# The SPL‐family transcription factor MpSPL3 orchestrates the proper regulation of vegetative and reproductive programs in Marchantia polymorpha

## TL;DR

This study shows how the MpSPL3 gene controls both vegetative growth and reproductive development in the liverwort Marchantia polymorpha.

## Contribution

The paper reveals the first functional insights into MpSPL3's role in regulating developmental programs in Marchantia polymorpha.

## Key findings

- Knockout of MpSPL3 causes growth retardation, disordered thallus morphology, and loss of gametangiophore formation.
- Overexpression of the longer MpSPL3.1 isoform delays and reduces gametangiophore production.
- Altered gene expression in MpSPL3 mutants affects vegetative development and germ cell specification.

## Abstract

SQUAMOSA PROMOTER BINDING PROTEIN‐LIKE (SPL) genes encode plant‐specific transcription factors that are widely distributed across the plant kingdom. In angiosperms, the multimember SPL family regulates various biological processes, including vegetative‐to‐reproductive phase transition, inflorescence architecture, and lateral organ development. In contrast, the liverwort Marchantia polymorpha genome encodes only four SPL genes, with functional studies available only for microRNA‐targeted members, MpSPL1 and MpSPL2. MpSPL1 was shown to control the meristem dormancy to modulate the thallus architecture, whereas MpSPL2 was found to promote the transition from vegetative‐to‐reproductive phase. Here, we investigate the impact of the MpSPL3 gene on M. polymorpha development. We demonstrate that MpSPL3 influences coordination of the vegetative growth and the reproductive phase transition. Knockout of MpSPL3 leads to strong growth retardation with disordered thallus morphology, reduced gemma cup number, and, most strikingly, complete loss of gametangiophore formation. Interestingly, overexpression of MpSPL3.2, the shorter isoform, has no detectable morphological effect, whereas the overexpression of MpSPL3.1, the longer isoform encoding a protein with an additional 61‐aa long fragment, results in a delay in timing and reduced efficiency of gametangiophore production. Moreover, all the observed developmental abnormalities might be a consequence of the altered expression of genes essential for proper vegetative development and responsible for germ cell specification in MpSPL3 knockout and overexpression plants. Altogether, our findings demonstrate that MpSPL3 is important in regulating gametophyte development and ensuring reproductive success in M. polymorpha.

We showed that the MpSPL3 influences coordination of the vegetative growth and the reproductive phase transition in Marchantia polymorpha, as its null mutation caused strong growth retardation with disordered thallus branching, reduced gemma cup number, and abolished gametangiophores production. Our study provides the first insights into the fundamental functions of MpSPL3 in the orchestration of developmental programs during M. polymorpha life cycle.