Hydantocidin is a phytotoxic natural product previously reported to inhibit purine biosynthesis at the site of adenylosuccinate synthetase. While hydantocidin does not inhibit this enzymein vitro, Arabidopsis thalianaplants subjected to hydantocidin treatment were found to contain an inhibitor of adenylosuccinate synthetase. Alkaline phosphatase treatment of the plant extract significantly reduced inhibition, suggesting that thein vivoactive molecule might be phosphorylated. 2α-Phosphohydantocidin was synthesized and proved to be a potent inhibitor of rabbit muscle adenylosuccinate synthetase. This inhibition exhibited a time-dependent component not seen with hadacidin, a known competitive inhibitor with respect to aspartate at this site. The unidentified inhibitor produced inArabidopsiseluted in the same fraction as 2α-phosphohydantocidin following reversed-phase HPLC separation. Electrospray ionization mass spectrometry confirmed the presence of a molecule with a mass identical to that of 2α-phosphohydantocidin in this same fraction. These data suggest that 2α-phosphohydantocidin is thein vivoadenylosuccinate synthestase inhibitor responsible for hydantocidin phytotoxicity.

Hydantocidin is a highly active phytotoxin that causes immediate cessation of plant growth, meristematic necrosis, and plant death. Injury symptoms and metabolite reversal studies in Arabidopsis thaliana suggested a novel mode of action, most likely in purine biosynthesis. Adenine supplementation alleviated symptoms associated with hydantocidin, while the addition of guanine had no effect. Adenine reversal resulted from the in vivo conversion of adenine to adenosine 5′-phosphate. Hydantocidin also inhibited the incorporation of radiolabeled glycine into adenine nucleotides, but not into guanine nucleotides. Adenosine 5′-triphosphate levels in Arabidopsis were shown to drop dramatically after treatment with hydantocidin. All of the aforementioned physiological effects were also elicited by hadacidin, a known inhibitor of the conversion of inosine 5′-phosphate to adenosine 5′-phosphate, at the site of adenylosuccinate synthetase. However, hy dantocidin was subsequently shown not to be an in vitro inhibitor of either adenylosuccinate synthetase or adenylosuccinate lyase (the two enzymes that mediate the conversion of inosine 5′-phosphate to adenosine 5′-phosphate). We hypothesized that hydantocidin might be converted in vivo to another molecule that inhibits the formation of adenosine 5′-phosphate. This hypothesis was supported when increasing amounts of an adenylosuccinate synthetase inhibitor were formed in a plant extract, over time, after adding hydantocidin, ATP, and MgCl2.

Acetolactate synthase (ALS) was isolated from a field population of cocklebur (Xanthium strumarium) that developed resistance to the herbicide Scepter following three consecutive years of application. The active ingredient of Scepter, imazaquin, gave an inhibitor concentration required to produce 50% inhibition of the enzyme activity that was more than 300 times greater for the resistant enzyme than for the wild-type cocklebur ALS. Tests with flumetsulam and chlorimuron show that the resistant ALS was not cross-resistant to these two other classes of ALS inhibitors.

Acetolactate synthase (ALS) was isolated from a field population of cocklebur (Xanthium strumarium) that developed resistance to the herbicide Scepter following three consecutive years of application. The active ingredient of Scepter, imazaquin, gave an inhibitor concentration required to produce 50% inhibition of the enzyme activity that was more than 300 times greater for the resistant enzyme than for the wild-type cocklebur ALS. Tests with flumetsulam and chlorimuron show that the resistant ALS was not cross-resistant to these two other classes of ALS inhibitors.

ChemInformVolume 21, Issue 35 Reviews ChemInform Abstract: Aryloxyphenoxypropanoate and Cyclohexanedione Herbicides. Inhibition of Acetyl Coenzyme A Carboxylase J. SECOR, J. SECOR Agric. Prod. Res., Dow Chem. Co., Walnut Creek, CA 94598-0902, USASearch for more papers by this authorC. CSEKE, C. CSEKE Agric. Prod. Res., Dow Chem. Co., Walnut Creek, CA 94598-0902, USASearch for more papers by this authorW. J. OWEN, W. J. OWEN Agric. Prod. Res., Dow Chem. Co., Walnut Creek, CA 94598-0902, USASearch for more papers by this author J. SECOR, J. SECOR Agric. Prod. Res., Dow Chem. Co., Walnut Creek, CA 94598-0902, USASearch for more papers by this authorC. CSEKE, C. CSEKE Agric. Prod. Res., Dow Chem. Co., Walnut Creek, CA 94598-0902, USASearch for more papers by this authorW. J. OWEN, W. J. OWEN Agric. Prod. Res., Dow Chem. Co., Walnut Creek, CA 94598-0902, USASearch for more papers by this author First published: August 28, 1990 https://doi.org/10.1002/chin.199035354Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References J. SECOR, C. CSEKE, W. J. OWEN, Aryloxyphenoxypropanoate and Cyclohexanedione Herbicides. Inhibition of Acetyl Coenzyme A Carboxylase, ACS Symp. Ser., 1989, 389, 265–276. 10.1021/bk-1989-0389.ch018 CASWeb of Science®Google Scholar Volume21, Issue35August 28, 1990 ReferencesRelatedInformation

Acetyl-coenzyme A (CoA) carboxylase from maize (Zea mays L.) is inhibited by nanomolar concentrations of both haloxyfop, an aryloxyphenoxypropionate, and tralkoxydim, a cyclohexanedione herbicide. These results suggest that acetyl-CoA carboxylase, which catalyzes the first committed step in fatty acid biosynthesis, may be the target of these herbicides, contrary to an earlier report suggesting that aryloxyphenoxypropionate herbicides do not inhibit acetyl-CoA carboxylase.

