The cost of glucosinolate biosynthesis

Highighted article: Michaël Bekaert, Patrick P. Edger, Corey M. Hudson, J.Chris Pires, Gavin C. Conant (2012) Metabolic and evolutionary costs of herbivory defense: systems biology of glucosinolate synthesis. New Phytologist 196:596–605.

Research published in a current New Phytologist paper uses a systems biology approach to demonstrate the metabolic and evolutionary costs of producing glucosinolates for defence.  Bekart et al. used AraGEM (Oliveira Dal’Molin et al., 2010) as a starting point. They collected data on Arabidopsis glucosinolate genes by scouring published papers and downloading their expression patterns from AtGenExpress. This information was integrated into the basic dataset from AraGEM. The complete list of genes involved in glucosinolate reactions, including references, is in Supplementary Table S1 of the paper.

The team performed flux balance analysis on the integrated database to estimate metabolic and energy flux through reactions in the system both with glucosinolate biosynthesis activity and with none. They found that glucosinolate biosynthesis affected flux incidentally through 241 reactions in addition to the 196 reactions which are only active when glucosinolates are being produced.

The main finding of the research is the heavy cost of glucosinolate biosynthesis. Sulphur import dramatically increased when glucosinolates were being synthesised, and demand for water, carbon dioxide, ammonia, and photons increased too. Despite the increase in substrate import, biomass synthesis fell by around 15% during glucosinolate production. This cost is reflected in other studies demonstrating that the evolutionary competitive edge glucosinolates give to plants is a disadvantage when there are no predators around (Mauricio, 1997), and reduces the number of seeds and flowers produced per plant compared to non-producers (Stowe and Marquis, 2011).

This research qualitatively confirms the observation that glucosinolate biosynthesis is costly to the plant, and the authors conclude that the pathway can only have evolved as defence against herbivory, as this is the only explanation for the maintenance of such a costly system. This conclusion is certainly valid, but the raw data presented in the paper and supplementary material may be out of date given that AraGEM was built in 2009 and AtGenExpress finished collecting data before that. However, the systems biology approach Bekaert et al. used to confirm this previously only visually observed concept may be applied to other areas of plant science where physical correlations have not yet been quantified, for example in symbiotic relationships.

Teaching resources: 

This set of teaching modules from The Institute for Systems Biology in Seattle is a comprehensive introduction into systems biology for secondary school students. They are intensive and each take several lessons, but this one on environmental influence on gene networks is a useful overview of experimental design, procedure and analysis.

Image credit: Arabidopsis flower courtesy of Shutterstock, Yellow and Blue by Christopher Bruno via stock.xchng.  

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