Abstract
Cinnamic acid is a known allelochemical that affects seed germination and plant root growth and therefore influences several metabolic processes. In the present work, we evaluated its effects on growth, indole-3-acetic acid (IAA) oxidase and cinnamate 4-hydroxylase (C4H) activities and lignin monomer composition in soybean (Glycine max) roots. The results revealed that exogenously applied cinnamic acid inhibited root growth and increased IAA oxidase and C4H activities. The allelochemical increased the total lignin content, thus altering the sum and ratios of the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignin monomers. When applied alone or with cinnamic acid, piperonylic acid (PIP, a quasi-irreversible inhibitor of C4H) reduced C4H activity, lignin and the H, G, S monomer content compared to the cinnamic acid treatment. Taken together, these results indicate that exogenously applied cinnamic acid can be channeled into the phenylpropanoid pathway via the C4H reaction, resulting in an increase in H lignin. In conjunction with enhanced IAA oxidase activity, these metabolic responses lead to the stiffening of the cell wall and are followed by a reduction in soybean root growth.
Highlights
Many higher plants release allelochemicals into the environment through root excretion or exudation, leaching, evaporation and decomposition of plant tissues/organs [1]
In agreement with the effects observed on root growth, the enzyme activity of seedlings treated with cinnamic acid was significantly different from those of controls
Roots exposed to cinnamic acid significantly increased indole-3-acetic acid (IAA) oxidase activity by 32% to 91% (0.1 to 0.5 mM); 76% (0.75 mM) and 30% (1.0 mM) in comparison with the control (2.96 ± 0.31 nmols min−1 g−1 fresh weight) (Figure 1)
Summary
Many higher plants release allelochemicals into the environment through root excretion or exudation, leaching, evaporation and decomposition of plant tissues/organs [1]. These compounds can accumulate in the soil, influencing (positively or negatively) the growth and development of other species. This process is termed allelopathy and is broadly defined as any chemically-mediated interaction among plants. Many secondary metabolites have been referred to as allelochemicals They are commonly found in soils at concentrations between 0.01 and 0.1 mM, and they affect plant growth at concentrations of up to 10 mM [4,5]. In higher plants, the cell wall is one of the first tissues affected by stress signals, which are transmitted to the cell interior and influence several processes [9]
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