Major components in the KARRIKIN INSENSITIVE2-ligand signaling pathway are conserved in the liverwort, Marchantia polymorpha

Yohei Mizuno, Aino Komatsu, Shota Shimazaki, Xiaonan Xie, Kimitsune Ishizaki, Satoshi Naramoto, Junko Kyozuka

Posted on: 1 December 2020

Preprint posted on 19 November 2020

Article now published in The Plant Cell at


The Physcomitrium (Physcomitrella) patens PpKAI2L receptors for strigolactones and related compounds highlight MAX2 dependent and independent pathways

Mauricio Lopez-Obando, Ambre Guillory, François-Didier Boyer, David Cornu, Beate Hoffmann, Philippe Le Bris, Jean-Bernard Pouvreau, Philippe Delavault, Catherine Rameau, Alexandre de Saint Germain, Sandrine Bonhomme

Posted on:

Preprint posted on 24 November 2020

Article now published in The Plant Cell at

Insight from bryophytes into the enigmatic evolution of strigolactone: elucidating the functional role of KAI2 signalling pathway components

Selected by Facundo Romani


Strigolactones (SLs) are one of the most recently discovered hormone signalling pathways in plants and their nature and evolution are still enigmatic. SLs are a complex group of carotenoid-derived terpenoid lactones unique to plants. As phytohormones, they control various aspects of plant growth, including shoot branching, root growth and senescence, in addition to their function as rhizosphere-signalling molecules for arbuscular mycorrhizal symbiosis.

In seed plants, SLs are perceived by the protein DWARF14 (D14). Structurally similar to SLs, karrikins are perceived by the paralog receptors KARRIKIN INSENSITIVE2 (KAI2). Karrikins are by products of burning plants and work as a signal to detect smoke in the environment. It is proposed that there is an endogenous compound similar to karrikins referred to as KAI2-ligand (KL) that acts as a hormone but its chemical nature is still elusive. The KL signalling pathway seems to be even more ancestral than SL perception and biosynthesis. Thus, KL and SL are two branches of two hormonal signalling pathways that evolved together and share several components. These pathways also involve MAX2 F-box proteins that interact with KAI2 and subsequently trigger the degradation of SMXL transcription factor-like proteins.

Most components of SL/KL biosynthesis, perception and signal transduction were characterized in flowering plants but are conserved across land plants (Walker et al., 2019). In the last few years, biochemical studies have characterized proteins of SL/KL pathway from non-flowering plants, particularly in the models bryophytes Marchantia polymorpha and Physcomitrium patens (Bürger et al., 2019; Waters et al., 2015). Both models lack some of the SL biosynthesis components identified in flowering plants. Especially, MAX1 is not present in both and a CCD8 ortholog is lacking in M. polymorpha. It is likely that other proteins could replace these components, challenging the notions of what is “canonical” in the pathway. In some cases, these proteins could have been replaced by homologous proteins as in the case of D14 and KAI2. Biochemical studies have provided important hints on how this signalling pathway may work in bryophytes but functional studies using mutant plants are lacking.

Figure 1. Schematic model for KL and SL signalling in P. patens growth in wild-type and mutant backgrounds and in response to two GR24 enantiomers (SL analogues). see Lopez-Obando et al. Figure 13 for full reference.

Major findings

In two recent pre-prints, Mizuno et al. and Lopez-Obando et al., provide novel insight about the KAI2-mediated signalling pathway in M. polymorpha and P. patens, respectively. In P. patens,  the importance of CCD8 SL-like biosynthesis activity in plant growth and development was previously demonstrated (Proust et al., 2011). Here, Lopez-Obando and colleagues take an original approach using Phelipanche ramosa, a parasitic plant that germinates in response to host plant SL/KL. This allows testing PpCCD8-derived compound activity using P. patens exudates from wild-type and Ppccd8 mutants. In one of the seed populations, only wild-type exudates presented germination stimulant activity. This greatly contributes to the idea that P. patens can synthesize active SLs in a CCD8 conserved pathway, despite lacking MAX1.

