Glyphosate is a widely applied broad-spectrum systemic herbicide that competitively inhibits the penultimate enzyme, 5-Enolpyruvylshikimate 3-Phosphate Synthase (EPSPS), of the shikimate pathway thereby causing deleterious effects. A glyphosate resistant Arabidopsis mutant (gre1) was isolated and genetic analyses indicated a dysfunctional red (R) and far-red (FR) light receptor, phytochrome B (phyB), causing this phenotype. This is consistent with increased glyphosate sensitivity and glyphosate-induced shikimate accumulation in low R:FR light, and the induction of genes encoding enzymes of the shikimate pathway in high R:FR light. Expression of the shikimate pathway genes exhibits diurnal oscillation and this is altered in the phyB mutant. Furthermore, transcript analysis suggests that this diurnal oscillation is not only dependent on phyB but also due to circadian regulatory mechanisms. Our data offer an explanation of the well-documented observation that glyphosate treatment at various times throughout the day, with their specific composition of light quality and intensity, results in different efficiencies of the herbicide.This article is protected by copyright. All rights reserved.