title: Phenylalanine ammonia-lyases and 4-coumaric acid coenzyme A ligases in Chara braunii, Marchantia polymorpha and Physcomitrium patens as extant model organisms for plant terrestrialization
title: Phenylalanin Ammoniak-Lyasen und 4-Cumarsäure Coenzym A-Ligasen in Chara braunii, Marchantia polymorpha und Physcomitrium patens als rezente Modelorganismen der pflanzlichen Terrestrialisierung
titleType: AlternativeTitle
Creators: name: Schwarze, Christoph Michael
Publisher: name: Philipps-Universität Marburg
Year: 2024
Subjects: Chara braunii
4-coumaric acid coenzyme A ligase
plant terrestrialization,
Physcomitrium patens
Phenylalanine ammonia-lyase
Botanical sciences
Pflanzen (Botanik)
Marchantia polymorpha

description: Plant terrestrialization represents one of the most momentous events in palaeobotany, necessitating remarkable adaptability and resilience of organisms having been adjusted to marine conditions for millions of years. A most significant epiphenomenon along the way of land colonization was the emergence of new pathways and novel abilities within plant specialized metabolism that facilitated embryophytic subsistence on land. The phenylpropanoid pathway introduced an array of metabolic compounds and processes leading to crucial mechanisms of protection and self-preservation for plants, including lignification of cell walls, setup of water conducting elements and herbivore repellence (Agorio et al. 2024; Kunz et al. 2024). This pathway is divided into the general and the lignin-specific phenylpropanoid pathway (Labeeuw et al. 2015; Barros and Dixon 2020).
Phenylalanine ammonia-lyase (PAL), the first enzyme in the general phenylpropanoid pathway, catalyzes the reaction of the amino acid L-phenylalanine via elimination of ammonia to t cinnamic acid (Barros and Dixon 2020). This imperative reaction has been found to be conserved in extant members of the embryophytes as well as in the green alga Chara braunii, a member of the Charophyceae. In C. braunii, the transformation of L-Phe occurs with high substrate specificity, yet low turnover, whereas in extant members of bryophytes, the versatility as well as catalytic efficaciousness of PAL increase. PALs derived from the liverwort Marchantia polymorpha and the moss Physcomitrium patens are capable of not only transforming L-Phe to a higher degree than CbPAL, but also possess affinity towards D phenylalanine and L-tyrosine. Here, MpPAL 3 possesses with 28000 s-1 mol-1 l the highest catalytic efficiency for L-Phe, whereas in PpPALs 1 to 4 the catalytic efficiency varies between 5000 and 22000 s-1 mol-1 l. The catalytic efficiencies for D‑Phe and L-Tyr are significantly lower (Schwarze and Petersen 2024). The transformation of L-Tyr leads directly to 4-coumaric acid, which is otherwise produced via cinnamic acid 4-hydroxylase (C4H) from t-cinnamic acid (Barros and Dixon 2020). Intrinsic TAL-activity of PAL thus represents an alternative pathway towards this particular product. The increase in catalytic efficiency with ongoing evolution, as well as the more widespread orientation towards different substrates might indicate that the investigated CbPAL could represent a less elaborated form of PAL with the putative capability of evolving into more efficient enzymes. The third enzyme in the general phenylpropanoid pathway, 4-coumarate CoA-ligase (4CL), transforms 4-coumaric acid further into 4-coumaroyl-CoA. 4CL is capable of catalysing the same reaction with a number of different substrates, such as caffeic, ferulic, cinnamic, sinapic and 4-hydroxybenzoic acids (Schneider et al. 2003; Wohl and Petersen 2020). Several isoforms of 4CL have been discovered in all three investigated organisms. In C. braunii, activity is present, especially for caffeic acid, but not to a significantly high degree. In contrast to that, kinetic experiments could be performed for 4-coumaric, caffeic, ferulic and cinnamic acids with 4CL isoforms derived from M. polymorpha and P. patens. In M. polymorpha, the majority of catalytic functionality seems to be carried out by Mp4CL 3, being extraordinarily active and especially affine towards 4-coumaric acid (with a catalytic efficiency of more than 2.5 million s-1 mol-1 l). The other three isoforms display no substantial catalytic activities. In P. patens, four isoforms have been kinetically examined, revealing rather heterogenous catalytic behaviour. The most active isoform, Pp4CL 4, reached catalytic activities of appr. 55000 s-1 mol-1 l for 4 coumaric acid and appr. 29000 s-1 mol-1 l for caffeic acid, respectively (Schwarze and Petersen 2024). 
The biochemical investigations of C. braunii, M. polymorpha and P. patens have shown that the metabolic toolkit for the production of specialized phenolic compounds necessary for plant survival on dry land is at least in some parts already present in extant members of streptophytes that are not yet part of the land plants. It has furthermore provided evidence that the variance and extent of these abilities have ostensibly increased as evolution continued. With the help of metabolites derived from the lignin-specific phenylpropanoid pathway, streptophytic organisms could unfold their potential to enhance vital characteristics, a development that contributed substantially to plant terrestrialization and further evolution on dry land.