Abstract
Cinnamic acid and its hydroxylated derivatives (p-coumaric, caffeic, ferulic and sinapic acids) are known allelochemicals that affect the seed germination and root growth of many plant species. Recent studies have indicated that the reduction of root growth by these allelochemicals is associated with premature cell wall lignification. We hypothesized that an influx of these compounds into the phenylpropanoid pathway increases the lignin monomer content and reduces the root growth. To confirm this hypothesis, we evaluated the effects of cinnamic, p-coumaric, caffeic, ferulic and sinapic acids on soybean root growth, lignin and the composition of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) monomers. To this end, three-day-old seedlings were cultivated in nutrient solution with or without allelochemical (or selective enzymatic inhibitors of the phenylpropanoid pathway) in a growth chamber for 24 h. In general, the results showed that 1) cinnamic, p-coumaric, caffeic and ferulic acids reduced root growth and increased lignin content; 2) cinnamic and p-coumaric acids increased p-hydroxyphenyl (H) monomer content, whereas p-coumaric, caffeic and ferulic acids increased guaiacyl (G) content, and sinapic acid increased sinapyl (S) content; 3) when applied in conjunction with piperonylic acid (PIP, an inhibitor of the cinnamate 4-hydroxylase, C4H), cinnamic acid reduced H, G and S contents; and 4) when applied in conjunction with 3,4-(methylenedioxy)cinnamic acid (MDCA, an inhibitor of the 4-coumarate:CoA ligase, 4CL), p-coumaric acid reduced H, G and S contents, whereas caffeic, ferulic and sinapic acids reduced G and S contents. These results confirm our hypothesis that exogenously applied allelochemicals are channeled into the phenylpropanoid pathway causing excessive production of lignin and its main monomers. By consequence, an enhanced stiffening of the cell wall restricts soybean root growth.
Highlights
Higher plants regularly release secondary metabolites into the soil environment that can influence the growth and development of neighboring plants both positively and negatively
By the sequential action of cinnamoyl-CoA reductase (CCR), ferulate 5-hydroxylase (F5H), caffeic acid 3-O-methyltransferase (COMT) and cinnamyl alcohol dehydrogenase (CAD), the CoA thioesters are converted into monolignols (p-coumaryl, coniferyl and sinapyl alcohols)
We examined the effects of cinnamic acid and its hydroxylated derivatives on soybean root growth, total lignin and the composition of H, G and S monomers
Summary
Higher plants regularly release secondary metabolites into the soil environment that can influence the growth and development of neighboring plants both positively and negatively. Allelochemicals typically suppress seed germination, causing disorders of root growth and inhibiting plant growth They alter several physiological and biochemical processes such as water utilization, mineral uptake, photosynthesis, amino acid metabolism, protein synthesis, glycolysis, mitochondrial respiration and ATP synthesis, among countless others [1]. Lignification, the metabolic process of sealing a plant cell wall by lignin deposition, occurs during the course of normal tissue development and is an important step during root growth. In the first steps of this metabolic pathway, the deamination of L-phenylalanine by phenylalanine ammonia-lyase (PAL) produces t-cinnamate This step is followed by hydroxylation of the aromatic ring, catalyzed by cinnamate 4-hydroxylase (C4H), to generate p-coumarate. The oxidative polymerization of these three monolignols, via the action of peroxidases (POD), generates the p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units of the lignin polymer [4]
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