Maize enzyme activity

Enzymes Involved in Starch Biosynthesis. Much of the eady data dealing with starch biosynthesis in plants are derived from the study of various mutants. The shrunken-2 and britde-2 mutants of maize have gready reduced levels of ADPGPP activity owing to the absence of one of the two subunits of this enzyme, and result in a shrunken seed appearance. Mendel s eady work on inheritance of traits was performed with a pea mutant deficient in branching enzyme activity (61). Mutations in plants affecting starch biosynthesis can have severe results to plant morphology and viability. 

Passera, C., Nicolao, L., Ferretti, M., Rascio, N., and Ghisi, R. (1991). Effect of humic substances of enzyme-activities of sulfate assimilation and chloroplast ultrastructure of maize leaves. Photosynthetica 25(1), 39-45. 

Figure 14.8. Thermograms for the volatilization of peptide-derived compounds in freeze-dried rhizodeposits leached from a soil cropped with maize after a daytime and a nighttime growth period and thermogram for the volatilization of L-glutamic acid. Reprinted from Leinweber, P., Jandl, G., Baum, C., Eckhardt, K.-U., and Kandeler, E. (2008). Stability and composition of soil organic matter control respiration and soil enzyme activities. Soil Biology and Biochemistry 40,1496-1505, with permission from Elsevier.

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. 

In C. reinhardtii, Ball and his collaborators (Mouille etal., 1996) generated seven independent alleles in the stal locus. All mutants lacked granular starch, but contained a water-soluble polysaccharide, similar to maize phytoglycogen, in an amount equivalent to 5% of the starch content of the wild type. This defect was associated with the disappearance of a specific debranching activity. All other starch-related enzyme activities were normal. 

Echeverria, E., Boyer, C. D., Thomas, P. A., Liu, K.-C., and Shannon, J. C. 1988. Enzyme activities associated with maize kernel amyloplasts. Plant Physiol. 86,786-792. 

Ou-Lee, T.-M., and Setter, T. 1985b. Enzyme activities of starch and sucrose pathways and growth of apical and basal maize kernels. Plant PhysioL 79, 848-851. 

It is likely that the regulatory mechanisms discussed in Section 5 apply to the regulation of CO2 fixation in C4 plants. In particular it is known that the ferre-doxin/thioredoxin system of light-linked enzyme activation (see Section 5.2.2) is present in C4 plants. NADP-malate dehydrogenase, FBPase and SBPase from maize leaves are regulated in this way [33]. 

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... 

There is evidence from several systems that a tissue s capacity to convert ABA to PA is induced by ABA itself [171-174]. In cultured maize cells, induction was blocked by cordycepin and cycloheximide, indicating that increased enzyme activity resulted from increased expression of the gene encoding ABA 8 -hydroxylase. After induction, the 8 -hydroxylase was rapidly degraded with a half-life of approximately two hours [172]. 

Figure 6. Increased extractable PAL in maize roots caused by root-feeding of glyphosate to intact plants (39). Dark-grown, 3-day-old maize seedlings were transferred to 1 mM glyphosate (%), or water (O) and enzyme activity was monitored over a 3-day time course during dark growth.

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