Fisiologia vegetal

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Evolution and diversity of green plant cell walls
¨ Zoe A Popper
Plant cells are surrounded by a dynamic cell wall that performs many essential biological roles, including regulation of cell expansion, the control of tissue cohesion, ion-exchange and defence against microbes. Recent evidence shows that the suite of polysaccharides and wall proteinsfrom which the plant cell wall is composed shows variation between monophyletic plant taxa. This is likely to have been generated during the evolution of plant groups in response to environmental stress. Understanding the natural variation and diversity that exists between cell walls from different taxa is key to facilitating their future exploitation and manipulation, for example by increasinglignocellulosic content or reducing its recalcitrance for use in biofuel generation.
Address Department of Botany, National University of Ireland Galway, Ireland ¨ Corresponding author: Popper, Zoe A (zoe.popper@nuigalway.ie)

biomass for the emerging biofuel industry. Cell walls have been used as a source of fuel for millennia, for example in the form of wood, derived from dicotyledonous trees,and peat, derived from Sphagnum moss. Algae, while being among the most efficient photosynthetic organisms and of interest for biofuel generation, will not be specifically discussed within this review because their greatest potential is as a source of oils (localised in the vacuole) for use in bio-diesel production, rather than cell wall biomass. This review will focus on the natural variation anddiversity that exists within land plant cell wall composition that is associated with land plant phylogeny (Table 1). A more comprehensive investigation of diverse land plant cell walls not only is essential to facilitate wall modification enabling production of biomass with improved qualities, for example an increased cellulose concentration; but may also suggest the potential of novel crop plants.Current Opinion in Plant Biology 2008, 11:286–292 This review comes from a themed issue on Physiology and metabolism Edited by Ken Keegstra and Markus Pauly Available online 10th April 2008 1369-5266/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2008.02.012

Types of cell wall
Cells walls are composed of three types of layers: the middle lamella, theprimary cell wall and the secondary cell wall. The middle lamella is deposited soon after mitosis creating a boundary between the two daughter nuclei and, once the cell plate is complete, the primary cell wall is deposited and continues to be deposited throughout cell growth and expansion. Therefore, although the primary cell wall is typically only 0.1– 10 mm thick, its composition is of importance forbiomass accumulation through controlling cell growth. The secondary cell wall is deposited internally to the primary cell wall at the onset of differentiation, once cell growth has ceased. However, secondary cell walls are not present in all cell types, parenchyma and collenchyma frequently have only a primary cell wall, or in all plant taxa. In spermatophytes, secondary cell wall composition isknown to vary from one cell type and reflects cell function. For example, many secondary cell walls, particularly xylem cells, contain lignin that increases wall strength.

Introduction
Cellulose-rich cell walls, one of the defining characteristics of plants, are of fundamental importance for normal plant growth and development. Despite recognition that distinct differences exist in wall chemistrybetween Angiosperm taxa [1,2], the main focus of cell wall research has been on Angiosperm, primarily crop, species. Recently, however, there has been increasing interest in the cell wall biochemistry of non-Angiosperm plants [3,4,5–8,9,10–14], and evidence has emerged to support the hypothesis [15] that changes in cell wall composition are fundamentally involved with plant evolution and...