Populus trichocarpa

Populus trichocarpa (Black Cottonwood; also known as Western Balsam Poplar or California Poplar) is a tree species native to western North America. It is used for timber, and is notable as a model organism in plant biology. Its full genome sequence was published in 2006. It is the first, and so far only, tree species ever to be sequenced and contains the largest number of genes ever discovered in any organism.

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Description

It is a large tree, growing to a height of 30-50 m and a trunk diameter of over 2 m, which makes it the largest poplar species in the Americas. The bark is grey and covered with lenticels, becoming thick and deeply fissured on old trees. It is normally fairly short-lived, but some trees may live for up to 400 years (Forbes 2006).

The stem is grey in the older parts and light brown in younger parts. The crown is usually roughly conical and quite dense. In large trees the lower branches droop downwards. Spur shoots are common. The wood has a light coloring and a straight grain.

The leaves are 7-20 cm long with a dark green upper side and glaucous light grey-green underside; larger leaves, up to 30 cm long, may be produced on stump sprouts and very vigorous young trees. The leaves are alternate, elliptic with a crenate margin and an acute tip, and reticulate venation (see leaf terminology). The petiole is reddish. The buds are conical, long, narrow and sticky, with a strong balsam scent in spring when they open.

P. trichocarpa has an extensive and aggressive root system, which can invade and damage drainage systems. Sometimes the roots can even damage the foundations of buildings by drying out the soil.

Reproduction

Flowering and Fruiting

P. trichocarpa is normally dioecious; male and female catkins are borne on separate trees. The species reaches flowering age at about 10 years. Flowers may appear in early March to late May in [Washington] and Oregon, and sometimes as late as mid-June in northern and interior British Columbia, Idaho, and Montana. Staminate catkins contain 30 to 60 stamens, elongate to 2 to 3 cm, and are deciduous. Pistillate catkins at maturity are 8 to 20 cm long with rotund-ovate, three carpellate subsessile fruits 5 to 8 mm long. Each capsule contains many minute seeds with long, white cottony hairs.

Seed Production and Dissemination

The seed ripens and is disseminated by late May to late June in Oregon and Washington, but frequently not until mid-July in Idaho and Montana. Abundant seed crops are usually produced every year. Attached to its cotton, the seed is light and buoyant and can be transported long distances by wind and water. Although highly viable, longevity of P. trichocarpa seed under natural conditions may be as short as 2 weeks to a month. This can be increased with cold storage.

Seedling development

Moist seedbeds are essential for high germination, and seedling survival depends on continuously favorable conditions during the first month. Wet bottom lands of rivers and major streams frequently provide such conditions, particularly where bare soil has been exposed or new soil laid down. Germination is epigeal. P. trichocarpa seedlings do not usually become established in abundance after logging unless special measures are taken to prepare the bare, moist seedbeds required for initial establishment. Where seedlings become established in great numbers, they thin out naturally by age 5 because the weaker seedlings of this shade-intolerant species are suppressed.

Vegetative reproduction

P. trichocarpa sprouts readily from stumps. After logging operations, it sometimes regenerates naturally from rooting of partially buried fragments of branches. Sprouting from roots occurs. The species also has the ability to abscise shoots complete with green leaves. These shoots drop to the ground and may root where they fall or may be dispersed by water transport. In some situations, abscission may be one means of colonizing exposed sandbars.

Range

The native range of P. trichocarpa covers large sections of western North America. It extends northeast from Kodiak Island along Cook Inlet to latitude 62° 30° N., then southeast in southeast Alaska and British Columbia to the forested areas of Washington and Oregon, to the mountains in southern California and northern Baja California (lat. 31° N.). It is also found inland, generally on the west side of the Rocky Mountains, in British Columbia, western Alberta, western Montana, and northern Idaho. Scattered small populations have been noted in southeastern Alberta, eastern Montana, western North Dakota, western Wyoming, Utah, and Nevada. It grows up to elevations of 2100 m.

Populus trichocarpa has been one of the most successful introduction of trees to the otherwise treeless Faroe Islands

Use as a model species

P. trichocarpa has several qualities which makes it a good model species for trees:

• Modes genome size (although signicantly larger than the other model plant, Arabidopsis thaliana)
• Rapid growth (for a tree)
• Reaches reproductive maturity 4-6 years
• Economically important
• It represents a phenotypically diverse genus

For these reasons the species has been extensively studied. Its genome sequence was published in 2006 (see "Genome" below). More than 121 000 expressed sequence tags (ESTs) have been sequenced from it. The wide range of topics studied by using P. trichocarpa include the effects of ethylene, lignin biosynthesis, draught tolerance and wood formation.

Economic use

P. trichocarpa wood is light-weight and although not particularly strong, is strong for its weight. The wood material has short, fine cellulose fibres which are used in the production of high-quality book and magazine paper. The wood is also excellent for production of plywood. Living trees are used as windbreaks. P. trichocarpa contains salicin, and has been used medicinally for fever, pain and inflammation.

P. trichocarpa grows very quickly; trees in plantations in Great Britain have reached 18 m tall in 11 years, and 34 m tall in 28 years (Mitchell 1996). It can reach suitable size for pulp production in 10-15 years and about 25 years for timber production.

It is also grown as an ornamental tree, valued for its fast growth and scented foliage in spring, detectable from over 100 m distance. The roots are however invasive, and it can damage the foundations of buildings on shrinkable clay soils if planted nearby (Mitchell 1996).

Genome

Characteristics

• Size: 485 million base pairs. (Human genome: 3 billion base pairs)
• Proportion of heterochromatin to euchromatin: 3:7
• Number of chromosomes: 19
• Number of putative genes: 45,555, the largest number of genes ever recorded (estimate in September 2006)
• Mitochondrial genome: 803,000 base pairs, 52 genes
• Chloroplast genome: 157,000 base pairs, 101 genes

General information

The sequence of P. trichocarpa is that of an individual female specimen "Nisqually-1", named after the Nisqually River in the Washington state in the USA, where the specimen was collected. The sequencing was performed at the Joint Genome Institute by using the shotgun method. The depth of the sequencing was approximately 7,5 x (meaning that each base pair was sequenced on average 7,5 times). Genome annotation was done by primarily by the Joint Genome Institute, the Oak Ridge National Laboratory, the Umeå Plant Science Centre and the Genome Canada.

Prior to the publication of P. trichocarpa genome the only available plant genomes were those of thale cress and rice, both of which are herbaceous. P. trichocarpa is the first woody plant genome to be sequenced. Considering the economic importance of wood and wood products, the availability of a tree genome was necessary. The sequence also allows evolutionary comparisons and the elucidation of basic molecular differences between herbaceous and woody plants.

Nomenclature

"Trichocarpa" is Greek for "hairy fruits". The following scientific names are now considered synonymous with Populus trichocarpa:

References

• P. tricocharpa information at the USDA Forest Service website, written by Dean S. DeBell
• The Populus Database
• Populus genome at the JGI website
• Plants for a Future: Populus trichocarpa
• Forbes, R. D. (2006). Morrisey Old Growth Cottonwood Forest (pdf file)
• Mitchell, A. F. (1996). Alan Mitchell's Trees of Britain. Collins. ISBN 0-00-219213-6.