Abstract

The troublesome agronomic weed, jimsonweed (Datura stramonium L.), is also a major poisonous plant. The toxicity of jimsonweed is due to tropane alkaloids, which contain a methylated nitrogen atom (N-CH₃) and include the anticholinergic drugs atropine, hyoscyamine, and scopolamine, as well as the narcotic cocaine. The enzyme and gene regulatory system of tropane alkaloid in plants is not clearly understood. The effects of the herbicides chlorsulfuron and glyphosate, and the growth regulator methyl jasmonate on the growth and alkaloid biosynthesis of jimsonweed were investigated. The growth responses of jimsonweed were characterized by determining the molar concentration of the herbicides required to inhibit plant root fresh weight by 50% (I₅₀). Chlorsulfuron and glyphosate inhibited root growth of jimsonweed seedlings; the I₅₀ was 8 × 10⁻⁹ and 10⁻⁷ M, respectively. Methyl jasmonate increased root growth of jimsonweed seedlings at 10⁻⁶ and 10⁻⁷ M, but was inhibitory at 10⁻⁵ M. Tropane alkaloids are produced by the condensation of tropine and tropic acid, a phenylalanine-derived intermediate. Phenylalanine levels in jimsonweed seedlings were reduced by glyphosate and chlorsulfuron. Methyl jasmonate at 10⁻⁸ M or greater concentrations induced phenylalanine levels in jimsonweed seedlings. Arginine and ornithine levels in jimsonweed seedlings were increased two to three times by chlorsulfuron and glyphosate, but reduced by methyl jasmonate. Contents of tropinone, -tropine, and tropine in jimsonweed seedlings were reduced by chlorsulfuron and glyphosate, but increased by methyl jasmonate at 10⁻⁷ and 10⁻⁶ M. Putrescine N-methyltransferase (PMT) is the first committed enzyme, which drives the flow of nitrogen away from polyamine biosynthesis to tropane alkaloid biosynthesis. The expression of PMT mRNA transcripts was reduced in jimsonweed roots treated with 10⁻⁶ M chlorsulfuron and glyphosate. PMT mRNA transcripts of jimsonweed roots were greatly induced following treatment with 10⁻⁷ M methyl jasmonate. These results indicate that methyl jasmonate enhanced PMT activity due to an increase of PMT mRNA. The effects of glyphosate and chlorsulfuron on PMT mRNA’s transcriptional control and the accumulation of amino acid arginine/ornithine contents, coupled with their inhibitory effect on phenylalanine, tropinone, -tropine, and tropine levels, suggest that these herbicides might cause a decrease in overall tropane alkaloid biosynthesis in jimsonweed. However, the relationship between plant growth and metabolism affected by these compounds is not clear.

Keywords: Alkaloid biosynthesis; Chlorsulfuron; Glyphosate; Jimsonweed; Methyl jasmonate; Putrescine N-methyltransferase; RNA gel blot analysis

Article Outline

1. Introduction

2. Materials and methods

2.1. Chemicals

2.2. Plant material

2.3. Determination of amino acid contents

2.4. Determination of tropine and ψ-tropine contents

2.5. RNA isolation and Northern blotting

3. Results and discussion

3.1. Effects of chemical treatments on jimsonweed growth

3.2. Amino acid content of jimsonweed seedlings

3.3. Tropine contents of jimsonweed seedlings

3.4. Northern blot analysis

4. Concluding remarks

Acknowledgements

References

Contribution 639 from the Department of Plant Pathology, Physiology and Weed Science, Virginia Agricultural Experiment Station, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0330, USA.