A Modified phosphate-carrier protein theory is proposed as a non-target site mechanism For glyphosate resistance in weeds

Abstract

-Glyphosate is an herbicide that inhibits the enzyme 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPs) (EC 2.5.1.19). EPSPs is the sixth enzyme of the shikimate pathway, by which plants synthesize the aromatic amino acids phenylalanine, tyrosine, and tryptophan and many compounds used in secondary metabolism pathways. About fifteen years ago it was hypothesized that it was unlikely weeds would evolve resistance to this herbicide because of the limited degree of glyphosate metabolism observed in plants, the low resistance level attained to EPSPs gene overexpression, and because of the lower fitness in plants with an altered EPSPs enzyme. However, today 20 weed species have been described with glyphosate resistant biotypes that are found in all five continents of the world and exploit several different resistant mechanisms. The survival and adaptation of these glyphosate resistant weeds are related toresistance mechanisms that occur in plants selected through the intense selection pressure from repeated and exclusive use of glyphosate as the only control measure. In this paper the physiological, biochemical, and genetic basis of glyphosate resistance mechanisms in weed species are reviewed and a novel and innovative theory that integrates all the mechanisms of non-target site glyphosate resistance in plants is presented.Keywords: membrane carrier proteins, herbicide, weed resistance.RESUMO -Glyphosate é uma glicina fosfonada e inibe a enzima 5-enolpiruvil-shikimato-3-fosfato sintase (EPSPS) (EC 2.5.1.19). EPSPS é a sexta enzima da rota do shikimato, na qual são sintetizados os compostos do metabolismo secundário e os aminoácidos aromáticos fenilalanina, tirosina e triptofano. Alguns autores hipotetizaram que seria improvável a evolução de plantas daninhas resistentes a este herbicida. As justificativas estariam relacionadas à limitada metabolização de glyphosate nas plantas, ao baixo nível de resistência obtido com a superexpressão do gene EPSPS e à alta penalidade de adaptação oriunda de mutações no gene EPSPS. Mas, atualmente estão descritas 18 espécies com biótipos resistentes distribuídas em 17 países. A sobrevivência e adaptação estão relacionadas aos mecanismos de resistência que ocorrem nas plantas daninhas selecionadas em função da intensa pressão de seleção do herbicida. Esta revisão objetiva apresentar e discutir evidências fisiológicas, bioquímicas e genéticas que fundamentam os principais mecanismos envolvidos na resistência ao herbicida glyphosate em plantas daninhas.Palavras-chave: herbicida, proteínas carregadoras de membrana, planta daninha.

In Vivo 31P-Nuclear Magnetic Resonance Studies of Glyphosate Uptake, Vacuolar Sequestration, and Tonoplast Pump Activity in Glyphosate-Resistant Horseweed

“…The ammonia challenge to GR horseweed demonstrates the remarkable ability of the plant cell to metabolically adjust to the alkaline challenge and thereby maintain nearly normal cellular pH. Consistent with other reports (Roso and Vidal, 2010;Staub et al, 2012), these observations also provide evidence that the tonoplast phosphate portal is not used by glyphosate as vacuole sequestration resumes during the recovery phase (Fig. 5C), where it left off at the end of the control phase (Fig.…”

Non-target-site glyphosate resistance inConyza bonariensisis based on modified subcellular distribution of the herbicide

“…). These findings combined with the high levels of resistance (more than 15‐fold) exhibited by the examined biotypes suggest a mechanism of non‐target‐site GR, as has been reported by others …”

Differential antioxidant enzyme activity in rapid‐response glyphosate‐resistant Ambrosia trifida

“…A plausible site of glyphosate recognition in RR A. trifida may be a cell membrane‐bound phosphate transporter, as glyphosate is one of only three herbicides for which an active transport system in plants exists . For example, low‐affinity phosphate transporters Pht1;6 and Pht2;1 have been identified in several plant species and may account for active glyphosate transport . Identifying the role these may have in glyphosate perception in the RR biotype will assist in elucidating the intricacies involved in this novel form of glyphosate resistance.…”