Maize enzymes

The variety of aldehyde oxidases discovered in other plants have similarities to the maize enzyme, but also have some very important differences. Enzymes contained in a cell wall fraction from barley seedlings were able to oxidize IAAld to form IAA at a pH optimum of 7 and Km of 5 pmol 1 1, which was very similar to the enzyme found in maize.113 Two aldehyde oxidases from potato have also been identified 101 they had a similar pH optimum (between 7 and 8), but preferred aliphatic aldehydes to aromatic aldehydes. Although oat and cucumber aldehyde oxidases have been shown to oxidize IAAld to produce IAA,102 114 the oat enzyme had a lower pH optimum and higher Km than the maize enzyme, and the cucumber enzyme was inhibited by synthetic auxin and activated by 2-mercaptoethanol, which was not true for the maize enzyme. The difference in the enzymes makes it difficult to envision a common evolutionary origin for the IAAld pathway in plants if these particular enzymes are involved in each case. 

Since the CKX activity has been discovered more than 30 years ago, numerous inhibitors of the enzyme were reported. Synthetic derivatives of diphenylurea, e.g, 7V-(2-chloro-4-pyridyl)-iV-phenylurea and 7V-phenyl-7V-l,2,3-thidiazol-5-urea, are strong CKX inhibitors [166-168], In the presence of oxygen as the only electron acceptor, the inhibitions were described as noncompetitive and uncompetitive, respectively. However, for the maize enzyme ZmCKXI and with 2,6-dichlorophenol indophenol as the electron acceptor, the inhibition appears to be competitive towards isopentenyladenine [142] and uncompetitive towards 2,6-dichlorophenol... 

Some but not all of the maize enzymes are encoded by small gene families. It will be critical to dissect the role of gene family members in contributing to tissue-... 

RR corn was commercialized in 1998. Resistance was achieved via expression of a glyphosate insensitive TIPS-EPSPS, which is a maize enzyme with two amino acid mutations that conferred glyphosate insensitivity (see Chapter 6.2, Section 6.2.2). A second-generation RR com trait with CP4-EPSPS showed improved glyphosate resistance and was introduced in 2001. Since its introduction in 1998, RR com has grown to 31% of the total corn acres in production for 2005 (Fig. 6.1.4). 

Several plant aspartate kinases are activated by other amino acids such as valine, alanine, and isoleucine (Table HI). Activation does not appear to be a general effect of hydrophobic amino acids, since neither methionine nor leucine influence the activity of the maize enzyme yet, leucine has been reported to activate the enzyme isolated from Sinapsis alba and inhibit the enzyme from Helianthus annus. Interaction of these secondary effectors with the various aspartate kinases has not been fully explored, but several observations suggest a considerable degree of complexity. Alanine partially relieves threonine inhibition of the enzyme isolated from pea seedlings (Aames and Rognes, 1974). Lysine inhibition of maize aspartokinase is diminished in the presence of isoleucine, alanine, or valine (Bryan e/ al., 1970) and threonine, even though it does not inhibit the maize enzyme, counteracts... 

Aspartate and norvaline have been reported to activate several plant threonine dehydratases (Kagan et al., 1969a Bleckman et al., 1971). Low concentrations of aspartate (1-5 mM) do not, however, affect the activity of the maize enzyme under any of a number of assay conditions that have been tested (E. Lissik and J. Bryan, unpublished). Although a role of aspartate as a feedforward pathway activator would be interesting, it has not been established that this amino acid is an important effector of any plant threonine dehydratase in vivo. 

Purification of the maize carboxylase, enzymatic assays, preparation of the small subunit as well as cleavage and sequencing of this polypeptide were performed as described recently (11). The components of the heterogeneous moiety of the small subunit of the maize carboxylase were resolved by two-dimensional gel electrophoresis using the O Farrell technique (12). Circular dichroism spectra of the spinach and maize Rubisco were recorded using a Jasco J-20 spectropolarimeter. Small angle X-ray scattering of the maize enzyme was measured as reported in (3,4). 

The enzyme which isomerizes cycloeucalenol (8-F) to obtusifoliol (9-F), that is, opens the cyclopropane ring, has been demonstrated in cell-free preparations from tissue cultures of bramble (Heintz and Benveniste, 1974) and microsomes of Zea mays embryos the maize enzyme is specific for cycloeucalenol (9-F) (Rahier et al., 1976, 1977). The fact that neither cycloar-... 

In plants, acetyl-CoA carboxylase levels seem to play an important role in the overall rates of fatty acid synthesis de novo. The purified enzyme is unaffected by citrate or a whole range of organic acids. There is evidence that in vivo it may be controlled by a combination of factors. Thus, the change in chloroplast pH, Mg ", ATP and ADP levels which occur during illumination have been shown by Hawke to cause an increase of about 24-fold in the activity of the maize enzyme in vitro. This is almost exactly the same as the observed increase in fatty acid synthesis in vivo when leaves are illuminated. 

---