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Isoprene is a volatile hydrocarbon with profound effects on atmospheric chemistry. This hydrocarbon is emitted by leaves of certain plants. The formation of isoprene takes place via a newly described pathway called the DOXP pathway. According to one hypothesis, isoprene functions to protect thylakoids from damage at high temperatures.
In a review article published in the November issue of Trends in Plant Science, Drs. Barry Logan of Bowdoin College, Russell Monson of the University of Colorado and Mark Potosnak of Columbia University discuss the most recent developments concerning isoprene biosynthesis, describe the evidence surrounding isoprene’s hypothesized role as a thermal protectant and advance an alternative hypothesis about the adaptive role that isoprene biosynthesis might play.
Isoprene emission is light-dependent and strongly positively correlated with temperature. It is now known that isoprene is produced via the DOXP pathway in the chloroplast, which is named after an intermediate product (1-deoxy-D-xylulose-5- phosphate) of the pathway. This has been demonstrated by several lines of evidence, including the fact that an inhibitor of DOXP reductoisomerase (an enzyme of the DOXP pathway) abolishes isoprene emission.
Since isoprene production takes place in chloroplasts, it is suggested that a link may exist between isoprene production and heat stress to the photosynthetic apparatus. A report that isoprene protected leaves against damage by the short-term higher temperature, confirms this belief. Furthermore externally supplied isoprene to leaves (17.5 mg/l) was found to delay the onset of thermal damage by 7.5 C.
The authors point out, however, that further attempts to observe this effect have failed. The authors describe an alternative hypothesis that may explain the phenomenon of isoprene emission. They propose that isoprene emission may serve as a metabolic “overflow valve” required to maintain appropriate pool sizes for cellular metabolites. Support for this hypothesis is circumstantial, at present, but includes the fact that patterns of isoprene emission from certain bacteria are consistent with metabolic regulation. Additionally, many plant species that emit isoprene possess a mechanism of sugar transport that results in high sugar concentrations in photosynthetic cells (perhaps necessitating metabolic regulation).
The authors conclude by expressing the hope that further research on the hypothesis that isoprene serves as a thermal protectant will be complemented by parallel research into alternative explanations for this trait.