TALE and Shape: How to Make a Leaf Different

Abstract

The Three Amino acid Loop Extension (TALE) proteins constitute an ancestral superclass of homeodomain transcription factors conserved in animals, plants and fungi. In plants they comprise two classes, KNOTTED1-LIKE homeobox (KNOX) and BEL1-like homeobox (BLH or BELL, hereafter referred to as BLH), which are involved in shoot apical meristem (SAM) function, as well as in the determination and morphological development of leaves, stems and inflorescences. Selective protein-protein interactions between KNOXs and BLHs affect heterodimer subcellular localization and target affinity. KNOXs exert their roles by maintaining a proper balance between undifferentiated and differentiated cell state through the modulation of multiple hormonal pathways. A pivotal function of KNOX in evolutionary diversification of leaf morphology has been assessed. In the SAM of both simple- and compound-leafed seed species, downregulation of most class 1 KNOX (KNOX1) genes marks the sites of leaf primordia initiation. However, KNOX1 expression is re-established during leaf primordia development of compound-leafed species to maintain transient indeterminacy and morphogenetic activity at the leaf margins. Despite the increasing knowledge available about KNOX1 protein function in plant development, a comprehensive view on their downstream effectors remains elusive. This review highlights the role of TALE proteins in leaf initiation and morphological plasticity with a focus on recent advances in the identification of downstream target genes and pathways.

KNAT3/4/5‐like class 2 KNOX transcription factors are involved in Medicago truncatula symbiotic nodule organ development

“…S1). KNOX genes have been largely characterized as key players in morphogenetic processes when boundaries between undifferentiated cells and organ outgrowth have to be established, such as during the formation of leaf primordia, leaf serrations and compound leaves (Di Giacomo et al, 2013;Bar & Ori, 2015). KNOX proteins are classified into two subfamilies, class 1 and class 2, based on intron number and position, structural similarities in the consensus domain outside the homeobox, and their different expression patterns (Kerstetter et al, 1994;Hake et al, 2004).…”

“…As class 1 KNOX proteins act in several developmental processes by maintaining an undifferentiated cell fate, their ectopic expression deeply affects shoot architecture and leaf shape (Sinha et al, 1993;Lincoln et al, 1994;Hareven et al, 1996;Tamaoki et al, 1997;Frugis et al, 2001). Several hormonal pathways act downstream of class 1 KNOX proteins, most notably cytokinins, gibberellins and brassinosteroids (Hake et al, 2004;Bolduc et al, 2012;Di Giacomo et al, 2013;Tsuda et al, 2014). By contrast, the function of class 2 KNOX proteins, as well as potential connections with hormonal pathways, has long remained unknown.…”

Transcription Factor Networks in Leaves of Cichorium endivia: New Insights into the Relationship between Photosynthesis and Leaf Development

“…A TALE homeobox homologous to the Arabidopsis BLH8 (CeTALE_22) was placed in cluster six. TALEs are encoded by two small subfamilies in the Arabidopsis, KNOX and BLH [88]. TALEs are master controllers of plant development that regulate the fine equilibrium between cell fate maintenance and differentiation in the lateral organ formation at the shoot apical meristem (SAM) and during leaf morphogenesis and leaf marginal outgrowth [88,89].…”

“…TALEs are encoded by two small subfamilies in the Arabidopsis, KNOX and BLH [88]. TALEs are master controllers of plant development that regulate the fine equilibrium between cell fate maintenance and differentiation in the lateral organ formation at the shoot apical meristem (SAM) and during leaf morphogenesis and leaf marginal outgrowth [88,89]. In Arabidopsis, BLH8 interact with the class 1 KNOX SHOOT MERISTEMLESS and BLH9 to restrict organogenesis at the SAM [90].…”

The KNOTTED-like genes of peach (Prunus persica L. Batsch) are differentially expressed during drupe growth and the class 1 KNOPE1 contributes to mesocarp development

“…Synoptically, the KNOX1 are necessary for the proper functioning of shoot apical meristem (SAM) by providing undifferentiated cells from the SAM to lateral organs [21]. A common role of KNOX1 is to prevent cell differentiation, which onsets after their down regulation [23], and regulate the blade margin in simpleor compound-leafed species [24]. As for the silique, the class 1 BREVIPEDICELLUS (BP a.k.a.…”