Very large amounts of isoprene are emitted from vegetation, especially from mosses, ferns, and trees. This hydrocarbon flux to the atmosphere, roughly equal to the flux of methane, has a large effect on the oxidizing potential of the atmosphere. Isoprene emission results from de novo synthesis by the deoxyxylulose phosphate/methyl erythritol 4-phosphate pathway in plastids. Dimethylallyl pyrophosphate made by this pathway is converted to isoprene by isoprene synthase. Isoprene synthase activity in plants has a high pH optimum and requirement for Mg2+ that is consistent with its location inside chloroplasts. Isoprene emission costs the plant significant amounts of carbon, ATP, and reducing power. Researchers hypothesize that plants benefit from isoprene emission because it helps photosynthesis recover from short high-temperature episodes. The evolution of isoprene emission may have been important in allowing plants to survive the rapid temperature changes that can occur in air because of the very low heat capacity of isoprene relative to water.

Isoprene is synthesized and emitted in large amounts by a number of plant species, especially oak (Quercus sp.) and aspen (Populus sp.) trees. It has been suggested that isoprene improves thermotolerance by helping photosynthesis cope with high temperature. However, the evidence for the thermotolerance hypothesis is indirect and one of three methods used to support this hypothesis has recently been called into question. More direct evidence required new methods of controlling endogenous isoprene. An inhibitor of the deoxyxylulose 5-phosphate pathway, the alternative pathway to the mevalonic acid pathway and the pathway by which isoprene is made, is now available. Fosmidomycin eliminates isoprene emission without affecting photosynthesis for several hours after feeding to detached leaves. Photosynthesis of fosmidomycin-fed leaves recovered less following a 2-min high-temperature treatment at 46°C than did photosynthesis of leaves fed water or fosmidomycin-fed leaves in air supplemented with isoprene. Photosynthesis of Phaseolus vulgaris leaves, which do not make isoprene, exhibited increased thermotolerance when isoprene was supplied in the airstream flowing over the leaf. Other short-chain alkenes also improved thermotolerance, whereas alkanes reduced thermotolerance. It is concluded that thermotolerance of photosynthesis is a substantial benefit to plants that make isoprene and that this benefit explains why plants make isoprene. The effect may be a general hydrocarbon effect and related to the double bonds in the isoprene molecule.

Isoprene synthase converts dimethylallyl diphosphate, derived from the methylerythritol 4-phosphate (MEP) pathway, to isoprene. Isoprene is made by some plants in substantial amounts, which affects atmospheric chemistry, while other plants make no isoprene. As part of our long-term study of isoprene synthesis, the genetics of the isoprene biosynthetic pathway of the isoprene emitter, kudzu (Pueraria montana), was compared with similar genes in Arabidopsis (Arabidopsis thaliana), which does not make isoprene. The MEP pathway genes in kudzu were similar to the corresponding Arabidopsis genes. Isoprene synthase genes of kudzu and aspen (Populus tremuloides) were cloned to compare their divergence with the divergence seen in MEP pathway genes. Phylogenetic analysis of the terpene synthase gene family indicated that isoprene synthases are either within the monoterpene synthase clade or sister to it. In Arabidopsis, the gene most similar to isoprene synthase is a myrcene/ ocimene (acyclic monoterpenes) synthase. Two phenylalanine residues found exclusively in isoprene synthases make the active site smaller than other terpene synthase enzymes, possibly conferring specificity for the five-carbon substrate rather than precursors of the larger isoprenoids. Expression of the kudzu isoprene synthase gene in Arabidopsis caused Arabidopsis to emit isoprene, indicating that whether or not a plant emits isoprene depends on whether or not it has a terpene synthase capable of using dimethylallyl diphosphate.Some plants (about one-third of angiosperms) can emit a significant fraction of recently fixed carbon as isoprene. Isoprene emission is an important biological process because it plays a large role in atmospheric chemistry (Trainer et al., 1987; Fehsenfeld et al., 1992;Thompson, 1992). It has been hypothesized that plants make isoprene for thermotolerance (Sharkey and Singsaas, 1995), especially protection from damage caused by rapid temperature fluctuation (Singsaas and Sharkey, 2000;Sharkey et al., 2001). This thermotolerance hypothesis has recently been independently confirmed (Peñ uelas et al., 2005;Velikova and Loreto, 2005). It has also been shown that isoprene can protect against ozone and other reactive oxygen species (Loreto and Velikova, 2001; Affek and Yakir, 2002). The use of isoprene by plants to help tolerate environmental stress contrasts with the use of mono-and diterpenes by plants for biological defense (Trapp and Croteau, 2001a).Isoprene is made from dimethylallyl diphosphate (DMAPP) by isoprene synthase (Silver and Fall, 1995;Schnitzler et al., 1996;Miller et al., 2001). DMAPP used for isoprene synthesis is a product of the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway (Schwender et al., 1997;Rodriguez-Concepció n and Boronat, 2002; Fig. 1), which is present in all plant chloroplasts and is the source of chloroplast terpenoids (Lichtenthaler, 1999). The first two enzymes in the MEP pathway, 1-deoxy-D-xylulose 5-phosphate (DXP) synthase (DXS) and DXP reductoisomerase (DXR), are likely key regulators of ...

Antifreeze proteins similar to two different chitinases accumulate during cold acclimation in winter rye (Secale cereale). To determine whether these cold-responsive chitinases require post-translational modification to bind to ice, cDNAs coding for two different full-length chitinases were isolated from a cDNA library produced from cold-acclimated winter rye leaves. CHT9 is a 1,193-bp clone that encodes a 31.7-kD class I chitinase and CHT46 is a 998-bp clone that codes for a 24.8-kD class II chitinase. Chitinase-antifreeze proteins purified from the plant were similar in mass to the predicted mature products of CHT9 and CHT46, thus indicating that there was little chemical modification of the amino acid sequences in planta. To confirm these results, the mature sequences of CHT9 and CHT46 were expressed in Escherichia coli and the products of both cDNAs modified the growth of ice. Transcripts of both genes accumulated late in cold acclimation in winter rye. Southern analysis of winter rye genomic DNA indicated the presence of a small gene family homologous to CHT46. In hexaploid wheat, CHT46 homologs mapped to the homeologous group 1 chromosomes and were expressed in response to cold and drought. We conclude that two novel cold-responsive genes encoding chitinases with ice-binding activity may have arisen in winter rye and other cereals through gene duplication.