As shown before, neither M. polymorpha nor P. patens phenotypically respond to the karrikins generated by burning vegetation as in the KL pathway of flowering plants. However, both pre-prints showed that KAI2 enzymes can potentially interact with SL analogues, possibly connecting both branches of the “canonical” signalling pathway in the common ancestor of land plants and during evolution. Mizuno et al. managed to create a full mutant plant for KAI2 thanks to the M. polymorpha genome only bearing two copies of the gene. Remarkably, Marchantia Mpkai2 mutant plants displayed developmental defects similar to Mpmax2 mutants. In the other hand, in P. patens with 13 KAI2-like genes, it represented a more difficult task. Thus, Lopez-Obando et al. generated multiple mutants of combinations of P. patens KAI2 corresponding to different clades. Particularly one of these clades, mutant plants phenocopy not on Ppmax2 but also Ppccd8. These results suggest that KAI2 and the F-box MAX2 participate in the genetic pathway regulating growth and photomorphogenesis in a conserved fashion, but in P. patens, some KAI2 genes probably diversified into MAX2 independent pathways. In M. polymorpha, Mpkai2 and Mpmax2 showed a slightly reduced sensibility to synthetic SL analogues.

In addition to functional studies of KAI2 and MAX2, Mizuno et al. also analyzed MpSMXL genes. MpSMXL is also degraded through ubiquitination as in flowering plants. Interestingly, they found that Mpsmxl double mutants can suppress mutant phenotypes in Mpkai2 or Mpmax2 backgrounds. Moreover, the complementation of Mpsmxl with a version of MpSMXL resistant to protein degradation showed similar phenotypes to Mpkai2 and Mpmax2, suggesting that SMXL degradation could be a conserved part of the SL/KL pathway despite the strong divergence of this family (Walker et al., 2019).

Future directions

Current knowledge supports the hypothesis that there is a SL/KL signalling pathway conserved in the common ancestor to land plants that probably works in a similar way in extant bryophytes. Nevertheless, many aspects of the evolution of SL/KL biosynthesis and signalling pathways are poorly understood. It is clear that bryophyte components are unable to replace their putative counterparts in flowering plants as shown by Lopez-Obando et al. and also before (Bürger et al., 2019; Waters et al., 2015), indicating that the involved interactions may have co-evolved differently in each lineage. It remains to be tested whether SMXL proteins could also participate in the pathway in P. patens. In P. patens, it seems that SL compounds can be synthesized through CCD8 as in flowering plants, while in M. polymorpha, only the KL branch may be functional. In that sense, Lopez-Obando et al. propose in this pre-print that the diversification of KAI2 receptors could be important to separate both branches in P. patens. In this context, the role of MAX2 may be restricted to KL signalling (Lopez-Obando et al., 2016). The main challenge is still the lack of a canonical KL compound that could be unequivocally associated with KAI2 receptors. KAI2 diversity and promiscuity to synthetic SL analogues are often problematic and controversial in experimental conditions. Another exciting open question is how arbuscular mycorrhizal symbiosis is affected in the bryophyte mutants of KL/SL signalling and biosynthesis components.

What I Liked

These two new studies reach to complementary conclusions and provide many insights (some of them were not mentioned here) that will help to complete the puzzle of the SL/KL pathway in bryophytes. They also highlight the importance of the use of multiple model systems to address complex evolutionary challenges. An evolutionary framework will certainly facilitate the generation of new hypotheses leading to a better understanding of this complex and diversified signalling pathway.


Bürger, M., Mashiguchi, K., Lee, H.J., Nakano, M., Takemoto, K., Seto, Y., Yamaguchi, S., Chory J. (2019). Structural basis of karrikin and non-natural strigolactone perception in Physcomitrella patens. Cell Reports, 26(4):855-865.e5.

Lopez‐Obando, M., de Villiers, R., Hoffmann, B., Ma, L., de Saint Germain, A., Kossmann, J., Coudert, Y., Harrison, C.J., Rameau, C., Hills, P. and Bonhomme, S. (2018), Physcomitrella patens MAX2 characterization suggests an ancient role for this F‐box protein in photomorphogenesis rather than strigolactone signalling. New Phytol, 219: 743-756.

Waters, M.T., Scaffidi, A., Moulin, S.L.Y., Sun, Y.K., Flematti, G.R., Smith S.M. (2015). A Selaginella moellendorffii ortholog of KARRIKIN INSENSITIVE2 functions in Arabidopsis development but cannot mediate responses to karrikins or strigolactones. The Plant Cell Jul 2015, 27(7):1925-1944.

Proust, H., Hoffmann, B., Xie, X., Yoneyama, K., Schaefer, D.G., Yoneyama, K., Nogué, F., Rameau, C. (2011). Strigolactones regulate protonema branching and act as a quorum sensing-like signal in the moss Physcomitrella patens. Development 138:1531-1539.

