All critical developmental and physiological events in a plant9s life cycle depend on the proper activation and repression of specific gene sets, and this often involves epigenetic mechanisms. Some Arabidopsis thaliana mutants with disorders of the epigenetic machinery exhibit pleiotropic defects, including incurved leaves and early flowering, due to the ectopic and heterochronic derepression of developmental regulators. Here, we studied one such mutant class, the incurvata11 (icu11) loss-of-function mutants. We have identified ICU11 as the founding member of a small gene family that we have named CUPULIFORMIS (CP). This family is part of the 2-oxoglutarate/Fe(II)-dependent dioxygenase superfamily. ICU11 and its closest paralog, CP2, have unequally redundant functions: although cp2 mutants are phenotypically wild type, icu11 cp2 double mutants skip vegetative development and flower upon germination. This phenotype is reminiscent of loss-of-function mutants of the Polycomb-group genes EMBRYONIC FLOWER1 (EMF1) and EMF2. Double mutants harboring icu11 alleles and loss-of-function alleles of genes encoding components of the epigenetic machinery exhibit synergistic, severe phenotypes, and some are similar to those of emf mutants. Hundreds of genes are misexpressed in icu11 plants, including SEPALLATA3 (SEP3), and derepression of SEP3 causes the leaf phenotype of icu11. ICU11 and CP2 are nucleoplasmic proteins that act as epigenetic repressors through an unknown mechanism involving histone modification, but not DNA methylation.

Polycomb Repressive Complex 2 (PRC2) catalyzes the trimethylation of lysine 27 of histone H3 (H3K27me3) and plays a key role in epigenetic repression of gene expression in plants and animals. PRC2 core components have all been identified in Arabidopsis thaliana , with an expanding list of accessory proteins, some of which facilitate the recruitment of PRC2 to specific targets. INCURVATA11 (ICU11) is a 2-oxoglutarate and Fe ²⁺ -dependent dioxygenase that was previously shown to be a likely PRC2 accessory protein. In Tandem Affinity Purification (TAP)-based screens for interacting partners of ICU11 and its redundant paralog CUPULIFORMIS2 (CP2), we discovered that ICU11 interacts with four PRC2 core components, including EMBRYONIC FLOWER 2 (EMF2), and with the accessory proteins EMF1, TELOMERE REPEAT BINDING 1 (TRB1), TRB2, and TRB3. CP2 did not interact with PRC2 core components, nor with TRB1, TRB2, or TRB3, but did interact with TRB4 and TRB5. Both ICU11 and CP2 interacted with the nuclear proteins NAC DOMAIN CONTAINING PROTEIN 50 (NAC050), NAC052 and COP9 SIGNALOSOME SUBUNIT 1 (CSN1). Bimolecular Fluorescence Complementation (BiFC) assays revealed that ICU11 and CP2 both interact with the PRC2 core components CURLY LEAF and SWINGER, and the accessory proteins LIKE HETEROCHROMATIN PROTEIN 1, TRB1, and TRB3. ICU11 and CP2 did not interact with each other. Beyond their phenotypes, transcriptomic profiles revealed strong similarities between emf2-3 and the double mutant icu11-5 cp2-1 , as well as with mutants in PRC2 core components. A significant proportion of the genes mis-regulated in icu11-5 cp2-1 are known to harbor H3K27me3 repressive marks in the wild type. Our results provide further evidence that ICU11 acts as a PRC2 accessory protein, and strongly suggest that CP2 plays a similar role.

The paralogous genes INCURVATA11 ( ICU11 ) and CUPULIFORMIS2 ( CP2 ) encode components of the epigenetic machinery in Arabidopsis and belong to the 2-oxoglutarate and Fe (II)-dependent dioxygenase superfamily. We previously inferred unequal functional redundancy between ICU11 and CP2 from a study of the synergistic phenotypes of the double mutant and sesquimutant combinations of icu11 and cp2 mutations, although they represented mixed genetic backgrounds. To avoid potential confounding effects arising from different genetic backgrounds, we generated the icu11-5 and icu11-6 mutants via CRISPR/Cas genome editing in the Col-0 background and crossed them to cp2 mutants in Col-0. The resulting mutants exhibited a postembryonic-lethal phenotype reminiscent of strong embryonic flower ( emf ) mutants. Double mutants involving icu11-5 and mutations affecting epigenetic machinery components displayed synergistic phenotypes, whereas cp2-3 did not besides icu11-5 . Our results confirmed the unequal functional redundancy between ICU11 and CP2 and demonstrated that it is not allele or genetic background specific. An increase in sucrose content in the culture medium partially rescued the post-germinative lethality of icu11 cp2 double mutants and sesquimutants, facilitating the study of their morphological phenotypes throughout their life cycle, which include floral organ homeotic transformations. We thus established that the ICU11-CP2 module is required for proper flower organ identity.

Screens for specific phenotypes have long been a cornerstone of biology. Here, we present an updated synthesis of our large-scale visual screens for Arabidopsis (Arabidopsis thaliana) mutants that exhibit leaf morphology defects. In our 2009 review, we used phenotypes to group the leaf mutants that we had isolated and characterized since 1992; here, by contrast, we functionally classified the mutations that we studied over the last 16 years based on the biological programs they disrupt. Since 2009, we have identified and analyzed 38 genes required for proper leaf development; these genes are involved in translation, chloroplast function, cell wall construction, auxin homeostasis, microRNA biogenesis, and epigenetic regulation. Many of the identified mutants have pleiotropic phenotypes, consistent with the central roles of the affected pathways in development. In this review, we systematically link morphological traits to specific molecular dysfunctions, highlighting the enduring utility of forward genetic approaches. We found that the Arabidopsis leaf is a model organ of a model organism, and we have used this model-in-a-model system to dissect whole-plant traits such as cell proliferation and expansion, and to improve our understanding of the genetic control of plant form and size.