Herbicide Symptomology and the Mechanism of Action of Methiozolin

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.