Walker, C.H., Siu-Ting, K., Taylor, A., et al (2019). Strigolactone synthesis is ancestral in land plants, but canonical strigolactone signalling is a flowering plant innovation. BMC Biol 17:70.

Tags: evodevo, marchantia, physcomitrella, strigolactones


(No Ratings Yet)

Have your say

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Sign up to customise the site to your preferences and to receive alerts

Register here

Also in the evolutionary biology category:

Fetal brain response to maternal inflammation requires microglia

Bridget Elaine LaMonica Ostrem, Nuria Dominguez Iturza, Jeffrey Stogsdill, et al.

Selected by 24 April 2024

Manuel Lessi


How the liver contributes to stomach warming in the endothermic white shark Carcharodon carcharias

David C. Bernvi, Geremy Cliff

Selected by 22 April 2024

Sarah Young-Veenstra


A long non-coding RNA at the cortex locus controls adaptive colouration in butterflies

Luca Livraghi, Joseph J. Hanly, Elizabeth Evans, et al.


The ivory lncRNA regulates seasonal color patterns in buckeye butterflies

Richard A. Fandino, Noah K. Brady, Martik Chatterjee, et al.


A micro-RNA drives a 100-million-year adaptive evolution of melanic patterns in butterflies and moths

Shen Tian, Tirtha Das Banerjee, Jocelyn Liang Qi Wee, et al.

Selected by 05 April 2024

Isabella Cisneros

Developmental Biology

Also in the plant biology category:

Generalized Biomolecular Modeling and Design with RoseTTAFold All-Atom

Rohith Krishna, Jue Wang, Woody Ahern, et al.

Selected by 24 January 2024

Saanjbati Adhikari


Plant plasmodesmata bridges form through ER-driven incomplete cytokinesis

Ziqiang P. Li, Hortense Moreau, Jules D. Petit, et al.


Plasmodesmata act as unconventional membrane contact sites regulating inter-cellular molecular exchange in plants

Jessica Pérez-Sancho, Marija Smokvarska, Marie Glavier, et al.

Selected by 09 January 2024

Gwendolyn K. Kirschner

Plant Biology

H2O2 sulfenylates CHE linking local infection to establishment of systemic acquired resistance

Lijun Cao, Heejin Yoo, Tianyuan Chen, et al.

Selected by 23 August 2023

Marc Somssich

Plant Biology

preLists in the evolutionary biology category:

‘In preprints’ from Development 2022-2023

A list of the preprints featured in Development's 'In preprints' articles between 2022-2023


List by Alex Eve, Katherine Brown

preLights peer support – preprints of interest

This is a preprint repository to organise the preprints and preLights covered through the 'preLights peer support' initiative.


List by preLights peer support

EMBO | EMBL Symposium: The organism and its environment

This preList contains preprints discussed during the 'EMBO | EMBL Symposium: The organism and its environment', organised at EMBL Heidelberg, Germany (May 2023).


List by Girish Kale

9th International Symposium on the Biology of Vertebrate Sex Determination

This preList contains preprints discussed during the 9th International Symposium on the Biology of Vertebrate Sex Determination. This conference was held in Kona, Hawaii from April 17th to 21st 2023.


List by Martin Estermann

EMBL Synthetic Morphogenesis: From Gene Circuits to Tissue Architecture (2021)

A list of preprints mentioned at the #EESmorphoG virtual meeting in 2021.


List by Alex Eve

Planar Cell Polarity – PCP

This preList contains preprints about the latest findings on Planar Cell Polarity (PCP) in various model organisms at the molecular, cellular and tissue levels.


List by Ana Dorrego-Rivas

TAGC 2020

Preprints recently presented at the virtual Allied Genetics Conference, April 22-26, 2020. #TAGC20


List by Maiko Kitaoka et al.

ECFG15 – Fungal biology

Preprints presented at 15th European Conference on Fungal Genetics 17-20 February 2020 Rome


List by Hiral Shah

COVID-19 / SARS-CoV-2 preprints

List of important preprints dealing with the ongoing coronavirus outbreak. See for additional resources and timeline, and for full list of bioRxiv and medRxiv preprints on this topic


List by Dey Lab, Zhang-He Goh


SDB 78th Annual Meeting 2019

A curation of the preprints presented at the SDB meeting in Boston, July 26-30 2019. The preList will be updated throughout the duration of the meeting.


List by Alex Eve

Pattern formation during development

The aim of this preList is to integrate results about the mechanisms that govern patterning during development, from genes implicated in the processes to theoritical models of pattern formation in nature.


List by Alexa Sadier