Adaptation of Pea Roots to Auxins and Homologues 125 



oxidase after preincubation of intact tissue with indoleacetic acid. 

 This, as he pointed out, could possibly be due to the induced forma- 

 tion of an enzyme as we had originally suggested. It could also be 

 due to an alteration of the cofactors which are known to be required 

 for the operation of this enzyme. 



One of my graduate students, Mr. Masaki Furuya, has carried out 

 a few experiments which have convinced us that the so-called induc- 

 tive or adaptive changes can be demonstrated very easily in vitro 

 under conditions in which protein synthesis is impossible. Thus, we 

 now think that at least part of the adaptive changes are due to 

 changes in the cofactors and not in the enzyme itself. 



As you know, there are two kinds of cofactors for the lAA-oxidase 

 of peas. One is the manganous ion. If an etiolated pea homogenate 

 containing lAA-oxidase activity is pre-incubated with ca. 10"^ Mn+^ 

 for various periods of time prior to the addition of the lAA sub- 

 strate, the activity of the enzyme will be found to rise markedly with 

 time. These changes leading to enhanced activity are temperature 

 and oxygen sensitive. We have evidence that they are due to destruc- 

 tion of an inhibitor of the lAA-oxidase contained in the homogenate. 



The second cofactor is a yet unknown material the action of 

 which can be imitated by 2,4-dichlorophenol (DCP). If the lAA- 

 oxidase is incubated with 10 ^ M DCP, the enzymatic activity be- 

 comes successively lower and lower with increasing pre-incubation 

 time. This inhibitory effect of pre-incubation with DCP is reversible 

 by a subsequent incubation with Mn++. If both cofactors are added 

 simultaneously to the pre-incubation mixture, the first effect noted 

 is the inhibitory action of DCP, which is later reversed by Mn+^ 

 These results lead us to the belief that at least part of the induced 

 changes in lAA-oxidase activity noted by Galston and Dalberg and 

 also by Audus are not true enzymatic inductions, but are, rather, due 

 to changes in the cofactor levels for the enzyme. 



Dr. Audus: I don't know how this affects Dr. Galston's ideas, but 

 all these observations of ours were made in the presence of DCP 

 added to the breis. 



Dr. Burstrom: You said that you measured the elongation from 

 the start of the elongation of the cells. I perhaps didn't quite see 

 the figures on your diagrams but I got the impression that you had 

 the intial cell lengths at apical ends of your sections of about 40 

 microns. Is that so? 



Dr. Audus: No; they are 10 microns long. The segment included 

 about 75 per cent of cells which hadn't started to elongate at the time 

 of excision. 



Dr. Burstrom: I agree with you completely that we must follow 

 the progress of elongation more carefully than is usually done. I 



