A comparative transcriptomic approach to understanding the formation of cork
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The transcriptome comparison of two oak species reveals possible candidates accounting for the exceptionally thick and pure cork oak phellem, such as those involved in secondary metabolism and phellogen activity.
Cork oak, Quercus suber, differs from other Mediterranean oaks such as holm oak (Quercus ilex) by the thickness and organization of the external bark. While holm oak outer bark contains sequential periderms interspersed with dead secondary phloem (rhytidome), the cork oak outer bark only contains thick layers of phellem (cork rings) that accumulate until reaching a thickness that allows industrial uses. Here we compare the cork oak outer bark transcriptome with that of holm oak. Both transcriptomes present similitudes in their complexity, but whereas cork oak external bark is enriched with upregulated genes related to suberin, which is the main polymer responsible for the protective function of periderm, the upregulated categories of holm oak are enriched in abiotic stress and chromatin assembly. Concomitantly with the upregulation of suberin-related genes, there is also induction of regulatory and meristematic genes, whose predicted activities agree with the increased number of phellem layers found in the cork oak sample. Further transcript profiling among different cork oak tissues and conditions suggests that cork and wood share many regulatory mechanisms, probably reflecting similar ontogeny. Moreover, the analysis of transcripts accumulation during the cork growth season showed that most regulatory genes are upregulated early in the season when the cork cambium becomes active. Altogether our work provides the first transcriptome comparison between cork oak and holm oak outer bark, which unveils new regulatory candidate genes of phellem development.
KeywordsCork Cork oak Phellem Rhytidome Suberin Wax
We would like to thank Professor M. Molinas (Departament de Biologia, UdG, Girona) for her useful advice and feedback during the analysis of the results and the drafting of the manuscript. The authors are grateful to Dr. R. Verdaguer, S. Fernández, S. Gómez and N. Salvatella for their help in cork harvesting. We thank Professor C. Pla (Departament de Biologia, UdG, Girona) for kindly lending the Thermocycler and Mr J. Blavia and Ms C. Carulla (Serveis Tècnics de Recerca, Universitat de Girona, Spain) for their highly skilled work with SEM. This work was supported by the Ministerio de Innovación y Ciencia [AGL2009-13745, FPI grant to P.B.], the Ministerio de Economía y Competitividad and FEDER funding [AGL2012-36725; AGL2015-67495-C2-1-R]. J.A.P.P. acknowledges the European Union’s Seventh Framework Programme for research, technological development and demonstration (EU FP7 Agreement No. 621321) and the Polish financial sources for education (2015–2019) allocated to Project No (W26/7.PR/2015).
PB, OS, JP and MF designed the experiment; PB extracted the RNA and purified the mRNA; PB, CH and CN performed bioinformatics; PB and AS performed qPCR; all authors analyzed and discussed the data. PB, MS, OS and MF wrote the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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