PubMed · 6361868
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Y Saeki. 1983. [Dioxygenases].. https://pubmed.ncbi.nlm.nih.gov/6361868/
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Mesotrione is a new herbicide being developed for the selective pre- and post-emergence control of a wide range of broad-leaved and grass weeds in maize (Zea mays). It is a member of the benzoylcyclohexane-1,3-dione family of herbicides, which are chemically derived from a natural phytotoxin obtained from the Californian bottlebrush plant, Callistemon citrinus. The compound acts by competitive inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD), a component of the biochemical pathway that converts tyrosine to plastoquinone and alpha-tocopherol. Mesotrione is an extremely potent inhibitor of HPPD from Arabidopsis thaliana, with a Ki value of c 6-18 pM. It is rapidly taken up by weed species following foliar application, and is distributed within the plants by both acropetal and basipetal movement. Maize is tolerant to mesotrione as a consequence of selective metabolism by the crop plant. Slower uptake of mesotrione, relative to susceptible weed species, may also contribute to its utility as a selective herbicide for use in maize.
This paper reviews the discovery of isoxaflutole (IFT), focusing on the chemical and physicochemical properties which contribute to the herbicidal behaviour of this new herbicide. IFT (5-cyclopropyl-1,2-isoxazol-4-yl alpha alpha alpha-trifluoro-2-mesyl-p-tolyl ketone) is a novel herbicide for pre-emergence control of a wide range of important broadleaf and grass weeds in corn and sugarcane. The first benzoyl isoxazole lead was synthesised in 1989 and IFT in 1990, and the herbicidal potential of the latter was identified in 1991. The decision to develop the molecule was taken after two years of field testing in North America. The biochemical target of IFT is 4-hydroxyphenylpyruvate dioxygenase (HPPD), inhibition of which leads to a characteristic bleaching of susceptible species. The inhibitor of HPPD is the diketonitrile derivative of IFT formed from opening of the isoxazole ring. The diketonitrile (DKN) is formed rapidly in plants following root and shoot uptake. The DKN is both xylem and phloem mobile leading to high systemicity. IFT also undergoes conversion to the DKN in the soil. The soil half-life of IFT ranges from 12 h to 3 days under laboratory conditions and is dependent on several factors such as soil type, pH and moisture. The log P of IFT is 2.19 and the water solubility is 6.2 mg litre-1, whereas the corresponding values for the DKN are 0.4 and 326 mg litre-1, respectively. These properties restrict the mobility of IFT, which is retained at the soil surface where it can be taken up by surface-germinating weed seeds. The DKN, which has a laboratory soil half-life of 20-30 days, is more mobile and is taken up by the roots. In addition to influencing the soil behaviour of IFT and DKN, the greater lipophilicity of IFT leads to greater uptake by seed, shoot and root tissues. In both plants and soil, the DKN is converted to the herbicidally inactive benzoic acid. This degradation is more rapid in maize than in susceptible weed species and this contributes to the mechanism of selectivity, together with the greater sowing depth of the crop.
Previous genealogical analyses of population structure in Coccidioides immitis revealed the presence of two cryptic and sexual species in this pathogenic fungus but did not clarify their origin and relationships with respect to other taxa. By combining the C. immitis data with those of two of its closest relatives, the free-living saprophytes Auxarthron zuffianum and Uncinocarpus reesii, we show that the C. immitis species complex is monophyletic, indicating a single origin of pathogenicity. Cryptic species also were found in both A. zuffianum and U. reesii, indicating that they can be found in both pathogenic and free-living fungi. Our study, together with a few others, indicates that the current list of known fungal species might be augmented by a factor of at least two. However, at least in the C. immitis, A. zuffianum, and U. reesii complexes, cryptic species represent subdivisions at the tips of deep monophyletic clades and thus well within the existing framework of generic classification. An analysis of silent and expressed divergence and polymorphism values between and within the taxa identified by genealogical concordance did not reveal faster evolution in C. immitis as a consequence of adaptation to the pathogenic habit, nor did it show positive Darwinian evolution in a region of a dioxygenase gene (tcrP gene coding for 4-HPPD) known to cause antigenic responses in humans. Instead, the data suggested relative stasis, indicative of purifying selection against mostly deleterious mutations. Two introns in the same gene fragment were considerably more divergent than exons and were unalignable between species complexes but had very low polymorphism within taxa.