Herbicide symptomology and the mechanism of action of methiozolin

Abstract

Methiozolin is a new herbicide for control of annual bluegrass (Poa annua L.) in several warm and cool season turfgrasses with an unknown mechanism of action (MOA). In the literature, methiozolin was proposed to be a pigment inhibitor via inhibition of tyrosine aminotransferases (TATs) or a cellulose biosynthesis inhibitor (CBI). Here, exploratory research was conducted to characterize the herbicide symptomology and MOA of methiozolin. Arabidopsis (Arabidopsis thaliana L.) and P. annua exhibited a similar level of susceptibility to methiozolin and arrestment of meristematic growth was the most characteristic symptomology. For example, methiozolin inhibited Arabidopsis root growth (GR50 8 nM), shoot emergence (GR80 ~50 nM), and at rates greater than 500 nM apical meristem growth was completely arrested. We concluded that methiozolin was neither a TAT nor a CBI inhibitor. Methiozolin had a minor effect on chlorophyll and alpha-tocopherol content in treated seedlings (< 500 nM) and supplements in the proposed TAT pathway could not lessen phytotoxicity. Examination of microscopy root images revealed methiozolin treated (100 nM) and untreated seedlings had similar root cell lengths. Thus, methiozolin inhibits cell proliferation and not elongation from meristematic tissue. Subsequently, we suspected methiozolin was an inhibitor of the mevalonic acid (MVA) pathway because its herbicidal symptomologies were nearly indistinguishable from those caused by lovastatin. However, methiozolin did not inhibit phytosterol production and MVA pathway metabolites did not rescue treated seedlings. Further experiments showed that methiozolin produced a very similar physiological profile across a number of assays as cinmethylin, a known inhibitor of fatty acid synthesis through inhibition of thioesterases (FATs). Experiments with Lemna showed that methiozolin also reduced fatty acid content in Lemna with a profile similar, but not identical, to cinmethylin. However, there was no difference in fatty acid content between treated (1 µM) and untreated Arabidopsis seedlings. Methiozolin also bound to both Arabidopsis and Lemna FATs in vitro. Modeling suggested that methiozolin and cinmethylin have comparable and overlapping binding sites to FAT. While there was a discrepancy in the effect of methiozolin on fatty acid content between Lemna and Arabidopsis, the overall evidence indicates that methiozolin is a FAT inhibitor and acts in a similar manner as cinmethylin.

Structure Simplification of Natural Products as a Lead Generation Approach in Agrochemical Discovery

“…After several failed attempts to discover the MoA of cinmethylin, it was reported in 2018, almost 35 years after being introduced, to inhibit the synthesis of medium-chain-length fatty acids by inhibition of acyl-ACP thioesterases, a new MoA . It is interesting that another herbicidal compound (the isoxazoline methiozolin (Figure ), commercialized in the Far East) that is structurally similar to cinmethylin had also been misinterpreted with cinmethylin to be phytotoxic through the inhibition of tyrosine amino transferase and was very recently found to also act through the inhibition of acyl-ACP thioesterases . Cinmethylin was not a significantly successful herbicide when first commercialized, and its analogs were apparently even less viable as potential herbicides; however, decades after its first introduction, the rapid increases in the herbicide resistance of weeds have caused renewed interest in cinmethylin, especially after its novel MoA was clearly determined.…”

Inhibition of acyl‐ACP thioesterase as site of action of the commercial herbicides cumyluron, oxaziclomefone, bromobutide, methyldymron and tebutam

“…Besides the reported cinmethylin binding to FAT, 7,25 this array of assays demonstrates a strong FAT binding affinity in the nanomolar range and inhibition of FAT enzymatic activity by cinmethylin. Studying the metabolic changes revealed that, amongst other effects, cinmethylin treatment led to an altered galactolipid composition.…”

“…This included the rice herbicides/safener oxaziclomefone, bromobutide, dymron, and dimepiperate, the turf herbicides methyldymron and orbencarb, the rape herbicides tebutam and clomazone, as well as the Brassica napus L. and cereal herbicide bixlozone. We also included the rice and turf herbicide methiozolin as a positive control, as we recently described FAT inhibition as its predominant mode of action 25 . An overview of the structures of discussed compounds is shown in Fig.…”

“…We also included the rice and turf herbicide methiozolin as a positive control, as we recently described FAT inhibition as its predominant mode of action. 25 An overview of the structures of discussed compounds is shown in Fig. 3(A)-(L).…”

“…Others 7,25,33 showed that different FAT inhibitor fingerprints of physiological profiles (P-Profiles), consisting of the inhibition values and symptoms of an array of 14 different bioassays, are strikingly similar. Tested FAT inhibitors led to strong growth inhibition of Galium mollugo cell suspension cultures and Lemna paucicostata, whereas the growth of photoautotrophic algae and germination of dicotyledonous Lepidium sativum seeds were only slightly affected ( 7,25,33 and Fig. S5 (A), (B)).…”