Desktop version

Home arrow Environment

  • Increase font
  • Decrease font

<<   CONTENTS   >>


Convergent evolution is an independent evolution resulting in similar traits in different organisms. For example, the biosynthesis of the same compound in different plant species may be catalyzed by phylogenetically unrelated enzymes. There are many examples of convergent evolution in the plant specialized metabolism. Phylogenetic analysis of caffeine N-methyltransferases (NMTs) in Coffea canephora showed that these genes expanded through tandem duplications, independently of NMTs from Theobroma cacao and Camellia sinensis, indicating a convergent evolution of caffeine biosynthesis in different plant lineages (Denoeud et al., 2014). Similarly, for decades it was believed that nearly all VOCs were produced through a limited number of pathways shared by all plants. However, there is more and more evidence that different pathways leading to similar or identical compounds have evolved independently in plants. These alternative pathways often occur through convergent evolution (Sun et al., 2016).

Functional Convergence: Distinct Enzyme Lineages Producing Similar Compounds

The monoterpene alcohol linalool is present in the floral scent of many species. Its diverse ecological functions have recently been reviewed (Raguso, 2016a). This compound may be both an attractant for pollinators (in orchids for example) and a repellant for various herbivores. Emission of linalool by N. attenuata leaves has been proposed to attract predators of the generalist herbivore Manduca sexta (He et al., 2019). When present in nectar, it could contribute to avoid microbe proliferation. Two enantiomeric forms of this molecule exist: (/?)-(—)-linalool and (S)-(+)-linalool. Both forms have distinct biochemical properties and impacts on insects. For instance, female moths oviposit more on Datura wrightii plants emitting (S)-(+)-linalool over control plants, while plants emitting (/?)-(—)-linalool are less preferred than control plants (Reisenman et al., 2010). Since the first characterization of C. breweri linalool synthase (Dudareva et al., 1996), TPSs able to catalyze the formation of linalool from GPP have been functionally characterized in many plant species, including A. thaliana (Ginglinger et al., 2013), F. x ananassa (Aharoni et al., 2004), and Rosa genus (Magnard et al., 2018). Phylogenetic analysis indicates that these genes belong to four TPS subfamilies (Figure 12.2). For example, in R. chinensis, three genes encoding potentially functional TPSs are expressed in petals, RcLINS, RcLIN-NERSl, and RcLIN-NERS2. The LIN-NERS1/2 genes are clustered on chromosome 5 of the rose genome (Raymond et al., 2018) whereas RcLINS gene is on chromosome 2. RcLINS is responsible for the small amounts of (/?)-(—)-linalool present in rose scent (Magnard et al., 2018). RcLIN-NERSl, and RcLIN-NERS2 genes, although weakly expressed, are probably not active in planta as neither nerolidol, nor (S)-(+)-linalool are present in rose petals. A comparison of roses with strawberries, show that these two members of the Rosaceae family have evolved different enzymes to produce linalool. F. x ananassa uses the (S)-(+)-linalool /nerolidol synthase FaNESl (Aharoni et al., 2004), whereas the orthologous RcLIN-NERS2 is ineffective in rose flowers. Conversely, the small quantity of linalool produced in rose flowers derives from the activity of the RcLINS (/?)-(—(-linalool synthase, which is unable to produce nerolidol. More generally, linalool or linalool/nerolidol synthase activities are well correlated with their belonging to a specific TPS subfamily (Figure 12.2). Monofunctional linalool synthases cluster into the TPS-b subfamily. Like RcLINS, most of these b class enzymes with linalool synthase activity generally produce (/?)-(—)-linalool. The bifunctional linalool/nerolidol synthases, including RcLIN-NERSl and RcLIN-NERS2 belong to the TPS-g subgroup together with some linalool/nerolidol/geranyllinalool

Diversity of linalool synthases characterized from different plant species

FIGURE 12.2 Diversity of linalool synthases characterized from different plant species. Unrooted Neighbor Joining tree depicting the classification of linalool synthases and linalool/nerolidol synthases from plants into terpene synthase (TPS) subfamilies. Linalool/nerolidol/geranyllinalool synthases were also included. Protein sequences were aligned with ClustalW and tree was constructed using Geneious software. Linalool isomers are indicated by colors of the GenBank accession letters. Black letters, unidentified linalool isomer; blue letters (R)-(-)-linalool synthase; orange letters (Л/SJ-linalool synthase (racemic); pink letters (5)-(+)-linalool synthase.

synthases. The linalool isomer produced by synthases belonging to this group is usually (£)-(+)- linalool, like RcLIN-NERSl and 2. Although not all studies report the linalool enantiomer that is produced by the different species, the general trend is that (S)-(+)- and (/()-(-)-!inalool synthases have evolved independently, probably under the selection pressures exerted by both herbivores and pollinators.

Another example of convergent evolution is the biosynthesis of phenylpropenes. These compounds are phenylpropanoids derived from phenylalanine, which participate in the unique aroma of many fruits, herbs and spices. In flowers, they attract pollinators and may be used as defense compounds against fungi and bacteria. Many plants synthesize volatile phenylpropenes in their floral and vegetative organs as a defense against insect pests and herbivores. For example, the stamens of roses emit eugenol (Yan et al., 2018). The last step of the biosynthesis pathway to eugenol and isoeugenol is performed by the NADPH-dependent reductases eugenol synthase and isoeugenol synthase, which catalyze the elimination of an acyl moiety from coniferyl acetate (reviewed in Koeduka, 2014). Phenylpropene synthases belong to the PIP family of reductases and are distributed in two distinct protein lineages in C. breweri and P. x hybrida (Koeduka et al., 2008), suggesting independent evolutionary origins. These independent origins have since been supported by studies in a number of other plants such as F. x ananassa and Daucus carota (Aragiiez et al., 2013; Yahyaa et al., 2019).

<<   CONTENTS   >>

Related topics