Mesembryanthemum crystallinum (common ice plant) belongs to the family
Aizoaceae in the order Caryophyllales. In this order very few
crop species are found, sugar beet and spinach are examples (both are relatively
salt- and low temperature tolerant). The different families in this order, however,
include a large number of species that are extremely tolerant to diverse abiotic
stresses – high salinity, drought, and temperature extremes. The best-known examples
may be the Cactaceae. Mesembryanthemum crystallinum is well studied
physiologically (Winter and Smith, 1996) and under
molecular biology aspects (Cushman and Bohnert, 1999;
Bohnert & Cushman, 2000;
Cushman 2001). The plant first attracted researchers because of a shift from C3
photosynthesis to CAM (Crassulacean Acid Metabolism) that occurs as young plants in
their natural habitat (Namibian Desert/South Africa) experience drought and increasing
salinity as the season progresses (Cushman &
Borland, 2002). This could be based on changes in the external conditions alone but
indications suggest the shift in metabolism is also part of the plant’s developmental program.
Mesembryanthemum crystallinum has provided many clues about the mechanisms employed
by stress-tolerant plants to survive extreme conditions - it is one of the few plant species
that have a model character with respect to stress tolerance. For this reason,
we think that Mesembryanthemum crystallinum must be one of the plant species
whose genome should be sequenced.
There are many supportive indicators for the value of a Mesembryanthemum
genome sequence to plant biology and agriculture (Bohnert,
et al., 1988; Adams, et al., 1998;
Cushman and Bohnert, 1999;
Bohnert & Cushman, 2000). These include:
A genome sequence for Mesembryanthemum crystallinum would be of immense value for
understanding the CAM pathway, the developmental peculiarities of the plant, and a number
of morphological and molecular specialties that characterize many stress-tolerant species.
From a standpoint of gene mining, we think that the plant has evolved not only stress-adapted
versions of many proteins, but possibly also a number of novel paralogs of common genes for
pathways that lead to stress tolerance.
- a genome size of 390 Mb. (DeRocher et al., 1990)
- a large number of genomic DNA (lambda) and cDNA libraries.
- approximately 22,000 EST sequences are already available; they represent
approximately 10,000 genes.
- collections of EMS, gamma and fast-neutron-radiation induced mutants.
- tissue culture and regeneration capability.
- transformation with Agrobacterium tumefaciens vectors has been achieved.
(The technology is not yet optimized, but transformation should not be a major
problem; developed by Dr. J.C. Cushman (U. Nevada, Reno).
- a comparison of approximately 8,000 transcripts (EST contigs for unigenes)
with the sequence of the Arabidopsis genome indicated a number
of sequences that seem to have no homologs in the Arabidopsis
genome. Extrapolated to the entire genome the numbers seem to indicate
that Mesembryanthemum includes several thousand genes that are
not present in the Arabidopsis genome (JC Cushman, unpublished;
- Mesembryanthemum is highly salt-tolerant as an adult plant. As a
young, pre-flowering plant, it is however not a eu-haloplyte (criterion:
accelerated growth in the presence of sodium). Under such conditions growth
of seedlings and young plants stops, but the plants do not degenerate.
Rather, they wait for better times and, once those have arrived, grow again.
- ploidy levels of nuclei differ in a cell-specific way and also influenced by
- during development, Mesembryanthemum shows clear phase changes,
changes in morphology and growth habit, and different leaf forms. The
difference can be used for typing development.
- Mesembryanthemum, like the species of most Caryophyllales
families, does not synthesize anthocyanins. The pigments replacing
anthocyanins are betalains.
- large seed number ~15,000/plant when grown in 1 liter pots (although not
comparable with Arabidopsis).
- plants can be miniaturized by growth conditions.
- a spectacular feature, which gave many species in the family the name ice plant,
are epidermal bladder cells (EBC). Developmentally-formed huge cells
fill during salt stress and then protrude from the epidermis. They
cause light dispersion such that the plants seem to be covered by
glittering ice crystals. EBC are modified trichomes and with the help
of EBCs, Mesembryanthemum crystallinum can accumulate sodium. It may be
used for bio-remediation.
- M. crystallinum is also a model for Crassulacean Acid Metabolism, CAM,
which characterizes some 8% of all angiosperm species
One Carbon Metabolism
Functional Genomics Project