PP7L is essential for MAIL1-mediated transposable element silencing and primary root growth.

Cloe De Luxán‐Hernández,Geert De Jaeger,Kerstin Wöltje,Senoch Seanpong,Olivier Mathieu,Magdalena Weingartner,Zoltan Magyar,Julia Lohmann,Thierry Pélissier,Wiebke Hellmeyer,Stefan Hoth,Aladár Pettkó‐Szandtner

doi:10.1111/tpj.14655

Abstract

The two paralogous Arabidopsis genes MAINTENANCE OF MERISTEMS (MAIN) and MAINTENANCE OF MERISTEMS LIKE1 (MAIL1) encode a conserved retrotransposon-related plant mobile domain and are known to be required for silencing of transposable elements (TE) and for primary root development. Loss of function of either MAIN or MAIL1 leads to release of heterochromatic TEs, reduced condensation of pericentromeric heterochromatin, cell death of meristem cells and growth arrest of the primary root soon after germination. Here, we show that they act in one protein complex that also contains the inactive isoform of PROTEIN PHOSPHATASE 7 (PP7), which is named PROTEIN PHOSPHATASE 7-LIKE (PP7L). PP7L was previously shown to be important for chloroplast biogenesis and efficient chloroplast protein synthesis. We show that loss of PP7L function leads to the same root growth phenotype as loss of MAIL1 or MAIN. In addition, pp7l mutants show similar silencing defects. Double mutant analyses confirmed that the three proteins act in the same molecular pathway. The primary root growth arrest, which is associated with cell death of stem cells and their daughter cells, is a consequence of genome instability. Our data demonstrate so far unrecognized functions of an inactive phosphatase isoform in a protein complex that is essential for silencing of heterochromatic elements and for maintenance of genome stability in dividing cells.

Highlights

Almost all cells of the plant body descend from small populations of self-renewing stem cells that are maintained within meristems
Loss of function of either MAIN or MAINLIKE 1 (MAIL1) leads to release of heterochromatic transposable elements (TE), reduced condensation of pericentromeric heterochromatin, cell death of meristem cells and growth arrest of the primary root soon after germination
We show that they act in one protein complex that contains the inactive isoform of PROTEIN PHOSPHATASE 7 (PP7), which is named PROTEIN PHOSPHATASE 7-LIKE (PP7L)

Summary

Introduction

Almost all cells of the plant body descend from small populations of self-renewing stem cells that are maintained within meristems. The cells directly adjacent to the QC are named root initials and maintain a stem cell-like character. They are able to renew themselves and to produce daughter cells, which undergo several rounds of rapid cell divisions until reaching the elongation zone, in which they gradually become differentiated (Wendrich et al, 2017a). The status and integrity of the DNA is constantly monitored and detection of DNA damage leads to activation of the two conserved checkpoint kinases ATAXIATELANGIECTASIA MUTATED (ATM) and ATM AND RAD3-RELATED (ATR), which are known to phosphorylate the transcription factor SUPPRESSOR OF GAMMARESPONSE 1 (SOG1) in plants (Sancar et al, 2004; Yoshiyama et al, 2013). SOG1 induces programmed cell death (PCD) in root initials to prevent accumulation and propagation of

Results

Discussion

Conclusion