Rhytidome- and cork- type barks of holm oak, cork oak and their hybrids highlight processes leading to cork formation
The periderm is basic for land plants due to its protective role during radial growth, which is achieved by the polymers deposited in the cell walls. Despite the research on the topic has unravelled the role of several enzymes and transcription factors, many questions remain open, especially those regarding cell development. Here we use the outer bark of cork oak (cork), holm oak (rhytidome), and their natural hybrids' to further understand the mechanisms underlying periderm development. Cork is an outstanding model as it consists of a thick and very homogeneous periderm produced by a permanent mother-cell layer (phellogen). Conversely, holm oak contains a more heterogeneous bark including several thin periderms mixed with phloem, also known as a rhytidome. The inclusion of hybrid samples showing rhytidome-type and cork-type barks is valuable to approach cork development, allowing an accurate identification of candidate genes and processes. The present study underscores that biotic stress and cell death signalling are enhanced in rhytidome-type barks while lipid metabolism and cell cycle are enriched in cork-type barks. Based on the DEGs most expressed related to development, we highlight that cell division, cell expansion, and cell differentiation could account for the differences found between cork and rhytidome-type barks. Overall design: Comparative gene expression profile analysis of RNA-seq data of outer bark from cork oak, holm oak and the hybrids. Sample: Outer barks of tree trunks from four adult cork oaks (Quercus suber L.), four holm oaks (Quercus ilex L.) and six Q. ilex x Q. suber hybrids. Trees were naturally grown in a mixed holm oak-cork oak forest in Fregenal de la Sierra (Extremadura, Spain). Outer bark of cork oak is named as cork, outer bark of holm oak is named as rhytidome, outer of the hybrid similar to that of cork oak is named as cork-like and outer bark of the hybrids similar to holm oak are named as rhytidome-like.  Replicates: 4 south orientation biological replicates from the cork oak, holm oak, and rhytidome-like bark hybrid and 4 samples coming from the north, south, west, and east of the cork-like bark hybrid