MpFEW RHIZOIDS1 miRNA-Mediated Lateral Inhibition Controls Rhizoid Cell Patterning in Marchantia polymorpha

Abstract

Lateral inhibition patterns differentiated cell types among equivalent cells during development in bacteria, metazoans, and plants. Tip-growing rhizoid cells develop among flat epidermal cells in the epidermis of the early-diverging land plant Marchantia polymorpha. We show that the majority of rhizoid cells develop individually, but some develop in linear, one-dimensional groups (chains) of between 2 and 7 rhizoid cells in wild-type plants. The distribution of rhizoid cells can be accounted for within a simple cellular automata model of lateral inhibition. The model predicted that in the absence of lateral inhibition, two-dimensional rhizoid cell groups (clusters) form. These can be larger than those formed with lateral inhibition. M. polymorpha rhizoid differentiation is positively regulated by the ROOT HAIR DEFECTIVE SIX-LIKE1 (MpRSL1) basic-helix-loop-helix (bHLH) transcription factor, which is directly repressed by the FEW RHI-ZOIDS1 (MpFRH1) microRNA (miRNA). To test if MpFRH1 miRNA acts during lateral inhibition, we generated loss-of-function (lof) mutants without the MpFRH1 miRNA. Two-dimensional clusters of rhizoids develop in Mpfrh1 lof mutants as predicted by the model for plants that lack lateral inhibition. Furthermore, two-dimensional clusters of up to 9 rhizoid cells developed in the Mpfrh1 lof mutants compared to a maximum number of 7 observed in wild-type groups. The higher steady-state levels of MpRSL1 mRNA in Mpfrh1 lof mutants indicate that MpFRH1-mediated lateral inhibition involves the repression of MpRSL1 activity. Together, the modeling and genetic data indicate that MpFRH1 miRNA mediates lateral inhibition by repressing MpRSL1 during pattern formation in the M. polymorpha epidermis.

A Comprehensive Toolkit for Quick and Easy Visualization of Marker Proteins, Protein–Protein Interactions and Cell Morphology in Marchantia polymorpha

“…We here show successful PI staining of cell walls of M. polymorpha (Figure 11), in agreement with former reports (Delmans et al, 2017;Jones and Dolan, 2017;Thamm et al, 2020). Strong fluorescence was observed already after short incubation times of 10 min.…”

“…In the last decade, the liverwort Marchantia polymorpha has emerged as a powerful model system to study early land plant evolution due to its early evolutionary divergence in the land plant phylogenetic tree ( Shaw et al, 2011 ; Harrison, 2017 ; Morris et al, 2018 ). Research deploying M. polymorpha has led to a series of insightful studies on the functional evolution of abscisic acid (ABA, Lind et al, 2015 ; Eklund et al, 2018 ) and jasmonic acid (JA) signaling mechanisms ( Monte et al, 2018 , 2019 ; Peñuelas et al, 2019 ), plant immunity ( Carella et al, 2019 ; Gimenez-Ibanez et al, 2019 ), reproductive and vegetative development ( Flores-Sandoval et al, 2015 ; Proust et al, 2016 ; Jones and Dolan, 2017 ; Rövekamp et al, 2016 ; Otani et al, 2018 ; Westermann et al, 2019 ; Thamm et al, 2020 ) and cell division ( Buschmann et al, 2016 ). It offers the advantage of genetic and morphological simplicity in combination with its dominant haploid vegetative life phase, allowing for fast generation of knockout mutants and subsequent phenotypic analyses, irrespectively of time-consuming homozygous mutant generation ( Ishizaki et al, 2015b ).…”

Gemma cup and gemma development in Marchantia polymorpha

“…Gemmae, mucilage cells, and rhizoids have a common feature in that they are formed from a single epidermal cell. It has been proposed that Mp FRH1 plays a role in the spatial specification of rhizoids via inhibition of Mp RSL1 in surrounding cells (Honkanen et al ., 2018; Thamm et al ., 2020); a similar system might regulate the spatial distribution of gemmae and mucilage cells in the gemma cup floor.…”

The hornworts: morphology, evolution and development

“…New Phytologist has recently been identified (Honkanen et al, 2018;Thamm et al, 2020). This demonstrated that the negative regulators of class 1 RSL genes, at least in liverworts, are different from those in A. thaliana where class 1 RSL genes are negatively regulated by a Class IV homeodomain-leucine-zipper protein, AtGL2 (Di Cristina et al, 1996;Bernhardt et al, 2003;Lin et al, 2015).…”

“…FEW RHIZOIDS1 (MpFRH1), a microRNA that acts as a negative regulator of the class 1 RSL gene in M . polymorpha , has recently been identified (Honkanen et al ., 2018; Thamm et al ., 2020). This demonstrated that the negative regulators of class 1 RSL genes, at least in liverworts, are different from those in A .…”