form shoots via the lateral root developmental pathway.
A recent study from the University of Helsinki, led by Professor Ari Pekka Mähönen, provides new insight into how these XPP cell fates are regulated. The research team discovered that plant hormones (i.e. phytohormones) auxin and cytokinins play antagonistic roles in determining the fate of XPP cell lineages - auxin promotes LR fate acquisition, while cytokinins favor cambium fate acquisition.
In the first part of the study, the researchers focused on understanding the LR fate regulation. LR formation requires tightly patterned cell divisions and the establishment of new meristem identity within the lateral root primordium (LRP). The team found that only about half of the LRPs successfully develop into mature LRs. The remaining LRPs gradually lose their developmental identity and instead acquire cambium identity. This transition is closely linked to auxin and cytokinin signaling: auxin reduces the ratio of arrested LRPs but cytokinins increase this ratio.
A key breakthrough in this research was the development of the LR Tracker, a lineage tracing system that permanently labels LRPs, allowing both developing and arrested primordia to be visualized and quantified under a fluorescence stereo microscope. “This system allows us to monitor the developmental outcomes of individual LRPs over time,” explained Dr. Xin Wang, the study’s first and co-corresponding author and now an assistant professor at the Shenzhen University of Advanced Technology.
In the second part of the study, the team explored the fate plasticity of non-LR-fated XPP cells. Even after these cells acquire procambium identity, their identity can still be redirected toward LR formation by auxin treatment. However, this reprogramming ability diminishes as the root matures, restricted by the activity of key cambium regulators. In contrast, cytokinins suppress auxin-induced LR formation and promotes cambium identity through these regulatory factors.
In summary, these findings not only underscore the notable cell fate plasticity of the XPP lineage but also revealed how auxin and cytokinins balance these two cell fates between LR and cambium, thereby influencing root architecture and secondary growth.