In Planta Recapitulation of Isoprene Synthase Evolution from Ocimene Synthases

Abstract

Isoprene is the most abundant biogenic volatile hydrocarbon compound naturally emitted by plants and plays a major role in atmospheric chemistry. It has been proposed that isoprene synthases (IspS) may readily evolve from other terpene synthases, but this hypothesis has not been experimentally investigated. We isolated and functionally validated in Arabidopsis the first isoprene synthase gene, AdoIspS, from a monocotyledonous species (Arundo donax L., Poaceae). Phylogenetic reconstruction indicates that AdoIspS and dicots isoprene synthases most likely originated by parallel evolution from TPS-b monoterpene synthases. Site-directed mutagenesis demonstrated invivo the functional and evolutionary relevance of the residues considered diagnostic for IspS function. One of these positions was identified by saturating mutagenesis as a major determinant of substrate specificity in AdoIspS able to cause invivo a dramatic change in total volatile emission from hemi- to monoterpenes and supporting evolution of isoprene synthases from ocimene synthases. The mechanism responsible for IspS neofunctionalization by active site size modulation by a single amino acid mutation demonstrated in this study might be general, as the very same amino acidic position is implicated in the parallel evolution of different short-chain terpene synthases from both angiosperms and gymnosperms. Based on these results, we present a model reconciling in a unified conceptual framework the apparently contrasting patterns previously observed for isoprene synthase evolution in plants. These results indicate that parallel evolution may be driven by relatively simple biophysical constraints, and illustrate the intimate molecular evolutionary links between the structural and functional bases of traits with global relevance.

Nutrient‐rich plants emit a less intense blend of volatile isoprenoids

“…On the other hand, several authors have embraced the hypothesis that the capacity to emit isoprene was gained and lost multiple times during evolution Sharkey et al, 2013). Li et al (2017) recently discovered that isoprene synthase neo-functionalization occurs by active site mutation triggered by a single amino acid mutation, supporting evolution of isoprene synthase from the large class of monoterpene synthases (TPS-b). Isoprene emission may be conserved only in the narrow range of environmental conditions in which it clearly benefits plant fitness (Monson et al, 2013), or when plant genera undergo extensive speciation, more typically in perennial plants and in many trees (Dani et al, 2014).…”

“…These results indicate that emission type (ISP or MTP, monoterpene storage or not) was better preserved in the phylogeny than emission rate, which is likely because specific mutations are required for isoprene and monoterpene emission (and storage) and, once a species acquires them, rates of emission may vary depending on the environment. The stronger phylogenetic signal for isoprene than monoterpene emission, though, seems counterintuitive because of the previously reported trade-off that exists between them (Harrison et al, 2013;Li et al, 2017). Some authors have argued that the difference in phylogenetic signal of isoprene and monoterpene emission was due to the lack of ecological pressure in isoprene emission, whereas monoterpene emission developed as an adaptation to xeric environments (Loreto et al, 2014a).…”

“…Some studies have analysed the emission of isoprenoids in various taxa and discussed the emission type of their ancestors (Loreto et al, 1998;Loreto, 2002). Recently, a broad reconstruction of the ancestral emission type of a large array of phylogenetically distant species has been attempted (Li et al, 2017). However, the mode of evolution of isoprenoids (e.g.…”

Shoot Characterization of Isoprene and Ocimene-Emitting Transgenic Arabidopsis Plants under Contrasting Environmental Conditions

“…The elevated frequency in gaining and losing the trait has been explained by the relatively few mutations necessary to produce or lose the IspS gene coupled with the evidence that isoprene emission is advantageous in a narrow range of environments. Recent phylogenetic reconstruction indicates that Arundo donax IspS (AdoIspS) and dicots IspS most likely originated by parallel evolution from Terpene Synthase b (TPS-b) monoterpene synthases, suggesting potentially different physiological roles of the two VOCs (isoprene and ocimene) under environmental stresses [13]. Therefore, understanding how isoprene affects plant growth and physiology and comparing the induced protection under abiotic stresses of isoprene and other monoterpenes will allow determination of whether isoprene emission is a beneficial trait to be reintroduced to plants, especially for the purpose of crop improvement.…”

“…To compare the role of isoprene and ocimene on environmental stress tolerance, we used some transgenic Arabidopsis produced in [13]. Wild-type plants and two transgenic lines per type of emitted VOC were compared in three independent experiments for their growth and stress tolerance by using a series of non-invasive shoot phenotyping techniques.…”