Mesembryanthemum

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 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; http://www.tigr.org/tdb/mcgi/).
• 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 stress conditions.
• 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 (Cushman, 2001).