title: Secondary Effects of Glyphosate Action inPhelipanche aegyptiaca: Inhibition of Solute Transport from the Host Plant to the Parasite.
abstract: It is currently held that glyphosate efficiently controls the obligate holoparasitePhelipanche aegyptiaca(Egyptian broomrape) by inhibiting its endogenous shikimate pathway, thereby causing a deficiency in aromatic amino acids (AAA). While there is no argument regarding the shikimate pathway being the primary site of the herbicide's action, the fact that the parasite receives a constant supply of nutrients, including proteins and amino acids, from the host does not fit with an AAA deficiency. This apparent contradiction implies that glyphosate mechanism of action inP. aegyptiacais probably more complex and does not end with the inhibition of the AAA biosynthetic pathway alone. A possible explanation would lie in a limitation of the translocation of solutes from the host as a secondary effect. We examined the following hypotheses: (a) glyphosate does not affectsP. aegyptiacaduring its independent phase and (b) glyphosate has a secondary effect on the ability ofP. aegyptiacato attract nutrients, limiting the translocation to the parasite. By using a glyphosate-resistant host plant expressing the "phloem-mobile" green fluorescent protein (GFP), it was shown that glyphosate interacts specifically withP. aegyptiaca, initiating a deceleration of GFP translocation to the parasite within 24 h of treatment. Additionally, changes in the entire sugars profile (together with that of other metabolites) ofP. aegyptiacawere induced by glyphosate. In addition, glyphosate did not impair germination or seedling development ofP. aegyptiacabut begun to exert its action only after the parasite has established a connection to the host vascular system and became exposed to the herbicide. Our findings thus indicate that glyphosate does indeed have a secondary effect inP. aegyptiaca, probably as a consequence of its primary target inhibition-via inhibition of the translocation of phloem-mobile solutes to the parasite, as was simulated by the mobile GFP. The observed disruption in the metabolism of major sugars that are abundant inP. aegyptiacawithin 48 h after glyphosate treatment provides a possible explanation for this inhibition of translocation and might reflect a critical secondary effect of the herbicide's primary action that results in loss of the parasite's superior sink for solutes.
authors.affiliation: Department of Plant Pathology and Weed Research, Agricultural Research Organization, Newe Ya'ar Research CenterRamat Yishay, Israel; The Robert H. Smith Faculty of Agriculture, Food and Environment, The Robert H. Smith Institute of Plant Sciences and Genetics, The Hebrew University of JerusalemRehovot, Israel.
authors.forename: Tal
authors.initials: T
authors.lastname: Shilo
authors.affiliation: The Robert H. Smith Faculty of Agriculture, Food and Environment, The Robert H. Smith Institute of Plant Sciences and Genetics, The Hebrew University of Jerusalem Rehovot, Israel.
authors.forename: Baruch
authors.initials: B
authors.lastname: Rubin
authors.affiliation: Department of Plant Pathology and Weed Research, Agricultural Research Organization, Newe Ya'ar Research Center Ramat Yishay, Israel.
authors.forename: Dina
authors.initials: D
authors.lastname: Plakhine
authors.affiliation: Department of Entomology, Agricultural Research Organization, Newe Ya'ar Research Center Ramat Yishay, Israel.
authors.forename: Shira
authors.initials: S
authors.lastname: Gal
authors.affiliation: Migal Galilee Technology Center Kiryat Shmona, Israel.
authors.forename: Rachel
authors.initials: R
authors.lastname: Amir
authors.affiliation: Migal Galilee Technology Center Kiryat Shmona, Israel.
authors.forename: Yael
authors.initials: Y
authors.lastname: Hacham
authors.affiliation: The Robert H. Smith Faculty of Agriculture, Food and Environment, The Robert H. Smith Institute of Plant Sciences and Genetics, The Hebrew University of Jerusalem Rehovot, Israel.
authors.forename: Shmuel
authors.initials: S
authors.lastname: Wolf
authors.affiliation: Department of Plant Pathology and Weed Research, Agricultural Research Organization, Newe Ya'ar Research Center Ramat Yishay, Israel.
authors.forename: Hanan
authors.initials: H
authors.lastname: Eizenberg
keywords: Egyptian broomrape; Orobanche; germination; phloem; sugars; tomato; translocation