Autoregulation of RCO by Low-Affinity Binding Modulates Cytokinin Action and Shapes Leaf Diversity

First author: Mohsen Hajheidari; Affiliations: Max Planck Institute for Plant Breeding Research (普朗克植物育种研究所): Cologne, Germany

Corresponding author: Miltos Tsiantis

Mechanisms through which the evolution of gene regulation causes morphological diversity are largely unclear. The tremendous shape variation among plant leaves offers attractive opportunities to address this question. In cruciferous plants, the REDUCED COMPLEXITY (RCO) homeodomain protein evolved via gene duplication and acquired a novel expression domain that contributed to leaf shape diversity. However, the molecular pathways through which RCO regulates leaf growth are unknown. A key question is to identify genome-wide transcriptional targets of RCO and the DNA sequences to which RCO binds. We investigate this question using Cardamine hirsuta, which has complex leaves, and its relative Arabidopsis thaliana, which evolved simple leaves through loss of RCO. We demonstrate that RCO directly regulates genes controlling homeostasis of the hormone cytokinin to repress growth at the leaf base. Elevating cytokinin signaling in the RCO expression domain is sufficient to both transform A. thaliana simple leaves into complex ones and partially bypass the requirement for RCO in C. hirsuta complex leaf development. We also identify RCO as its own target gene. RCO directly represses its own transcription via an array of low-affinity binding sites, which evolved after RCO duplicated from its progenitor sequence. This autorepression is required to limit RCO expression. Thus, evolution of low-affinity binding sites created a negative autoregulatory loop that facilitated leaf shape evolution by defining RCO expression and fine-tuning cytokinin activity. In summary, we identify a transcriptional mechanism through which conflicts between novelty and pleiotropy are resolved during evolution and lead to morphological differences between species.

基因调控演化导致形态多样性的机制在很大程度上尚不清楚。植物叶片之间巨大的形状变化为研究该问题提供了十分好的机会。在十字花科植物中，通过基因复制而出现的RCO同源结构域蛋白，获得了作用于叶形多样性的新功能。但是，RCO调节叶片生长的分子途径尚不清楚。其中，一个关键问题就是确定RCO的全基因组转录靶标和RCO结合的DNA序列。本文中，作者使用叶形较复杂的碎米荠及相对具有简单叶形的拟南芥来研究此问题，拟南芥通过失去RCO基因演化出简单的叶片。本文的研究显示RCO通过直接调控植物激素细胞分裂素的内稳态来抑制叶片基部的生长。在RCO表达域中增加细胞分裂素信号转导能够将拟南芥的简单叶片转化为复杂叶片，并且能够部分绕开了碎米荠复杂叶片对于RCO的需求。作者还发现RCO基因是其自身的靶基因。RCO通过一系列低亲和力结合位点直接抑制其自身的转录，这些位点是RCO基因从祖先序列复制后演化而来的。RCO基因需要这种自我抑制来限制本身的表达。因此，低亲和力结合位点的演化产生了一个负的自调节回路，从而通过影响RCO基因的表达和精细调控细胞分裂素的活性来促进叶形的演化。总之，本文鉴定了一种转录机制，通过解决演化过程中新颖性和多效性之间的冲突，导致物种之间的形态差异演化。

通讯：Miltos Tsiantis（https://www.mpipz.mpg.de/226344/tsiantis-dpt）

研究方向：植物形态建成的遗传基础。

doi: https://doi.org/10.1016/j.cub.2019.10.040

Journal: Current Biology

Published date: November 21, 2019