Colenquima

Annals of Botany 110: 1083– 1098, 2012 doi:10.1093/aob/mcs186, available online at www.aob.oxfordjournals.org

INVITED REVIEW

Collenchyma: a versatile mechanical tissue with dynamic cell walls
Olivier Leroux1,2,*
Botany and Plant Science and Ryan Institute, School of Natural Sciences, National University of Ireland Galway, University Road, Galway, Ireland and 2Pteridology, Department ofBiology, Ghent University, KL Ledeganckstraat 35, B-9000 Ghent, Belgium * E-mail: Olivier.Leroux@UGent.be
Received: 29 April 2012 Returned for revision: 11 June 2012 Accepted: 6 July 2012 Published electronically: 29 August 2012
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† Background Collenchyma has remained in the shadow ofcommercially exploited mechanical tissues such as wood and fibres, and therefore has received little attention since it was first described. However, collenchyma is highly dynamic, especially compared with sclerenchyma. It is the main supporting tissue of growing organs with walls thickening during and after elongation. In older organs, collenchyma may become more rigid due to changes in cell wallcomposition or may undergo sclerification through lignification of newly deposited cell wall material. While much is known about the systematic and organographic distribution of collenchyma, there is rather less information regarding the molecular architecture and properties of its cell walls. † Scope and conclusions This review summarizes several aspects that have not previously been extensivelydiscussed including the origin of the term ‘collenchyma’ and the history of its typology. As the cell walls of collenchyma largely determine the dynamic characteristics of this tissue, I summarize the current state of knowledge regarding their structure and molecular composition. Unfortunately, to date, detailed studies specifically focusing on collenchyma cell walls have not been undertaken.However, generating a more detailed understanding of the structural and compositional modifications associated with the transition from plastic to elastic collenchyma cell wall properties is likely to provide significant insights into how specific configurations of cell wall polymers result in specific functional properties. This approach, focusing on architecture and functional properties, is likely toprovide improved clarity on the controversial definition of collenchyma. Key words: Collenchyma, histology, plant anatomy, mechanical tissue, plant cell wall, primary and secondary cell walls, plant biomechanics.

IN T RO DU C T IO N The emergence of mechanical tissues was a key innovation in the evolution of land plants and a prerequisite for the appearance of large terrestrial species. By themiddle Devonian, many plant species developed a hypodermal sterome consisting of heavily thickened sclerenchyma cells (Rowe and Speck, 2004). Biomechanical investigations indicated that the sterome significantly contributed to the stiffness of stems and allowed them to reach great heights and evolve diverse branched architectures compared with plants with turgor-based support systems (Rowe and Speck,2004). While sclerenchyma tissues confer rigidity and tensile and shear strength to many plant organs (Niklas, 1992; Jarvis, 2007), their properties are incapable of supporting growing plant organs which undergo extensive turgor-driven elongation. Indeed, sclerified tissues generally consist of dead cells with non-extensible rigid cell walls which are unable to undergo mitotic divisions. In smallslowly growing plant organs, turgor pressure generated in parenchyma cells may provide sufficient support, but many plant stems grow fast and are fragile, and therefore they cannot fully rely on turgor pressure for support. Partly because they are nonsclerified and only minimally lignified, young plant tissues are preferentially selected by grazing animals and plant bugs. For this reason, supporting...