Phylogenetic Variation in the Silicon Composition of Plants M. J. HODSON 1 , P. J. WHITE 2 , A. MEAD 2 and M. R. BROADLEY 3, * 1 School of Biological and Molecular Sciences, Oxford Brookes University, Headington, Oxford OX3 0BP, UK, 2 Warwick HRI, University of Warwick, Wellesbourne, Warwick CV35 9EF, UK and 3 Plant Sciences Division, School of Biosciences, University of Nottingham, Sutton Bonington, Leicestershire LE12 5RD, UK Received: 7 April 2005 Returned for revision: 8 June 2005 Accepted: 13 July 2005  Background and Aims Silicon (Si) in plants provides structural support and improves tolerance to diseases, drought and metal toxicity. Shoot Si concentrations are generally considered to be greater in monocotyledonous than in non-monocot plant species. The phylogenetic variation in the shoot Si concentration of plants reported in the primary literature has been quantified.  Methods Studies were identified which reported Si concentrations in leaf or non-woody shoot tissues from at least two plant species growing in the same environment. Each study contained at least one species in common with another study.  Key Results Meta-analysis of the data revealed that, in general, ferns, gymnosperms and angiosperms accumulated less Si in their shoots than non-vascular plant species and horsetails. Within angiosperms and ferns, differences in shoot Si concentration between species grouped by their higher-level phylogenetic position were identified. Within the angiosperms, species from the commelinoid monocot orders Poales and Arecales accumulated substantially more Si in their shoots than species from other monocot clades.  Conclusions A high shoot Si concentration is not a general feature of monocot species. Information on the phylogenetic variation in shoot Si concentration may provide useful palaeoecological and archaeological informa- tion, and inform studies of the biogeochemical cycling of Si and those of the molecular genetics of Si uptake and transport in plants. Key words: Biogeochemistry, fertilizer, phytoliths, silica, transporter, uptake. INTRODUCTION Silicon is the second most abundant element in the soil after oxygen (reviewed by Epstein, 1999; Richmond and Sussman, 2003). Most Si is present in the soil as insoluble oxides or silicates, although soluble silicic acid occurs in the range of 01–06mM. Silicon is also one of the most abundant mineral elements in plant tissues and shoot con- centrations in excess of 10 % d. wt have been reported (Epstein, 1999). Plants growing under natural conditions do not appear to suffer from Si deficiencies. However, Si-containing fertilizers are routinely applied to several crops including rice (Pereira et al., 2004) and sugar cane (Savant et al., 1999) to increase crop yield and quality. Increased Si supply improves the structural integrity of crops and may also improve plant tolerance to diseases, drought and metal toxicities (reviewed by Epstein, 1999; Richmond and Sussman, 2003; Ma, 2004). For example, Si deposition in the cell walls of root endodermal cells may contribute to the maintenance of an effective apoplastic barrier and thereby improve plant resistance to disease and drought stresses (Lux et al., 2002, 2003a, b; Hattori et al., 2005), whilst intra- and extracellular deposition of aluminosilicates in roots and shoots is thought to protect some species from potential Al toxicity (Hodson and Evans, 1995; Hodson and Sangster, 1999; Britez et al., 2002; Jansen et al., 2003; Wang et al., 2004). Early studies of Si in plants noted that species of Poaceae contained between 10 and 20 times the concentration of Si found in non-monocotyledonous species (de Saussure, 1804; Jones and Handreck, 1967). Recent reviews report that Si accumulation is, in general, higher in monocot than in non-monocot species (Epstein, 1999; Richmond and Sussman, 2003). However, detailed sampling of speci- mens from botanical gardens by Takahashi and colleagues (reviewed in Ma and Takahashi, 2002, and references therein) indicates that Si accumulation is largely restricted to primitive land plants and to certain monocot clades, namely the Poaceae, Cyperaceae and Commelinaceae. There is also evidence from these detailed studies that Si may accumulate in certain dicot clades such as the Urticaceae and Cucurbitaceae. In this study, the phylo- genetic variation in shoot Si concentration amongst plant species has been quantified by analysing all of the appro- priate literature data that could be sourced. These include the extensive data compiled by Takahashi and colleagues (Ma and Takahashi, 2002, and references therein). Using a recent consensus angiosperm phylogeny, it was thus possible to test the hypothesis that high shoot Si concen- tration is a general feature of monocot species, and it was also possible to identify Si accumulation features in several other well-represented clades of plant species. MATERIALS AND METHODS Data from 125 studies, contained in 54 papers in the primary literature, were identified that reported Si concentrations of leaf or non-woody shoot tissues in at least two species growing in the same environment, and which contained at * For correspondence. E-mail martin.broadley@nottingham.ac.uk Annals of Botany doi:10.1093/aob/mci255, available online at www.aob.oxfordjournals.org Ó The Author 2005. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. 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