Abstract
The phenylpropanoid pathway converts the aromatic amino acid phenylalanine into a wide range of secondary metabolites. Most of the carbon entering the pathway incorporates into the building blocks of lignin, an aromatic polymer providing mechanical strength to plants. Several intermediates in the phenylpropanoid pathway serve as precursors for distinct classes of metabolites that branch out from the core pathway. Untangling this metabolic network in Arabidopsis was largely done using phenylpropanoid pathway mutants, all with different degrees of lignin depletion and associated growth defects. The phenotypic defects of some phenylpropanoid pathway mutants have been attributed to differentially accumulating phenylpropanoids or phenylpropanoid-derived compounds. In this perspectives article, we summarize and discuss the reports describing an altered accumulation of these bioactive molecules as the causal factor for the phenotypes of lignin mutants in Arabidopsis.
Highlights
The general phenylpropanoid pathway (PPP) is a central metabolic pathway in plants involved in the synthesis of a broad range of secondary metabolites that consist of aromatic ring structures with particular sidechain modifications (Vogt, 2010; Figure 1)
The first enzyme of the pathway, PHENYLALANINE AMMONIA LYASE (PAL) deaminates the aromatic amino acid phenylalanine. This results in the formation of trans-cinnamic acid (t-Cinnamic acid (CA)), which is subsequently converted to p-coumaric acid by CINNAMATE-4-HYDROXYLASE (C4H)
4-HYDROXYCINNAMATE-CoA LIGASE (4CL) converts p-coumaric acid to p-coumaroyl-CoA, which is subsequently converted to p-coumaroyl-shikimate by HYDROXYCINNAMOYLCoA:SHIKIMATE HYDROXYCINNAMOYL TRANSFERASE (HCT). p-COUMAROYL SHIKIMATE/QUINATE 3 -HYDROXYLASE (C3 H) hydroxylates the shikimate conjugate and the product of this reaction, caffeoyl shikimate, is subsequently converted to caffeoylCoA by HCT
Summary
The general phenylpropanoid pathway (PPP) is a central metabolic pathway in plants involved in the synthesis of a broad range of secondary metabolites that consist of aromatic ring structures with particular sidechain modifications (Vogt, 2010; Figure 1). The first enzyme of the pathway, PHENYLALANINE AMMONIA LYASE (PAL) deaminates the aromatic amino acid phenylalanine This results in the formation of trans-cinnamic acid (t-CA), which is subsequently converted to p-coumaric acid by CINNAMATE-4-HYDROXYLASE (C4H). The other model suggests a shift in cell wall integrity to trigger a stress response, resulting in growth defects Besides these two models, a third proposes the differential accumulation of soluble pathway intermediates or derivatives thereof as the causal factor of the growth phenotypes. The literature is scattered with studies claiming evidence for bioactivity of nearly every intermediate or derivative of the PPP on plant growth and development (Vanholme B. et al, 2019) Many of these studies should, be interpreted with care as they were never subjected to rigorous independent scrutiny and only a handful of these studies remains significant when taking physiological relevance in mind.
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