Arabidopsis thaliana (L.) Heynh., Thale Cress

Account Summary

Growth form and preferred habitats

A small, slender, variable, rather straggly grey-green leaved, winter- or summer-annual weed, or very occasionally a biennial (Salisbury 1964), with unexciting tiny white flowers in a raceme up to 30 cm tall, Thale Cress sporadically colonises bare ground in numerous types of open, disturbed, usually unshaded, acid, lowland habitats where there is little or no plant competition. It most frequently occurs on dry or well-drained, sunny, light sandy, or sometimes stony soils in wayside situations. Populations are seldom large. It thrives in well-drained soils, but is prone to phosphorus deficiencies. Poor nutrition frequently causes reduced height, induces early flowering and causes low set set (Wilson et al. 1991). Salisbury (1964) reckoned that ripe seed could be produced within four weeks of germination. Typical habitats include on rocks, walls and ledges, in river shingle, in pavements, on waste ground, along the edges of paths and roadsides, or in garden plots and pots.

Flowering reproduction and use in genetic and physiological research

A. thaliana is a small, self-compatible crucifer with a relatively simple genome comprising 2n=10 chromosomes. In suitable growing conditions, it displays a rapid life cycle that makes it an ideal laboratory organism and it has been very extensively used for plant molecular genetics and physiological studies. Some races of the plant can flower within two or three weeks of germination and set seed after a further two weeks, so generations can be rapidly advanced. While individual plants will set seed by self-pollination, seed can also be obtained by experimental cross-pollination by hand. Because a single plant can produce thousands of seeds in less than eight weeks it is very easy to create segregating populations and to amplify mutant seed stocks in experimental studies.

A. thaliana has the smallest plant genome known to science it being only five times that of the yeast genome. A majority of the 76 mapped mutations confer readily apparent phenotypes such as loss of surface wax, leaf morphology or colour, siliqua morphology, growth habit, seed colour or photo-periodic response (Estelle & Somerville 1986). Projects are in progress to isolate genes of potential use to plant breeders and include genes for disease resistance and male sterility (which ensures seed occurs by cross- rather than self-fertilisation). It is hoped that these genes may be used to induce similar traits in plants of commercial value (Wilson et al. 1991).

In the wild, a proportion of flowers are visited by short-tongued insects including hover-flies, midges and bees and cross pollination and fertilisation achieved. However, insect visits probably are rare, being limited by the presence of only rudimentary floral nectaries (Wilson et al. 1991). Any nectar that is produced is confined to the first day of opening (Cruden et al. 1996). Either way, seed production is usually described as 'copious', each fruit siliqua (or silique) normally releasing 30-60 seeds. A study by Salisbury (1964) in England found a mean seed number of 33 seeds per cylindrical seed pod. A mature plant under favourable growing conditions can produce around 200 siliquae and thus give rise to several thousand small seeds, each less than 1 mm long (Lawrence 1976).

In common with many other members of the cabbage family, the A. thaliana seed coat possesses a mucilaginous layer, so that it sticks to surfaces when wetted, and can thus achieve some measure of secondary dispersal in mud or on tyres or other passing objects.

Seed dormancy, germination and longevity

Physiological strains of A. thaliana occur that display differing photo-periodic responses determining the timing of germination. Seed from wild populations of A. thaliana tends to remain dormant, failing to germinate without a prior cold period of overwintering stratification, thus behaving as spring annuals. However, some plants behave as winter annuals, germinating in the autumn after a period of warm weather, overwintering as leaf rosettes and then flowering and seeding in the early months of the following year. This growth pattern avoids possible drought periods which the fibrous root system of the plant probably would not survive. Other physiological forms of the plant do not require cold treatment to germinate or induce flowering, and they behave as summer annuals. The differing forms mean the species can produce seed almost all year round, although Thale Cress remains most common in the spring. Most of the plants used in laboratory research studies of molecular biology are summer annuals (Wilson et al. 1991).

A persistent seed bank is also formed in cultivated soil, an unknown proportion of buried seed surviving for up to five years and serving as a buffer against local extinction (Lawrence 1976; Roberts & Boddrell 1983; Grime et al. 1988; Thompson et al. 1997). In terms of its ecological strategy (a measure of its competitive ability), A. thaliana is classed as a stress-tolerant ruderal by Grime et al. (1988).

