Cell-type specific transcriptional networks in root xylem adjacent cell layers

Transport of water, ions and signals from roots to leaves via the xylem vessels is essential for plant life and needs to be tightly regulated. The final composition of the transpiration stream before passage into the shoots is controlled by the xylem-adjacent cell layers, namely xylem parenchyma and pericycle, in the upper part of the root. To unravel regulatory networks in this strategically important location, we generated Arabidopsis lines expressing a nuclear tag under the control of the HKT1 promoter. HKT1 retrieves sodium from the xylem to prevent toxic levels in the shoot, and this function depends on its specific expression in upper root xylem-adjacent tissues. Based on FACS RNA-sequencing and INTACT ChIP-sequencing, we identified the gene repertoire that is preferentially expressed in the tagged cell types and discovered transcription factors experiencing cell-type specific loss of H3K27me3 demethylation. For one of these, ZAT6, we show that H3K27me3-demethylase REF6 is required for de-repression. Analysis of zat6 mutants revealed that ZAT6 activates a suite of cell-type specific downstream genes and restricts Na+ accumulation in the shoots. The combined Files open novel opportunities for ‘bottom-up’ causal dissection of cell-type specific regulatory networks that control root-to-shoot communication under environmental challenge.