Consequences of administration of indoleacetic acid 



THE INDUCTION OF PEROXIDASE ACTIVITY BY lAA AND 



ITS MORPHOGENETIC SIGNIFICANCE 



Under certain conditions, it may be clearly shown that the peroxidase 

 activity of the tissue, like the lAA oxidase activity and peroxide-generating 

 capacity, rises in response to pretreatment with lAA. Dr. Siegel and I were 

 able to show this with the excised 5 mm apices of 2-day-old pea seedling roots 

 and also with young epicotyl tissue of week-old etiolated peas (Siegel and 

 Galston, 1953). These experiments are carried out by incubating the 

 appropriate tissue with ca. 10~" M lAA for ca. 2 hours, then homogenizing 

 the tissue and assaying the brei for peroxidase activity, using some appropriate 

 substrate such as pyrogallol. In our best experiments, we were able to show 

 about a 75 per cent increase in total peroxidase activity of pea root apices 

 [Figure 12). 



These experiments have been repeated and beautifully extended by 

 Dr. W. A. Jensen (1955) working in our laboratories. Dr. Jensen had 



WO 

 170 



^ 150 



Figure 12. The effect of lAA pre- 

 treatment on the peroxidase activity of 

 excised 5 mm apices of Pisum roots. 



110 



100 



- log mo/an f J I A/^ 



previously made a detailed anatomical analysis of the root of Vicia faba 

 and so chose to work with this organ. Using successive 200 /^ slices of the 

 root, he was able to assay each for total peroxidase activity by the use of 

 pyrogallol as a substrate. He was also able to carry out a histochemical 

 localization of the peroxidase activity in the various slices of the root by 

 employing as a substrate either benzidine or guaiacol, both of which produce 

 insoluble, coloured end products of peroxidation. By the judicious applica- 

 tion of both these techniques, he was able to determine which cells have 

 peroxidase activity and how much activity each cell type has. 



Dr. Jensen's results may be summarized as follows {Figures 13 and 14) : 



(a) The cells of the root cap are high in peroxidase activity, and this 

 activity is not altered by preincubation of the root with lAA. 



(b) The cells in the region of the general meristem are somewhat lower in 

 peroxidase activity, but are likewise insensitive to lAA, 



V 229 



16 



