Abstract
Aromatic amino acids (AAAs) synthesized in plants via the shikimate pathway can serve as precursors for a wide range of secondary metabolites that are important for plant defense. The goals of the current study were to test the effect of increased AAAs on primary and secondary metabolic profiles and to reveal whether these plants are more tolerant to abiotic stresses (oxidative, drought and salt) and to Phelipanche egyptiaca (Egyptian broomrape), an obligate parasitic plant. To this end, tobacco (Nicotiana tabacum) plants were transformed with a bacterial gene (AroG) encode to feedback-insensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase, the first enzyme of the shikimate pathway. Two sets of transgenic plants were obtained: the first had low expression of the AroG protein, a normal phenotype and minor metabolic changes; the second had high accumulation of the AroG protein with normal, or deleterious morphological changes having a dramatic shift in plant metabolism. Metabolic profiling analysis revealed that the leaves of the transgenic plants had increased levels of phenylalanine (up to 43-fold), tyrosine (up to 24-fold) and tryptophan (up to 10-fold) compared to control plants having an empty vector (EV) and wild type (WT) plants. The significant increase in phenylalanine was accompanied by higher levels of metabolites that belong to the phenylpropanoid pathway. AroG plants showed improved tolerance to salt stress but not to oxidative or drought stress. The most significant improved tolerance was to P. aegyptiaca. Unlike WT/EV plants that were heavily infected by the parasite, the transgenic AroG plants strongly inhibited P. aegyptiaca development, and only a few stems of the parasite appeared above the soil. This delayed development of P. aegyptiaca could be the result of higher accumulation of several phenylpropanoids in the transgenic AroG plants and in P. aegyptiaca, that apparently affected its growth. These findings indicate that high levels of AAAs and their related metabolites have the potential of controlling the development of parasitic plants.
Highlights
The shikimate pathway is initiated by combining phosphoenolpyruvate (PEP) and erythrose-4-phosphate (E4P) with the activity of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHPS)
To study the impact of high aromatic amino acids (AAAs) levels on primary metabolic profile and on the ability of tobacco plants to cope with stresses, we overexpressed the E. coli AroG175 gene targeted to the chloroplast under the control of the 35S CaMV promoter and octopine synthase terminator (Tzin et al, 2012; Supplementary Figure S2)
Thirty kanamycin-resistant T0 tobacco lines were screened for expression of the AroG gene by immunoblot analysis using antibodies against the 3HA epitope-tag that was fused to the AroG gene (Supplementary Figure S2)
Summary
The shikimate pathway is initiated by combining phosphoenolpyruvate (PEP) and erythrose-4-phosphate (E4P) with the activity of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHPS). It ends with the production of chorismate, which is used to generate three aromatic amino acids (AAAs): tryptophan, phenylalanine, and tyrosine (Yoo et al, 2013) These AAAs are used for protein synthesis and for the synthesis of different secondary metabolites that play a major role in protecting plants from biotic and abiotic stresses (Supplementary Figure S1). Studies have shown that under abiotic stresses, the expression levels of genes in the AAAs biosynthesis pathway are up-regulated, leading to higher levels of AAAs and their secondary metabolites, including flavonoids, anthocyanins, betalains, and phenolic acids (reviewed by Tzin and Galili, 2010; Maeda and Dudareva, 2012). The induction of AAAs could potentially contribute directly and indirectly to the stress tolerance of the plants
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
