The RopGEF KARAPPO Is Essential for the Initiation of Vegetative Reproduction in Marchantia polymorpha

First author: Takuma Hiwatashi; Affiliations: Kobe University (神户大学): Kobe, Japan

Corresponding author: Kimitsune Ishizaki

Many plants can reproduce vegetatively, producing clonal progeny from vegetative cells; however, little is known about the molecular mechanisms underlying this process. Liverwort (Marchantia polymorpha), a basal land plant, propagates asexually via gemmae (芽孢), which are clonal plantlets formed in gemma cups (孢子芽杯) on the dorsal side of the vegetative thallus (叶状体). The initial stage of gemma development involves elongation and asymmetric divisions of a specific type of epidermal cell, called a gemma initial, which forms on the floor of the gemma cup. To investigate the regulatory mechanism underlying gemma development, we focused on two allelic mutants in which no gemma initial formed; these mutants were named karappo, meaning “empty.” We used whole-genome sequencing of both mutants and molecular genetic analysis to identify the causal gene, KARAPPO (KAR), which encodes a ROP guanine nucleotide exchange factor (RopGEF) carrying a plant-specific ROP nucleotide exchanger (PRONE) catalytic domain. In vitro GEF assays showed that the full-length KAR protein and the PRONE domain have significant GEF activity toward MpROP, the only ROP GTPase in M. polymorpha. Moreover, genetic complementation experiments showed a significant role for the N- and C-terminal variable regions in gemma development. Our investigation demonstrates an essential role for KAR/RopGEF in the initiation of plantlet development from a differentiated cell, which may involve cell-polarity formation and subsequent asymmetric cell division via activation of ROP signaling, implying a similar developmental mechanism in vegetative reproduction of various land plants.

许多植物可以通过营养繁殖，从营养细胞产生克隆子代；然而，目前对该过程潜在的分子机制了解甚少。地钱（Marchantia polymorpha），是一个基础的陆地植物，通过芽孢进行无性繁殖，即营养叶状体背侧的孢子芽杯中形成的克隆小植株。芽孢发育的起始阶段涉及一个特定类型表皮细胞的伸长和不对称分裂，该过程称为芽孢起始，会形成孢子芽杯的底层。为了研究芽孢发育的潜在调控机制，作者重点关注了两个不能形成芽孢起始的等位基因突变体，这些突变体被命名为karappo，意味着“空”。作者通过对两个突变体的全基因组测序和分子遗传学的研究鉴定了关键基因KARAPPO，该基因编码一个ROP鸟嘌呤核苷酸交换因子RopGEF，该蛋白包含一个植物特异性的ROP核苷交换催化结构域PRONE。体外GEF试验显示，KAR蛋白全长和PRONE结构域均具有对地钱中唯一一个ROP GTPase蛋白MpROP的GEF活性。此外，遗传互补试验显示该基因C端和N端区域在芽孢发育过程中均具有重要作用。本文的研究揭示了KAR/RopGEF在起始小植株发育中起着至关重要的作用，该过程从细胞分化开始，可能涉及到细胞极性的形成和随后通过ROP信号激活控制的不对称细胞分裂，这意味着可能不同的陆地植物会共享类似的营养生殖发育机制。

通讯：Kimitsune Ishizaki (http://www.edu.kobe-u.ac.jp/fsci-biol/faculty/ishizaki_en.html)

个人简介：2000-2003年，京都大学，博士。

研究方向：地钱营养生长和配子发育的分子调控机制。

doi: https://doi.org/10.1016/j.cub.2019.08.071

Journal: Current Biology

Published date: October 10, 2019