VI 



CELL ENLARGEMENT 



783 



itors to be studied, and indeed one of the most effective, was iodoacetate. When 

 added to Avena coleoptiles in a simple growth solution iodoacetate can inhibit 

 growth down almost to zero. The concentration causing 50% inhibition is about 

 ^■lo'^M. This concentration has almost no effect on the respiration, so that the 

 inhibition is not due to general damage but is specifically exerted on the elongation 

 process. The response depends to some extent on the age of the coleoptiles and 

 the pH of the solution. Strikingly, lower concentrations clearly promote growth, 

 and below io"^A/ promotions of 50% or so can be observed, though so great a 

 promotion is not always easily duplicated, and values obtained by other workers 

 are commonly not over 30%. 



Pea stem and potato tuber sections show a similar inhibition though with little 

 or no promotion at low concentrations. Even Nenrospora cultures respond similarly 

 and the absolute iodoacetate levels effective are almost the same as in higher 

 plant tissues (Ryan et al., 1944). lodoacetamide has effects similar to those of the 

 free acid. 



The promotion of coleoptile growth by iodoacetate at io~^M is completely 

 prevented by lipoic acid (6,8-dithio-?<-octanoic acid) at about 5-io'^M (Slocum 

 and Little, 1956) but not by related dithio- or monothio- compounds. Similarly 

 the inhibition by higher iodoacetate levels could be prevented by lipoic acid. 

 Since lipoic acid is an intermediate in the oxidation of pyruvate, this relationship 

 is interpreted in terms of a balance between two paths for pyruvate — to be oxidized, 

 yielding energy for growth, or to be converted to sugars, yielding material for 

 growth. 



Organic mercurials, especially phenylmercuric salts and parachloromercuri- 

 benzoic acid, which are also powerful -SH reagents, inhibit cell enlargement in 

 even lower concentrations than iodoacetate. They act on all tissues which have 

 been investigated, and at nearly the same absolute concentration levels. Here 

 also there is promotion at low concentrations, at least with coleoptiles (see Fig. 9) . 



Arsenite is a highly effective inhibitor which has been used a good deal in com- 



Fig. g. Examples of the ac- 

 tion of enzyme inhibitors on 

 cell enlargement in auxin 

 solutions. A. Avena coleop- 

 tile sections cut from plants 

 of four different ages (in h.) 

 in j&-chloromercuribenzoa- 

 te. Arithmetical plot. Note 

 clear growth promotion at 

 low concentrations (the ex- 

 tent of which varies slightly 

 with age). From Thimann 

 and Bonner, 1949. 



140 



^100 



I 



o 

 01 80 



60- 



10 



Literature p. Si 6 



