SummaryMulticellular organisms are composed of many cell types that acquire their specific fate through a precisely controlled pattern of gene expression in time and space dictated in part by cell typespecific promoter activity. Understanding the contribution of highly specialized cell types in the development of a whole organism requires the ability to isolate or analyze different cell types separately. We have characterized and validated a large collection of root cell type-specific promoters and have generated cell type-specific marker lines. These benchmarked promoters can be readily used to evaluate cell type-specific complementation of mutant phenotypes, or to knockdown gene expression using targeted expression of artificial miRNA. We also generated vectors and characterized transgenic lines for cell type-specific induction of gene expression and cell type-specific isolation of nuclei for RNA and chromatin profiling. Vectors and seeds from transgenic Arabidopsis plants will be freely available, and will promote rapid progress in cell typespecific functional genomics. We demonstrate the power of this promoter set for analysis of complex biological processes by investigating the contribution of root cell types in the IRT1-dependent root iron uptake. Our findings revealed the complex spatial expression pattern of IRT1 in both root epidermis and phloem companion cells and the requirement for IRT1 to be expressed in both cell types for proper iron homeostasis. *For correspondence (yvon.jaillais@ens-lyon.fr, françois.roudier@biologie.ens.fr, or Gregory.Vert@i2bc.paris-saclay.fr). Significance statementIn multicellular organisms, different cell types often perform discrete functions. Here we present a collection of benchmarked cell type-specific promoters for functional genomics of the Arabidopsis root, and use this resource as a test case to spatially deconstruct iron uptake.

(2015) 'PRC2 represses dedi erentiation of mature somatic cells in Arabidopsis.', Nature plants., 1 (7). p. 15089.Further information on publisher's website:http://dx.doi.org/10.1038/nplants.2015.89Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. subscreened for those that initiate ectopic proliferation of mature cells. We reasoned that root hairs represent a suitable system for this study since they have a unicellular structure on the root 3 epidermis with highly specialised functions in water and nutrient uptake 4 . As shown in Fig. 1a (Fig. 1), pointing to the requirement of PRC2 activity in suppressing ectopic proliferation of differentiated cells. Remarkably, sustained divisions ultimately lead to the formation of calli, some of which further develop into somatic embryos that show typical accumulation of lipids (Fig. 1a).Several lines of evidence demonstrate that PRC2 deficiency leads to mitotic reactivation in terminally differentiated root hairs. First, the root hair-specific differentiation marker pEXP7:NLS-GFP 8 shows a similar expression pattern in WT, heterozygous and homozygous emf2-3 vrn2-1 roots, indicating that in the absence of PRC2 root hair differentiation occurs indistinguishably from WT (Fig. 1b). In addition, root hairs of 7-day-old emf2-3 vrn2-1 plants are initially unicellular and only become multicellular in older plants (Fig. 1c). Serial observations of emf2-3 vrn2-1 roots expressing plasma membrane (LTI6-GFP) and nucleus (H2B-YFP) markers 9further indicate that only fully elongated root hairs undergo nuclear and cellular division (Fig. 1d).Multicellularisation of root hairs does not appear to follow a typical gradient along the root axis, suggesting that it is not correlated with the timing of their initial differentiation ( Supplementary Fig. 1). Time-lapse imaging of emf2-3 vrn2-1 root hairs expressing LTI6-GFP and H2B-YFP confirms that these nuclear divisions are accompanied by the formation of a new cell plate and are therefore clearly distinct from the nuclear fragmentation occasionally observed in WT root hairs 10 (Fig. 1e, Supplementary Video 1).As part of the differentiation program, Arabidopsis root hairs undergo several rounds of 4 endoreduplication, a modified cell cycle in which cells replicate nuclear DNA without mitoses and concomitantly increase nucleus and cell size 11 . Entry into the endoreduplication cycle is generally accepted as a commitment for terminal differentiation since cells that have endoreduplicated do not nor...