Populus: arabidopsis for forestry. Do we need a model tree?

Abstract

Trees are used to produce a variety of wood-based products including timber, pulp and paper. More recently, their use as a source of renewable energy has also been highlighted, as has their value for carbon mitigation within the Kyoto Protocol. Relative to food crops, the domestication of trees has only just begun; the long generation time and complex nature of juvenile and mature growth forms are contributory factors. To accelerate domestication, and to understand further some of the unique processes that occur in woody plants such as dormancy and secondary wood formation, a 'model' tree is needed. Here it is argued that Populus is rapidly becoming accepted as the 'model' woody plant and that such a 'model' tree is necessary to complement the genetic resource being developed in arabidopsis. The genus Populus (poplars, cottonwoods and aspens) contains approx. 30 species of woody plant, all found in the Northern hemisphere and exhibiting some of the fastest growth rates observed in temperate trees. Populus is fulfilling the 'model' role for a number of reasons. First, and most important, is the very recent commitment to sequence the Populus genome, a project initiated in February 2002. This will be the first woody plant to be sequenced. Other reasons include the relatively small genome size (450-550 Mbp) of Populus, the large number of molecular genetic maps and the ease of genetic transformation. Populus may also be propagated vegetatively, making mapping populations immortal and facilitating the production of large amounts of clonal material for experimentation. Hybridization occurs routinely and, in these respects, Populus has many similarities to arabidopsis. However, Populus also differs from arabidopsis in many respects, including being dioecious, which makes selfing and back-cross manipulations impossible. The long time-to-flower is also a limitation, whilst physiological and biochemical experiments are more readily conducted in Populus compared with the small-statured arabidopsis. Recent advances in the development of large expressed sequence tagged collections, microarray analysis and the free distribution of mapping pedigrees for quantitative trait loci analysis secure Populus as the ideal subject for further exploitation by a wide range of scientists including breeders, physiologists, biochemists and molecular biologists. In addition, and in contrast to other model plants, the genus Populus also has genuine commercial value as a tree for timber, plywood, pulp and paper.

Fig. 1. Summary schematic of molecular genetic map of Populus and the occurrence of some QTL for leaf morphological and wood‐yield traits. The genome is diploid and consists of 19 linkage groups, here labelled A–Y. The inset shows the candidate gene approach used to map candidate gene PhyB2 on the Populus genetic map and co‐locating for the QTL ‘date of bud‐flush’. Other traits are for QTL located either in contrasting growing environments of the UK and USA (LS, leaf size; VOL, stem volume) or for traits confirmed through other analyses (Cell Area, leaf epidermal cell area). Data modified from Bradshaw et al. (1994), ¹ Frewen et al. (2000), ² Taylor et al. (2001b), ³ Wu (1998), ⁵ Bradshaw (1996) and ⁴ G.Taylor et al. (pers. comm.).

Fig. 2. Contrasting parents of the mapping pedigree ‘family 331’, where a cross between a female P. trichocarpa (A) and male P. deltoides (B) was used to generate an F₁ and then F₂ pedigree. Black square = 1 cm². Images of adaxial epidermal cells are also shown for both parents (C, P. trichocarpa; D, P. deltoides). Bar = 100 µm. The table illustrates the contrasting morphophysiological characteristics that have been used to detect a wide range of QTL in this pedigree. ¹ Taylor et al. (2001a), ² Ferris et al. (2002). Photographic image reproduced from Ferris et al. (2002), with permission.

Fig. 3. Classification of ESTs from the cambial region (A) and developing xylem (B) libraries. Modified from the BLASTX scores > 100, modified from Sterky et al. (1998).