Fermanagh occurrence

It is hard to believe that this currently widespread weed was not recorded anywhere in Fermanagh until 1976. Currently we have records from 62 tetrads, over 11.7% of the squares in the VC. It is thinly spread in suitably open, disturbed habitats throughout the lowlands, but with a noticeable linear roadside pattern. Prior to 1976, A. thaliana was known to occur elsewhere in NI on the exposed, sloping, rocky ground of the basalt scarps in Cos Antrim (H39) and Londonderry (H40), and in other open habitats providing sunny, dry soils.

The previous lack of Fermanagh records might possibly have been a consequence of its relatively early spring flowering maximum – ie generally before any visiting field botanists became active in the county. The majority of A. thaliana plants (certainly those in the more nearly natural habitats which might possibly be distinct ecotypes), follow a winter annual reproductive strategy with a rapid flowering and fruiting cycle from April to June (Grime et al. 1988; Rich 1991). In addition, Thale Cress frequently occurs as isolated individuals or in small quantity and only rarely does it become abundant, even in disturbed fertile garden soils.

Taken together, this behaviour might allow the species to be overlooked. However, the spreading, elongate dead seed heads are very distinctive and long persistent, making it rather difficult to imagine that if the species were previously present, no one in Fermanagh noticed it about the garden, or in other open, urban areas, including roadsides and waste ground earlier than 1976. Indeed, like other small, annual cruciferous weeds, when it appears in the most disturbed habitats the plant is free of day-length and seed vernalisation constraints, and it can germinate and flower virtually all year round (Grime et al. 1988).

These arguments lead Robert Northridge and the current author (RSF) to conclude that the apparent recent arrival and spread of Thale Cress must be real, and it could possibly be associated with the rise of Garden Centres around the county in the 1970s and 1980s. A. thaliana is strictly a non-competitive pioneer therophyte and, like Cardamine hirsuta (Hairy Bitter-cress), it is very much associated nowadays with garden and nursery cultivation, especially on light, sandy or stony, often calcareous and generally shallow, dry soils (Sinker et al. 1985; Rich 1991).

British and Irish occurrence

A. thaliana is common throughout most of Britain, thinning northwards into Scotland, but absent from a few areas on the E coast of England. In Ireland, it is much more frequent in the eastern half, becoming scarce or absent towards the W of the island (New Atlas). It is probably still spreading in Ireland, possibly with the assistance of the horticultural trade.

European and world occurrence

In common with numerous other members of the ruderal weed community, A. thaliana is thought to have most probably originated in temperate Eurosiberia where it is very widespread. A. thaliana is regarded as native throughout most of Europe and the Mediterranean basin (including most islands), the Macronesian isles, N Africa and the Far East, Asia and E Africa (Rich 1991; Jalas & Suomnen 1994, Map 2141). Thale Cress has spread along with man and his agricultural systems and become naturalised and Circumpolar in its distribution. It has been introduced in N America, C & S Africa, S Australia and New Zealand (Hultén & Fries 1986, Map 909; Webb et al. 1988). Rather surprisingly, in the NE United States and adjacent Canada it is regarded as a pernicious, aggressive, invasive weed of moist woodlands (Cruden et al. 1996).

Names

The genus name 'Arabidopsis' is Greek meaning 'like or resembling Arabis' (Gilbert-Carter 1964). The Latin specific epithet 'thaliana' may either be derived as a memorial to the Greek Muse Thalia, who personified 'luxuriant, blooming' features (Melderis & Bangerter 1955), or Dr Johannus Thal (or Thalius), a sixteenth century German physician whom Linnaeus commemorated in the given name since he had published a catalogue of the plants of the Hartz mountains (Gilbert-Carter 1964; Gledhill 1985; Prior 1879). Druce (1932) gives the first record of the species (which he calls Sisymbrium Thalianum (L.) Gay), as 'Johns. Merc., 59, 1634', which refers to Thomas Johnson (1595/1597-1644), and his account of one of his botanical journeys published in 'Mercurius botanicus' in 1634, the year after his famous greatly improved edition of 'Gerard's Herball' (Henrey 1975, Volume 1, p. 94).

English Common names for the plant include 'Thale Cress', which is a mere book-name translation by Lightfoot of the Latin epithet (Britten & Holland 1886), or 'Common Wall Cress', 'Mouse-ear Cress' (Wilson et al. 1991), or just plain 'Wall Cress', although this name is more usually given to species of Arabis, another genus within which this species has occasionally resided in the past.

Threats

None to it, since it appears to be on the increase.

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