Activities catalyzing the synthesis and degradation of fructose 2,6-bisphosphate-6-phosphofructo-2-kinase (ATP:D-fructose-6-phosphate-2-phosphotransferase, EC 2.7.1.105) and fructose-2,6-bisphosphatase (D-fructose-2,6-bisphosphate 2-phosphohydrolase, EC 3.1.3.46)-were isolated from spinach leaves by an improved procedure and separated on the basis of both charge and molecular weight. The separated activities showed no detectable cross-contamination, indicating, in contrast to all previous data, that they are not present on a single bifunctional protein of the classical type in liver. The fructose-2,6-bisphosphatase-a newly discovered phosphatase enzyme-differed from previous mixed preparations by showing greater specificity but lower affinity for fructose 2,6-bisphosphate, greater sensitivity to inhibition by inorganic phosphate, and in being sensitive to inhibition by Mg(2+). The 6-phosphofructo-2-kinase was found to be inhibited by low levels of inorganic pyrophosphate and, in addition, to be regulated by the metabolites described previously. Similar results were obtained with preparations from lettuce leaves. The results support the view that, through individual regulation of the activities catalyzing its synthesis and breakdown, cytosolic metabolites are key factors in controlling the fructose 2,6-bisphosphate content of leaves.

In photosynthetic eukaryotes, there are two well-characterized fructose-1,6-bisphosphatases (FBPases): the redox-insensitive cytosolic FBPase (cyFBPase), which participates in gluconeogenesis, and the redox-sensitive chloroplastic FBPase (cpFBPase1), which is a critical enzyme in the Calvin cycle. Recent studies have identified a new chloroplastic FBPase, cpFBPase2; however, its phylogenetic distribution, evolutionary origin, and physiological function remain unclear. In this study, we identified and characterized these three FBPase isoforms in diverse, representative photosynthetic lineages and analyzed their phylogeny. In contrast to previous hypotheses, we found that cpFBPase2 is ubiquitous in photosynthetic eukaryotes. Additionally, all cpFBPase2s from diverse lineages form a monophyly, suggesting cpFBPase2 is not a recently evolved enzyme restricted to land plants but rather evolved early in the evolution of photosynthetic organisms, and most likely, in the common ancestor of photosynthetic eukaryotes. cyFBPase was probably first duplicated to produce cpFBPase2, and then the latter duplicated to produce cpFBPase1. The ubiquitous coexistence of these two cpFBPases in chloroplasts is most likely the consequence of adaptation to different redox conditions of photosynthesis, especially those caused by recurrent changes in light conditions.

The regulation of sucrose breakdown and synthesis and the partitioning of carbon in sink tissues is not well understood. We present here a summary of what has been learned about these processes in both source and sink tissues and describe how our recent results add to our understanding of carbon metabolism in plant tissues.

TREATMENT OF CARROT ROOTS WITH ETHYLENE LED TO: (a) a doubling of the fructose-2,6-bisphosphate content; (b) a general increase in the concentration of glycolytic intermediates; and (c) an increase in the extractable activity of fructose-6-phosphate,2-kinase, the enzyme synthesizing fructose-2,6-bisphosphate from fructose-6-phosphate and adenosine triphosphate.

Fructose-6-phosphate 2-kinase and fructose-2,6-bisphosphatase have been partially purified from spinach leaves and their regulatory properties studied. Fructose-6-phosphate 2-kinase was activated by phosphate and fructose 6-phosphate, and inhibited by 3-phosphoglycerate and dihydroxyacetone phosphate. Fructose-2,6-bisphosphatase was inhibited by fructose 6-phosphate and phosphate. The interaction between these effectors was studied when they were varied, alone or in combination, over a range of concentrations representative of those in the cytosol of spinach leaf cells. In conditions when dihydroxyacetone phosphate or 3-phosphoglycerate rise, as is typical during photosynthesis, the fructose 2,6-bisphosphate level will decrease, which will favour sucrose synthesis. In conditions when fructose 6-phosphate accumulates, fructose 2,6-bisphosphate should rise, which will favour a restriction of sucrose synthesis and promotion of starch synthesis.

Metabolites known to occur in the cytosol of photosynthetic leaf cells were found to mediate the reversible conversion of pyrophosphate—D‐fructose‐6‐phosphate 1‐phosphotransferase (PFP) to phosphofructokinase (PFK) in partially purified preparations from spinach leaves. Preincubation of PFP with fructose 2,6‐bisphosphate, ATP or fructose 6‐phosphate converted PFP to PFK. The reverse reaction (PFK → PFP) was promoted by UDP‐glucose plus pyrophosphate. These conversions in catalytic capability were accompanied by changes in molecular mass and charge. The results are in accord with the view that the alterations in PFP and PFK activity, provisionally called ‘metabolite‐mediated catalyst conversion’, represent a regulatory mechanism to direct left cytosolic carbon flux in either the biosynthetic or degradatory direction.

Fructose 2,6-bisphosphate, a regulatory metabolite discovered in animal cells, plays a central role in regulating carbon metabolism in leaves.

An enzyme catalyzing the ATP and fructose 6‐phosphate‐dependent synthesis of fructose 2,6‐bisphosphate, a regulator of glycolysis and gluconeogenesis, has been identified and partially purified from plants, specifically the cytoplasmic fraction of spinach leaf parenchyma cells. The enzyme, designated fructose 6‐phosphate, 2‐kinase, showed no response to a protein phosphorylation system known to inhibit the corresponding enzyme in mammalian cells, but it responded strikingly to metabolite effectors (Pi, an activator/PGA, an inhibitor) through changes in substrate affinity and maximal velocity. The observed pattern of regulation suggests a role for chloroplasts in controlling cytoplasmic carbon processing.