How Holm Oak, Cork Oak, and Their Hybrids Create Cork

Jenn Hoskins
4th June, 2024

How Holm Oak, Cork Oak, and Their Hybrids Create Cork

Microscopic analysis of bark anatomy reveals that Quercus suber and its backcrossed hybrid produce a thick, continuous cork layer (a, d), in contrast to the thin, layered rhytidome characteristic of Quercus ilex and the F1 hybrid (b, c).

Image adapted from: Armendariz et al. / CC BY (Source)

Key Findings

  • The study, conducted by researchers from the Universitat de Girona, focused on the differences in periderm formation between cork oak and holm oak
  • Cork oak forms a persistent periderm, resulting in a uniform outer bark, while holm oak forms sequential periderms, creating a heterogeneous outer bark
  • Genetic analysis revealed that cork oak's ability to produce thick, homogeneous cork is due to the upregulation of specific genes related to suberin production and cell wall biogenesis
The periderm is a crucial tissue in land plants, providing protection during radial growth through the deposition of specific polymers in cell walls. Typically, in many trees such as the holm oak, the initial periderm is replaced by subsequent internal periderms, creating a heterogeneous outer bark known as rhytidome. However, cork oak is unique in forming a persistent periderm that results in a homogeneous outer bark composed of thick phellem cell layers, known as cork. This study, conducted by researchers from the Universitat de Girona[1], sheds light on this distinctive characteristic of cork oak and its implications. The study begins by highlighting the differences in periderm formation between cork oak and holm oak, despite their overlapping distribution ranges and potential for hybridization. In holm oak, the outer bark's heterogeneity arises from sequential periderms interspersed with dead secondary phloem. Conversely, cork oak maintains a long-lived periderm, resulting in a uniform outer bark. This distinction is significant as it underpins cork oak's ability to produce commercially valuable cork. Previous research has delved into the periderm's protective role and its formation mechanisms. For instance, the periderm comprises a meristematic tissue called the phellogen, or cork cambium, and its derivatives: the lignosuberized phellem and the phelloderm[2]. The focus on the chemical composition of the phellem has been due to its industrial relevance. However, there is a growing interest in understanding the regulatory networks underlying periderm development to enhance plant resilience and carbon sequestration. Comparative transcriptome studies between cork oak and holm oak have revealed significant insights into the genetic basis of their differing bark structures. For example, the transcriptome comparison of these two oak species identified candidate genes involved in secondary metabolism and phellogen activity, which contribute to the exceptionally thick and pure cork oak phellem[3]. These findings suggest that the upregulation of suberin-related genes and regulatory genes in cork oak leads to the increased number of phellem layers, distinguishing it from holm oak. Further research on cork-producing and non-cork-producing hybrids of Quercus cerris × suber has identified a significant number of genes exclusively associated with cork production[4]. These genes are involved in processes such as cell wall biogenesis, lipid metabolism, and metal ion binding, indicating that specific regulatory mechanisms are necessary for cork formation. This study advances our understanding of the genetic regulation behind cork production, highlighting the unique and essential nature of these mechanisms. The current study by the Universitat de Girona ties together these previous findings by focusing on the persistent periderm formation in cork oak. The researchers examined the genetic and biochemical processes that enable cork oak to maintain a long-lived periderm, resulting in a homogeneous outer bark. They found that the regulatory networks and gene expressions involved in cork oak's periderm formation are distinct from those in holm oak, despite their potential for hybridization. This persistence of the periderm in cork oak is crucial for its ability to produce high-quality cork, which has significant economic, ecological, and social importance, particularly in Mediterranean countries like Portugal and Spain. In conclusion, this study enhances our understanding of the regulatory mechanisms behind periderm formation in cork oak. By comparing the genetic and biochemical processes in cork oak and holm oak, the researchers have identified key differences that explain the unique ability of cork oak to produce a homogeneous outer bark. These findings not only contribute to our knowledge of plant biology but also have potential applications in improving plant resilience and carbon sequestration.

GeneticsBiochemPlant Science

References

Main Study

1) Rhytidome- and cork-type barks of holm oak, cork oak and their hybrids highlight processes leading to cork formation

Published 3rd June, 2024

https://doi.org/10.1186/s12870-024-05192-4

Related Studies

2) The Making of Plant Armor: The Periderm.

https://doi.org/10.1146/annurev-arplant-102720-031405

3) A comparative transcriptomic approach to understanding the formation of cork.

https://doi.org/10.1007/s11103-017-0682-9

4) Characterization of the cork formation and production transcriptome in Quercus cerris × suber hybrids.

https://doi.org/10.1007/s12298-018-0526-3


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