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

• Published in: Current biology Vol. 29; no. 24; pp. 4183 - 4192.e6
• Main Authors: Hajheidari, Mohsen, Wang, Yi, Bhatia, Neha, Vuolo, Francesco, Franco-Zorrilla, José Manuel, Karady, Michal, Mentink, Remco A., Wu, Anhui, Oluwatobi, Bello Rilwan, Müller, Bruno, Dello Ioio, Raffaele, Laurent, Stefan, Ljung, Karin, Huijser, Peter, Gan, Xiangchao, Tsiantis, Miltos
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 16.12.2019
• Subjects: cis-regulatory evolution; complex leaves; cytokinin; evo-devo; Evolutionary Biology; Evolutionsbiologi; leaf shape; low-affinity binding sites; plant homeobox gene; RCO; REDUCED COMPLEXITY; evo-devo; complex leaves; cytokinin; cis-regulatory evolution; leaf shape; REDUCED COMPLEXITY; plant homeobox gene; low-affinity binding sites; RCO
• ISSN: 0960-9822; 1879-0445; 1879-0445
• DOI: 10.1016/j.cub.2019.10.040

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. [Display omitted] •Identification of genome-wide target genes for the RCO transcription factor•RCO delimits its own expression through autorepression by low-affinity binding•RCO represses local leaf growth via regulating multiple cytokinin (CK)-related genes•RCO negative autorepression fine-tunes CK activity and regulates leaf shape Hajheidari et al. identify target genes for the RCO homeodomain protein that drove leaf shape diversity. They show that RCO regulates growth via orchestrating homeostasis for the hormone cytokinin and that it also represses its own transcription via low-affinity binding sites. This autorepression helps delimit RCO expression and shape leaf form.