Mechanism of glyphosate control of Phelipanche aegyptiaca

Abstract

Despite its total reliance on its host plant, the holoparasite Phelipanche aegyptiaca suffers from a deficiency of aromatic amino acids upon exposure to glyphosate. The herbicide glyphosate inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), a key enzyme in the biosynthesis of aromatic amino acids. However, the functionality of the EPSPS pathway in the obligate root holoparasite Phelipanche aegyptiaca is not straightforward because of the parasite's total dependence on the host plant. Despite the importance of glyphosate as a means of controlling P. aegyptiaca, the mechanism of action of the herbicide in this parasite is not clearly understood. We characterized glyphosate control of P. aegyptiaca by using a glyphosate-resistant tomato (GRT) genotype as the host plant and evaluating the activity of EPSPS and the levels of free aromatic amino acids in the parasite. The viability of the parasite's tissues deteriorated within the first 40 h after treatment (HAT) with glyphosate. In parallel, shikimate accumulation in the parasite was first detected at 24 HAT and increased over time. However, shikimate levels in the GRT host did not increase, indicating that the host was indeed glyphosate tolerant. Free phenylalanine and tyrosine levels decreased by 48 HAT in the parasite, indicating a deficiency of aromatic amino acids. The use of GRT as the host enabled us to observe, in an in situ experimental system, both endogenous EPSPS inhibition and a deficiency of aromatic amino acids in the parasite. We thus provided evidence for the presence of an active EPSPS and aromatic amino acid biosynthesis pathway in P. aegyptiaca and pinpointed this pathway as the target of glyphosate action in this parasite.

Recognition of Orobanche cumana Below-Ground Parasitism Through Physiological and Hyper Spectral Measurements in Sunflower (Helianthus annuus L.)

“…The relative increase in carbohydrate content can be explained by the increased demand for carbohydrates in the newly created sink, which leads to higher production of carbohydrates in the leaves (Nandula et al, 2000). Similar results were demonstrated in Phelipanche aegyptiaca parasitism of carrot and tomato (Nandula et al, 2000; Shilo et al, 2016). Another possible explanation is that a decrease in mineral content in the tissue leads to relative increases in the carbohydrate content.…”

The Effects of Herbicides Targeting Aromatic and Branched Chain Amino Acid Biosynthesis Support the Presence of Functional Pathways in Broomrape

“…Based on the results obtained with ALS, we expected to detect EPSPS activity in P. aegyptiaca , since glyphosate has been found to control P. aegyptiaca and Orobanche crenata parasitizing parsley and O. crenata parasitizing faba bean, pea, lentil, vetch, celery, carrot, and glyphosate-resistant tomato, without causing damage to the host (Kasasian, 1973; Jacobsohn and Kelman, 1980; Mesa-García and García-Torres, 1985; Arjona-Berral et al, 1988; Goldwasser et al, 2003; Shilo et al, 2016). Therefore, the parasite is probably more sensitive than those hosts to glyphosate (Goldwasser and Kleifeld, 2004).…”

“…Applied to P. aegyptiaca callus, glyphosate blocked EPSPS activity, and accumulation of the precursor shikimic acid was positively correlated with glyphosate concentration ( Figure 6 ). Accordingly, shikimate accumulation in P. aegyptiaca parasitizing glyphosate-resistant tomato following glyphosate application has been recently shown (Shilo et al, 2016). …”

“…Recently, shikimate accumulation and a decrease in free aromatic amino acids have been shown in Egyptian broomrape [ Phelipanche aegyptiaca (Pers.) Pomel] attached to the roots of glyphosate-resistant tomato following foliar glyphosate application (Shilo et al, 2016). This suggests the presence of active EPSPS in parasite tissues.…”

Phelipanche aegyptiaca parasitism impairs salinity tolerance in young leaves of tomato

“…To examine the salt accumulation in the tomato rhizosphere under P. aegyptiaca infection, tomato seedlings were grown in transparent polyethylene bags into which a piece of GF/A filter paper (Whatman, GE Healthcare, Maidstone, UK) had been inserted, as described by Shilo et al (2016). The polyethylene bags were held in a controlled conditioned growth room with 16 h/8 h day/night at a light intensity of 350 μmol photons m −2 s −1 and a constant temperature of 25 ∘ C. After establishment of the root system, 10 bags were opened, and sterilized P. aegyptiaca seeds where spread on half of the paper to infect only a small part of the root system.…”