Neil Ashton and David Cove identified roles for auxin in promoting and cytokinin in suppressing plant spread across a substrate in Physcomitrella through pharmacological experiments and mutant screens the 1970s [1]. Both hormones affect the activity of the stem cells at the tip of protonemal filaments, with auxin biasing tip cell identity towards foraging caulonemal fate and cytokinin biasing identity towards photosynthetic chloronemal fate [2]. Although the tip cells are the site of growth responses, Thelander et al. (2019) showed that auxin signalling is strongest away from the tip cells, posing a conundrum about how tip cells function [3]. Zoe Nemec Venza’s PhD with me has investigated roles for a key regulator of Arabidopsis stem cell fate in Physcomitrella and addressed this question [4]. She has found that components of the CLAVATA receptor-like kinase pathway are active in tip cells, regulating auxin homeostasis and auxin transporter expression to determine tip cell fate and plant spread. We have BioRXiVed a manuscript presenting her findings here, and we would very much welcome your feedback prior to publication.
[1] Ashton et al. (1979). Analysis of gametophytic development in the moss, Physcomitrella patens, using auxin and cytokinin resistant mutants. Planta 144: 427-435.
[2] Jang and Dolan (2011). Auxin promotes the transition from chloronema to caulonema in moss protonema by positively regulating PpRSL1and PpRSL2 in Physcomitrella patens. New Phytologist 192: 319-327.
[3] Thelander et al. (2019). Minimal auxin sensing levels in vegetative moss stem cells revealed by a ratiometric reporter. New Phytologist 224: 775-788.
[4] Nemec Venza et al. (2021). PpRPK2 modulates auxin homeostasis and transport to specify stem cell identity and plant shape in the moss Physcomitrella. https://doi.org/10.1101/2021.06.24.449551
