Phellem translational landscape throughout secondary development in Arabidopsis roots

Summary The phellem is a specialized boundary tissue providing the first line of defense against abiotic and biotic stresses in organs undergoing secondary growth. Phellem cells undergo several differentiation steps, which include cell wall suberization, cell expansion and programmed cell death. Yet, the molecular players acting particularly in phellem cell differentiation remain poorly described, particularly in the widely used model plant Arabidopsis thaliana.Using specific marker lines we followed the onset and progression of phellem differentiation in A. thaliana roots, and further targeted the translatome of new developed phellem cells using Translating Ribosome Affinity Purification followed by mRNA sequencing (TRAP-SEQ).We showed that phellem suberization is initiated early after phellogen (cork cambium) division. The specific translational landscape was organized in three main domains related to energy production, synthesis and transport of cell wall components, and response to stimulus. Novel players in phellem differentiation, related to suberin monomer transport and assembly, as well as novel transcription regulators were identified.This strategy provided an unprecedented resolution of the transcriptome of developing phellem cells, giving a detailed and specific view on the molecular mechanisms controlling cell differentiation in periderm tissues of the model plant Arabidopsis. Significance statementTo improve the understanding of phellem differentiation into a suberized protective layer, we followed the establishment of periderm in Arabidopsis roots and sequenced the phellem-specific translatome. We found that phellem suberization occurs shortly after pericycle cell divisions with the induction of pivotal suberin biosynthesis genes. In parallel, we detected the activation of three central genetic modules acting throughout the phellem differentiation. This study provides a unique and targeted genetic resource for further functional studies of phellem tissues.