Effectiveness and mode of action of phosphonate inhibitors of plant glutamine synthetase

Abstract

BACKGROUND: Aiming at the rational design of new herbicides, the availability of the threedimensional structure of the target enzyme greatly enhances the optimisation of lead compounds and the design of derivatives with increased activity. Among the most widely exploited herbicide targets is glutamine synthetase. Recently, the structure of a cytosolic form of the maize enzyme has been described, making it possible to verify whether steric, electronic and hydrophobic features of a compound are in agreement with inhibitor-protein interaction geometry.

Physiologic Activity of Bisphosphonates – Recent Advances

“…This inhibitory activity was observed when studying bleaching herbicides influencing biosynthesis of caretonoids. Further studies have shown that herbicidal aminomethylene-bisphosphonates may be considered multi-target substances, which was documented by their inhibitory activities towards glutamine synthetase, [198,199] 3-deoxy-D-arabinoheptulosonate-7-phosphate (DAHP) synthase, [200] δ 1 -pyrroline-5-carboxylate (P5CR) reductase [201,202] and pyrophosphatase [203]. Thus, these compounds may be considered a heterogeneous group of compounds with variable modes of action (Scheme 11).…”

Phytotoxicity of aminobisphosphonates targeting bothδ1-pyrroline-5-carboxylate reductase and glutamine synthetase

“…Moreover, as the rate of a double mutation conferring target‐site tolerance to an inhibitor acting on two different enzymes is the product of the probability of each single mutation, for dual‐target inhibitors herbicide resistance could be avoided or, at least, delayed. Some substituted derivatives of phenylaminomethylenebisphosphonic acid inhibit the activity of both P5C reductase and glutamine synthetase at micromolar concentrations . Unequivocal data supported the occurrence of GS inhibition in the plant cell .…”

“…However, in the absence of glutamine and/or proline starvation, it would be unclear what in this case may be the factor(s) causing cell growth inhibition. For a natural and potent inhibitor of GS, phosphinothricin, it has been proven that glutamate depletion is not the primary cause of plant cell death; instead, GS inhibition rapidly leads to the accumulation of ammonia which acts as an uncoupler, causing cytotoxicity . To verify whether in this case ammonia also accumulates in seedlings in response to the treatment with Br 2 PAMBPA, its levels were determined in rapeseed tissues.…”

“…Although none of these compounds was more potent, the availability of several active structures allowed a proper SAR analysis, leading us to hypothesise about the steric and electronic requirements for maintenance or enhancement of the inhibitory properties . Further studies indicated that the most active aminobisphosphonates are also able to inhibit glutamine synthetase (GS) (EC 6.3.1.2), the enzyme that plays a key role in ammonia assimilation . Because glutamate starvation deriving from GS inhibition is expected to block the synthesis of any nitrogen‐containing metabolite, and in all cases lower concentrations were required to reduce in vitro GS activity to 50% (supporting information Fig.…”

“…21 Further studies indicated that the most active aminobisphosphonates are also able to inhibit glutamine synthetase (GS) (EC 6.3.1.2), the enzyme that plays a key role in ammonia assimilation. 22 Because glutamate starvation deriving from GS inhibition is expected to block the synthesis of any nitrogen-containing metabolite, and in all cases lower concentrations were required to reduce in vitro GS activity to 50% (supporting information Fig. S1), the question arose as to the significance in planta of P5C reductase inhibition.…”