Secondary Effects of Glyphosate Action in Phelipanche aegyptiaca: Inhibition of Solute Transport from the Host Plant to the Parasite Article Swipe
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· 2017
· Open Access
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· DOI: https://doi.org/10.3389/fpls.2017.00255
· OA: W2590340313
It is currently held that glyphosate efficiently controls the obligate holoparasite <i>Phelipanche aegyptiaca</i> (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 in <i>P. aegyptiaca</i> is 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 affects <i>P. aegyptiaca</i> during its independent phase and (b) glyphosate has a secondary effect on the ability of <i>P. aegyptiaca</i> to 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 with <i>P. aegyptiaca</i>, 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) of <i>P. aegyptiaca</i> were induced by glyphosate. In addition, glyphosate did not impair germination or seedling development of <i>P. aegyptiaca</i> but 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 in <i>P. aegyptiaca</i>, 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 in <i>P. aegyptiaca</i> within 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.
