The appearance of root was a great step for plants toward better adaptation to growth on land. Fossil evidences indicate that the common ancestor of ferns and seed plants experienced the root origin event in the Middle Devonian. However, our knowledge about the molecular evolutionary route of root organogenesis is limited. The key scientific question is when and how the auxin signaling pathway was recruited in root evolution. In May 2020, XU Lin's group at CAS center for Excellence in Molecular Plant Sciences/Institute of Plant Physiology and Ecology, Chinese Academy of Sciences published the research entitled "Molecular evolution of auxin mediated root initiation in plants" in Molecular Biology and Evolution, and revealed the molecular route of root origin and evolution using Evo-Devo approaches.

By studying the fern Ceratopteris richardii, Xu's group found that auxin is the key hormone controlling adventitious and lateral root formation. The application of auxin could induce adventitious rooting. The auxin signaling pathway directly activates the expression of CrWOXA to form root founder cells, and then CrWOXA directly activates the expression of CrWUL for fate transition from root founder cells to root tips. Both CrWOXA and CrWUL are WOX family transcription factor genes.

Compared with the results from studies in the seed plant Arabidopsis thaliana, Xu's group noticed that the “auxin-WOX” molecular pathway also controls the development of root founder cells in Arabidopsis thaliana (Xu, 2018, Curr. Opin. Plant Biol. 41:39-45). This finding suggests that the "auxin-WOX" molecular pathway might be recruited by the common ancestors of ferns and seed plants in the root founder cells during the root origin event. With the evolution of plants, the increased gene members of the auxin signaling pathway and the WOX family were recruited by ferns and seed plants to initiate different types of roots (Liu and Xu, 2018, Trends Plant, Sci. 23: 490-496).

Many questions remain elusive regarding the origin and evolution of roots in vascular plants. The extant vascular plants are the descendants of two lineages of their ancestors: the euphyllophytes (including ferns and seed plants) and the lycophytes. Fossil evidences indicate that there were two independent root origin events in the two lineages: the first root origin event occurred in the lycophyte lineage in the early Devonian, and the second root origin event occurred in the common ancestor of ferns and seed plants in the euphyllophyte lineage in the Middle Devonian. Although this research work confirmed the conserved molecular function of auxin in controlling root organogenesis in euphyllophytes, the molecular mechanism of auxin in root organogenesis in lycophytes is still unclear.

YU Jie and ZHANG Yuyun are the co-first authors, and XU Lin is the corresponding author. The work is the collaboration among research groups of Dr. XU Lin and Dr. ZHANG Yijing from CAS center for Excellence in Molecular Plant Sciences/Institute of Plant Physiology and Ecology, Chinese Academy of Sciences. The research was supported by the grants from Chinese Academy of Sciences, NSFC, and National Key Laboratory of Plant Molecular Genetics.

Link: https://doi.org/10.1093/molbev/msz202
Contact:
Dr. Lin Xu, Professor
National Key Laboratory of Plant Molecular Genetics, CAS Center for Excellence in Molecular Plant Sciences/Shanghai Institute of Plant Physiology and Ecology (SIPPE), Chinese Academic of Sciences
Tel: 86-21-54924101
Email: xulin@cemps.ac.cn