Phellem translational landscape throughout secondary development inArabidopsisroots

Abstract

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.

Spatiotemporal development of suberized barriers in cork oak taproots

“…In fact, in sections taken at 13 and 16 cm, two layers of suberized cells were visible in some regions. At this stage we found no clear evidence for a collapse of the endodermis layer, upon the establishment of the root secondary development, as described for Arabidopsis (Wunderling et al 2018, Leal et al 2021. With the establishment of a suberized phellem, the presence of the endodermis would be redundant.…”

“…Plant roots display a gradient of cellular development, with primary development detected near the root tip and secondary development at fully matured regions (Machado et al 2013, Wunderling et al 2018, the latter being detected at the earliest near the root collar of the primary root (Leal et al 2021). In the present paper we followed the early stages of cork oak taproot development, highlighting the onset of secondary…”

“…Remarkably, the secondary development of the Arabidopsis root was observed to start after one week of germination in the most mature part of the seedling root (Leal et al 2021). Also, homologs of previously identified markers of cork oak phellem differentiation at stem level were found to be expressed in Arabidopsis newly formed root phellem cells (Wunderling et al 2018, Leal et al 2021, indicating a regulatory process that is conserved not only across species but also between different organs.…”

“…Cork oak roots are also known to develop phellem in mature lateral roots (Machado et al 2013). In A. thaliana, the root phellogen forms from the pericycle cells located beneath the endodermis (Wunderling et al 2018, Leal et al 2021, and the same has been suggested to occur in cork oak roots (Machado et al 2013), contrasting with the process described in stems Pereira 2004, Crang et al 2018). The root endodermis is the first suberized layer to be developed during primary development (Andersen et al 2015).…”

“…In order to predict putative regulatory pathways regulating suberin biosynthetic genes in roots, we overlapped the list of DEGs enriched in SD roots zones, with the phellemassociated translatome recently profiled in Arabidopsis roots (Leal et al 2021). This cellspecific translatome was identified using translating ribosome affinity purification in mature root segments, based on GPAT5-driven expression (Leal et al 2021). From the 5,098 DEGs enriched in SD, 4,997 could be assigned to 3,356 candidate Arabidopsis homologs (since more than one cork oak gene could be assigned to a single Arabidopsis gene).…”