This small cruciferous plant has taken on the mantle of being a ubiquitous research tool in studies of many aspects of plant and, increasingly, animal biology. Proofs of concept studies almost invariably utilize A. thalianaand, in consequence, there is an enormous literature describing every facet of its germination, growth and reproduction in genetic and molecular terms. It was originally identified as an experimental tool by Laibach (1943) in Germany following detailed investigations of its genetics that started in 1907. Its haploid chromosome number was identified as n= 5; a collection of ecotypes
was established and mutant forms resulting from exposure to chemical and physical mutagens were catalogued.
There is a set of recessive mutants that cause homeiotic variations that can be used in studies of developmental biology (Pruitt et al., 1987), and a group of colour variants make useful visible markers. Lethal recessive mutants can be employed for developmental, biochemical and physiological studies. Resistance to herbicides and a range of plant pathogens offers practical agricultural and horticultural applications. Arabidopsis thaliana is thus well characterized genetically; a wealth of useful and interesting mutations have been induced, analysed and mapped. These characteristics alone do not distinguish A.
thalianafrom other experimentally useful and genetically well studied plants such as the tomato (Lycopersicon esculentum) and maize (Zea mays).
It is the small size, short generation time, high seed set and ease of mutagenesis in Arabidopsis thalianathat make it easier and faster to induce, select and characterize new mutations which attracts scientists of many biological disciplines. Added to this are very attractive cellular and nuclear characteristics. Arabidopsis thaliana is attractive for both quantitative and qualitative genetics because of its small genome size and flexible genomic organization (Somerville, 1989; Griffiths and Scholl, 1991; Meyerowitz and Somerville, 1994; Meyerowitz, 1997). It has the honour of being the first plant where the entire genome has been sequenced.
The advantages offered by A. thalianainclude the following.
● Small size – hence ease and simplicity of cultivation in controlled environments where space is at a premium for availability and cost. More than 100 plants can be grown in the space occupied by this open book.
● Short life cycle – regeneration from seed to seed takes between 4 and 6 weeks, which offers short repeatable and reproducible experiments.
● Perfect flowers – these are self-fertile so that pollen transfer is simplified with no requirement for complex procedures to ensure cross-pollination, and the flowers tend not to be open pollinated. Self-fertilization exposes recessive mutations in homozygous form in the M2 (mutation two) generations following the application of mutagens.
Table 2.2. Examples of the morphological mutants of Arabidopsis thaliana.
Character Effect
AngustifoliaandAsymmetricleaves Narrow or asymmetric leaves when homozygous Glabraanddistortedtrichomes Remove or change the shape of the leaf hairs Erectaandcompacta Alter the disposition and size of stems Brevipedicellus Reduces the pedicels when homozygous
Apetala-1 Reduces petals
Eceriferum Changes the morphology of the epidermal wax
● Ability to produce large amounts of seed – up to 10,000 seeds can be harvested from one plant. This provides large populations very quickly from a single cross. In turn, this minimizes space requirements and makes the statistical analysis of populations easier with the large numbers of individuals available.
● A very compact genome – 70,000–100,000 kb per haploid genome with only 10% of highly repetitive sequences. The genome is amongst the smallest of the higher plants with the haploid size of about 100 Mb of DNA. This makes A. thalianaan ideal organism for genetic and molecular studies such as mutant analysis, molecular cloning of genes and the detailed construction of a physical map of the genome.
● Arabidopsis thalianacan be genetically engineered with relative simplicity using either A. tumefaciens-mediated transformation or direct particle bombardment of the tissues.
Arabidopsis thaliana has the smallest known genome among the higher plants. Both DNA reassociation kinetics and quantitative genome blotting experiments indicate a haploid genome size of approximately 70,000 kbp. This genome size is consistent with data from studies of the nuclear volume.
Arabidopsis thaliana is more resistant to ionizing radiation than many other angiosperms. This property correlates with its genome size. The genome size is only five times larger than that of the yeast fungus (S. cerevisiae) and only 15-fold larger than the common intestinal bacterium, Escherichia coli. The contrast of the A. thalianagenome with those of other higher plants frequently used in molecular and genetic studies is striking (Table 2.3). The ‘genetic baggage’ of many higher plants is thought to be an evolutionary insurance against environmental changes. Fewer than 25% of a complex organism’s genes may be required for growth and reproduction. Possession of multiple copies increases the chances that it will posses a variant (allele) that will make the difference between surviving or succumbing to an environmental stress such as drought.
The significance of this small DNA content for molecular genetics is that a genomic library of A. thalianachromosomal fragments is easier to make and simpler and more economical to screen. Only 16,000 random clones of 20 kb average insert size must be screened to offer a 99% chance of obtaining any A. thalianafragment. In contrast, in tobacco (Nicotiana tabacum), 370,000 clones from a similar library would have to be screened to have a similar chance of success; the comparable number for pea (Pisum sativum) is 1,000,000 and for wheat (Triticum aestivum) is 1,400,000 clones.
It is thus rapid and inexpensive to screen A. thaliana genomic libraries repeatedly, and this is important for experiments involving chromosome walking. A second advantage of the small genome size is the enhancement of signal relative to noise in genetic gel blotting experiments. This enhancement allows the detection of weak signals from heterologous probe hybridizations
and, in consequence, permits the detection of hybridization between very divergent probes and an angiosperm genome.
Arabidopsis thaliana has a low content of repeated sequences. Those elements that are repeated are set at far distances from each other. This characteristic is different from that of other angiosperms studied so far; for example, in tobacco (N. tabacum), the mean length of uninterrupted single- copy DNA is 1.4 kb and in pea (P. sativum) it is 0.3 kb. Thus, chromosome walking experiments are made conveniently with A. thaliana, whereas with other angiosperms the interspersed repeats make it difficult to select single- copy probes for each step in chromosome walking.
The ability to transform A. thalianagenetically allows for detailed analysis of gene function and expression. In addition, it permits the functional complementation of a mutant phenotype with cloned genes which is the critical final stage in isolating genes by mutational analysis. Many A. thaliana genes have been cloned and characterized; this provides genes for many different types of experiment and it is possible to develop a picture of genomic organization and its evolution.