High-throughput screening for in planta characterization of VOC biosynthetic genes by PTR-ToF-MS

A Novel Isoprene Synthase from the Monocot Tree Copernicia prunifera (Arecaceae) Confers Enhanced Drought Tolerance in Transgenic Arabidopsis

The capacity to emit isoprene, among other stresses, protects plants from drought, but the molecular mechanisms underlying this trait are only partly understood. The Arecaceae (palms) constitute a very interesting model system to test the involvement of isoprene in enhancing drought tolerance, as their high isoprene emissions may have contributed to make them hyperdominant in neotropical dry forests, characterized by recurrent and extended periods of drought stress. In this study we isolated and functionally characterized a novel isoprene synthase, the gene responsible for isoprene biosynthesis, from Copernicia prunifera, a palm from seasonally dry tropical forests. When overexpressed in the non-emitter Arabidopsis thaliana, CprISPS conferred significant levels of isoprene emission, together with enhanced tolerance to water limitation throughout plant growth and development, from germination to maturity. CprISPS overexpressors displayed higher germination, cotyledon/leaf greening, water usage efficiency, and survival than WT Arabidopsis under various types of water limitation. This increased drought tolerance was accompanied by a marked transcriptional up-regulation of both ABA-dependent and ABA-independent key drought response genes. Taken together, these results demonstrate the capacity of CprISPS to enhance drought tolerance in Arabidopsis and suggest that isoprene emission could have evolved in Arecaceae as an adaptive mechanism against drought.

Evolution of isoprene emission in Arecaceae (palms)

Isoprene synthase (IspS) is the sole enzyme in plants responsible for the yearly emission in the atmosphere of thousands of tonnes of the natural hydrocarbon isoprene worldwide. Species of the monocotyledonous family Arecaceae (palms) are among the highest plant emitters, but to date no IspS gene from this family has been identified. Here, we screened with PTR-ToF-MS 18 genera of the Arecaceae for isoprene emission and found that the majority of the sampled species emits isoprene. Putative IspS genes from six different genera were sequenced and three of them were functionally characterized by heterologous overexpression in Arabidopsis thaliana, demonstrating that they encode functional IspS genes. Site-directed mutagenesis and expression in Arabidopsis demonstrated the functional relevance of a novel IspS diagnostic tetrad from Arecaceae, whose most variable amino acids could not preserve catalytic function when substituted by a putatively dicotyledonous-specific tetrad. In particular, mutation of threonine 479 likely impairs the open-closed transition of the enzyme by altering the network of hydrogen bonds between helices H1α, H, and I. These results shed new light on the evolution of IspS in monocots, suggesting that isoprene emission is an ancestral trait within the Arecaceae family. The identification of IspS from Arecaceae provides promising novel enzymes for the production of isoprene in heterologous systems and allows the screening and selection of commercially relevant palm varieties with lower environmental impact.

Overexpression of Isoprene Synthase Affects ABA- and Drought-Related Gene Expression and Enhances Tolerance to Abiotic Stress

Isoprene is the most abundant single biogenic volatile compound emitted by plants. Despite the relevance of this molecule to plant abiotic resistance and its impact on global atmospheric chemistry, little is known about the details of its mechanism of action. Here, we characterized through both physiological and molecular methods the mechanisms of action of isoprene using model transgenic arabidopsis lines overexpressing a monocot isoprene synthase gene. Our results demonstrated the effect that isoprene had on ABA signaling at different tissue-specific, spatial, and temporal scales. In particular, we found that isoprene enhanced stomatal sensitivity to ABA through upregulation of RD29B signaling gene. By contrast, isoprene decreased sensitivity to ABA in germinating seeds and roots, suggesting tissue-specific mechanisms of action. In leaves, isoprene caused the downregulation of COR15A and P5CS genes, suggesting that the enhanced tolerance to water-deprivation stress observed in isoprene-emitting plants may be mediated chiefly by an enhanced membrane integrity and tolerance to osmotic stress.