A new Light on Plant Metabolism

Research on plant metabolism has entered a second golden age, made possible by combination of biochemical methods and genome-enabled technologies. Among the high-flying goals are the balancing of the world’s population diet for optimal health, renewable chemical feedstocks and the generation of biofuels.

Plant metabolites are unequalled in number and complexity by any other life form. Recent estimates conclude that a single plant species can produce 5000 to 25 000 different compounds and that about 100 000 chemical structures already known represent just a fraction of all the compounds that exist in the plant kingdom. The known chemical structures will, of course, increase with the improvement of analytical methods and the number of species analysed.

Comparative genomics between bacterial and plant genomes uses the fact that genes of related physiological function often co-localise on bacterial chromosomes, in operons, in gene fusions forming multifunctional enzymes, or simply in close proximity. Such observations have led to the discovery of missing pathway steps and sometimes even whole pathways like the isopentenyl pyrophosphate synthesis by the methylerythritol phosphate pathway and vitamin synthesis.

Vitamin biosynthesis in plants reflects the range of evolutionary mosaics in the plant metabolic pathways. While the synthesis of pantothenate (vitamin B5) and biotin (vitamin B7) are identical in plants and bacteria, tocopherols (vitamin E) may only be produced by photosynthetic eukaryotes and some cyanobacteria. Four out of five plant tocopherol biosynthetic genes share explicit similarities in sequence to cyanobacterial orthologues. The fifth is of archestral origin. The evolutionary origins of thiamine (vitamin B1) biosynthesis in plants are even more diverse.

But the most interesting plant metabolites, the so-called "secondary metabolites", have no known parallels in any microbes. They are often produced excessively in specific tissues or cell types of plant species or groups of taxonomically related plants. Examples are alkaloids like morphine and codeine or latex from poppy or rubber trees. The alkaloids and latex as well as terpenes, saponins and resins represent economically important secondary metabolites.

The plant cell wall is the most complex polymer in nature. Manipulation of the cell wall has become more important because of the cellulosic ethanol production from crops. Two novel classes of cell wall biosynthetic enzymes have been lately identified: Mannan synthase from the guar bean which produces high levels of galactomannan, and a beta-1,4 glucan synthase from developing nasturtium seeds, which make large amounts of xyloglucan. The sequences of both enzymes may be used to annotate some Arabidopsis genes that resemble cellulose synthases.

Glucosinolates, sulphur-containing defense compounds of plants, turned out to exhibit cancer-protective effects in animals. Two R2R3-Myb transcription factors have been discovered and possess a strong positive correlation with established glucosinolate biosynthetic genes. In plant cell culture, the transcription factors positively regulate glucosinolate accumulation. They therefore seem to be transcriptional regulators of the pathway.

The future of research in plant metabolism looks extremely bright in the upcoming years. But the success of DNA and protein sequence analyses brings some of the more basic problems back into view: the requirement to identify and quantify small molecules that allow a fast association of gene expression and end products. Biochemists, chemists, and genome scientists will have to work together to discover new pathways or to engineer plants with the potential to solve some of the most relevant problems of the world.

Related antibodies on antibodies-online.com:

Pantothenate (vitamin B5)

Biotin (vitamin B7)

α-tocopherols (vitamin E)

Thiamine (vitamin B1)

Morphine

Codeine

Mannan

c-Myb

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