ELIMÄKI Locus Is Required for Vertical Proprioceptive Response in Birch Trees

First author: Juan Alonso-Serra; Affiliations: University of Helsinki (赫尔辛基大学): Helsinki, Finland

Corresponding author: Kaisa Nieminen

Tree architecture has evolved to support a top-heavy above-ground biomass, but this integral feature poses a weight-induced challenge to trunk stability. Maintaining an upright stem is expected to require vertical proprioception through feedback between sensing stem weight and responding with radial growth. Despite its apparent importance, the principle by which plant stems respond to vertical loading forces remains largely unknown. Here, by manipulating the stem weight of downy birch (Betula pubescens) trees, we show that cambial development is modulated systemically along the stem. We carried out a genetic study on the underlying regulation by combining an accelerated birch flowering program with a recessive mutation at the ELIMÄKI locus (EKI), which causes a mechanically defective response to weight stimulus resulting in stem collapse after just 3 months. We observed delayed wood morphogenesis in eki compared with WT, along with a more mechanically elastic cambial zone and radial compression of xylem cell size, indicating that rapid tissue differentiation is critical for cambial growth under mechanical stress. Furthermore, the touch-induced mechanosensory pathway was transcriptionally misregulated in eki, indicating that the ELIMÄKI locus is required to integrate the weight-growth feedback regulation. By studying this birch mutant, we were able to dissect vertical proprioception from the gravitropic response associated with reaction wood formation. Our study provides evidence for both local and systemic responses to mechanical stimuli during secondary plant development.

林木演化出了地上部分承担最大生物量的整体结构，该特性对于树干稳定性提出了由重量引起的巨大挑战。维持茎干的向上生长需要一个垂直方向的本体感觉，主要由感知茎干重量以及径向生长响应的反馈组成。除了其显而易见的重要性，究竟是植物茎干的哪一部分响应于垂直方向负荷力还不清楚。本文中，通过人为控制毛桦的茎干重量，作者发现形成层的发育沿着茎干方向被系统性调节。野生型的毛桦具有单个、笔直的树干，而毛桦栽培品种Elimäki的分枝极不规则，作者获得了表型类似于野生型的Elimäki品种开放授粉F1代，然后在2年生时诱导该F1代植株开花，并与野生型毛桦进行回交，共得到了60株回交BC1代个体，其中分离比大概为1:1，即33棵植株具有野生型的茎干结构，而另外27棵毛桦（eki突变体）在前3个月完全正常生长，而后在24小时之内发生倒伏现象。作者发现与野生型相比比，eki突变体的木材形态建成延后，且形成层区域机械弹性更大、径向的木质部细胞大小被压缩，说明快速的组织分化对于机械应力下形成层的生长是非常重要的。此外，eki突变体中触碰诱导的机械感知通路转录失调，说明ELIMÄKI位点对于整合重量-生长反馈调控是必需的。通过研究毛桦的eki突变体，作者能够将林木垂直本体感觉从与木材形成相关的重力响应中解析出来。本文的研究揭示了植物次生发育过程中同时存在区域化和整体系统层面的对于机械刺激的响应。

p.s. 弹性模量（elastic modulus），指的是物体在外界施加应力时，抵抗弹性变形所产生的抵抗力，它定量了物体抵抗变形能力的大小。弹性模量越大，说明该物体越坚硬，在外力作用下越不容易变形；相反，弹性模量越小，说明该物体越柔软，在外力作用下越容易变形。

通讯：Kaisa Nieminen (https://www.luke.fi/en/henkilosto/kaisa-p-nieminen/)

个人简介：2003-2009年，赫尔辛基大学，博士；2009-2013年，洛桑大学，博士后。

研究方向：林木生长与木材形成的遗传学。

doi: https://doi.org/10.1016/j.cub.2019.12.016

Journal: Current Biology

Published date: January 30, 2020