Isoprene emission from plants: why and how

Abstract

Background: Some, but not all, plants emit isoprene. Emission of the related monoterpenes is more universal among plants, but the amount of isoprene emitted from plants dominates the biosphere-atmosphere hydrocarbon exchange.

Scope: The emission of isoprene from plants affects atmospheric chemistry. Isoprene reacts very rapidly with hydroxyl radicals in the atmosphere making hydroperoxides that can enhance ozone formation. Aerosol formation in the atmosphere may also be influenced by biogenic isoprene. Plants that emit isoprene are better able to tolerate sunlight-induced rapid heating of leaves (heat flecks). They also tolerate ozone and other reactive oxygen species better than non-emitting plants. Expression of the isoprene synthase gene can account for control of isoprene emission capacity as leaves expand. The emission capacity of fully expanded leaves varies through the season but the biochemical control of capacity of mature leaves appears to be at several different points in isoprene metabolism.

Conclusions: The capacity for isoprene emission evolved many times in plants, probably as a mechanism for coping with heat flecks. It also confers tolerance of reactive oxygen species. It is an example of isoprenoids enhancing membrane function, although the mechanism is likely to be different from that of sterols. Understanding the regulation of isoprene emission is advancing rapidly now that the pathway that provides the substrate is known.

Fig. 1.

Method for measuring leaf temperature. Copper and constantan wires of 0·079 mm diameter were made into a thermocouple that was threaded through adjacent veins so that the thermocouple measuring joint was pressed against the leaf. The use of very small diameter wire increased the response time and decreased radiation errors. This system is similar to one used by Drake et al. (1970) but different from the suggestion of Ehleringer (1991) who recommended that the thermocouples be inserted into the part of the leaf to be measured.

Fig. 2.

Temperature of a white oak leaf. Measurement was made at the top of a 30-m Quercus alba tree in Duke Forest, North Carolina.

Fig. 3.

Thermoprotection of photosynthetic capacity by isoprene. Photosynthesis of detached kudzu leaves was measured at the indicated temperatures. One leaf was fed water and so made isoprene from endogenous sources. Two other leaves were fed 4 µm fosmidomycin and isoprene emission was monitored until >90% of the isoprene emission capacity was lost. One of these leaves was then provided with 2 µL L⁻¹ isoprene in the air stream (exogenous isoprene treatment).

Fig. 4.

The methylerythritol 4-phosphate pathway. G3P = glyceraldehyde 3-phosphate; DXS = deoxyxylulose 5-phosphate (DXP) synthase; DXR = DXP reductoisomerase; MEP = methylerythritol 4-phosphate; CMS = diphosphocytidylyl methylerythritol (CDP-ME) synthase; CMK = CDP-ME kinase; CDP-MEP = CDP-ME 2-phosphate; MCS = methylerythritol 2,4-cyclodiphosphate (ME-cPP) synthase; HDS = hydroxymethylbutenyl diphosphate (HMBPP) synthase; HDR = HMBPP reductase; IDP = isopentenyl diphosphate; DMADP = dimethylallyl diphosphate; IDI = IDP isomerase; IspS = isoprene synthase.

Fig. 5.

Isoprene emission and photosynthesis rates, and IspS mRNA and protein levels for developing P. trichocarpa leaves. Emission rates were measured at 30 °C and 1000 µmol m⁻² s⁻¹ light. Experimental methods were similar to those reported in Wiberley et al. (2005) for kudzu.