Metabolism of phenolic natural compounds at the threshold to life on land

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Plants changing from an aquatic to a terrestrial lifestyle encounter various biotic (pathogens, herbivores) and abiotic (UV irradiation, loss of water) stresses which can be overcome with help of phenolic compounds that have deterring or antibiotic as well as UV-absorbing properties or are incorporated into lipophilic barriers. The basis for the biosynthesis of phenolic compounds is the phenylpropanoid pathway which forms an activated hydroxycinnamic acid from L-phenylalanine or L-tyrosine by the action of three enzymes: phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H) and 4-coumarate CoA-ligase (4CL). 4-Coumaroyl-CoA then is further metabolized to monolignols and hydroxycinnamic acid esters/amides under participation of hydroxycinnamoyltransferases (HCT), cytochrome P450 monooxygenases (CYP98) and caffeoylshikimate esterases (CSE). All these enzymes are well known in seed plants, but our knowledge on them in early diverged land plants or algae is much more limited. In the first funding period, our main focus was on the above-mentioned genes and enzymes in extant members of the earliest land plants using the bryophytes Anthoceros agrestis, Marchantia polymorpha and Physcomitrium patens as model organisms. Chara braunii as green alga of the Characeae was also investigated to some extent. Gene sequences from these plants for the enzymes of interest were selected, heterologously expressed and biochemically characterized. Generally, our results showed that annotations in the bryophyte databases are mostly correct and the biochemical characteristics are similar to those of the respective enzymes from seed plants. This was different for Chara braunii. Here, firstly genes can not be identified as easily using seed plant sequences as baits, and secondly, the annotations often are not correct or confusing. This shows us, that it is important to concentrate on the algal model systems to follow the evolution of genes/enzymes of the phenylpropanoid pathway as organisms where this evolution might have occurred. Besides Chara braunii, we will therefore concentrate on another algal model organism, Mesotaenium endlicherianum (Zygnematophyceae), to look there for the presence of genes/enzymes of phenolic metabolism and to characterize the encoded enzymes. First inspections of Chara braunii and Mesotaenium endlicherianum genome databases have shown that the identification of our target genes is more tedious and alternative biosynthetic pathways must be taken into consideration.