130 Fundamentals of Auxin Action 



The effects of auxins on specific enzyme systems have been in- 

 vestigated in many instances. It is very difficult to evaluate the results 

 obtained from most enzyme studies in terms of the actual mechanism of 

 auxin action in the plant. Studies of the effects of auxins on enzymes 

 in a semi-purified state almost invariably demonstrate that the auxins 

 inhibit these enzymes (cf. review of Bonner and Bandurski, 1952). 

 The complexity of the problem is illustrated by the fact that auxin 

 applied to intact Airena coleoptiles evokes large increases in dehydro- 

 genase activity, whereas in vitro application of auxin to the partly 

 purified dehydrogenases only inhibits their activity (Berger and Avery, 

 1943). Eyster (1946) suggested that physical phenomena may confuse 

 the results of such studies. He has shown that alpha amylase is in- 

 hibited by many auxins but that the inhibition is largely removed 

 when the enzyme is suspended on colloidal particles. He was able to 

 show that, instead of inhibiting amylase activity, auxins in a colloidal 

 suspension could actually stimulate the enzyme, although the effect 

 of the auxins on the acidity of the mixture may have accounted for 

 a large part of the effect. These observations leave us with a certain 

 feeling of uneasiness about the physiological significance of demonstra- 

 tions of the inhibitions of semi-purified enzymes by auxins in vitro. 



The applications of auxins to whole plants or plant parts and the 

 subsequent determinations of enzyme activities may be a more reliable 

 source of information concerning the means by which auxins may alter 

 metabolism of the plant. The starch-hydrolyzing activity of bean stem 

 sections has been shown to be enormously increased by the presence of 

 small amounts of 2,4-D (Gall, 1948). A remarkable increase in ascorbic 

 acid oxidase activity of tobacco segments with auxin has been shown 

 by Newcomb (1951). He found that small quantities of auxin would 

 stimulate apparent ascorbic acid oxidase activity as much as 300 per 

 cent in a period of 6 to 10 days. Some of his data are shown in figure 

 59. Supplying the excised plant pieces with ascorbic acid substrate 

 and then measuring oxygen uptake revealed this large increase in 

 activity. One of the most interesting aspects of this phenomenon is 

 that the enzymatic response appears to precede the growth response. 

 This suggests the real possibility of a causal relationship. A variety of 

 other enzymes have been found to increase in activity after auxin treat- 

 ment, including pectinase (Neely et al, 1950), phosphorylase, ^-amyl- 

 ase, catalase (Wort and Cowie, 1953), and possibly proteinase and 

 polypeptidase (Freiburg, 1952). Many enzymes have been found to be 

 inhibited by auxin applications (see review of Bonner and Bandurski, 

 1952). 



Mitochondrial preparations containing all of the enzymes neces- 



