Consequences of administration of indoleacetic acid 



(c) The cells in the region of elongation and early differentiation have a 

 low level of peroxidase activity which may be dramatically increased by 

 preincubation of the tissue with ca. 10~" M lAA. Specifically, in these 

 regions it is the potentially lignified cells of the xylem and phloem which 

 show the marked increase in peroxidase activity, while cortex, epidermis, 

 and pith show essentially no peroxidase activity either before or after 

 treatment with lAA. 



These results conform well with other recent discoveries. It has been 

 demonstrated (Torrey, 1953) that administration of I AA to pea roots results 

 in premature lignification, i.e. the occurrence of lignified cells closer to the 

 root apex. It is also clear that lAA may promote the differentiation of 

 xylem strands in a wounded stem of Coletis (Jacobs, 1952). 



7-^n 



=5: Cofffro/ 10'^ 10'^ 10''^ 

 lAA Concentration 



Figure 14. The effect of various lAA pretreatments on the subsequent peroxidase activity of successive 

 sections o/Vicia roots. Note the marked effect of lAA on activity in the posterior sections, and the shift of 

 the optimum from young to older tissues. 



How may the induced formation of peroxidase explain the lignogenic 

 action of lAA? The answer is suggested by the work of Siegel (1955), who 

 has demonstrated clearly that lignin is synthesized in certain plant cells by 

 the action of peroxidase and HgOg on hydroxyphenylpropane precursors such 

 as eugenol. Fortified by this knowledge, Jensen was able to show, in his 

 Vicia roots, that the same cells which are induced to higher peroxidase 

 activity by lAA, have, after such induction, a greatly increased ability to 

 convert eugenol to a lignin-like compound. Thus, it appears that the morpho- 

 genetic action of lAA in promoting lignification is mediated by the induced formation, 

 in certain cells, of a lignin-synthesi zing peroxidase. 



Recently, Dr. I. B. Perils, also working in our laboratories, has been making 

 a study of the peroxidases of the etiolated pea seedling. He has found, as 

 have others (Jermyn and Thomas, 1954; Kondo and Morita, 1952), that if 

 one subjects the brei to chromatography or to paper electrophoresis, several 

 (probably five) distinct peroxidases appear. These peroxidases differ not 

 only in chromatographic and electrophoretic mobility, but also in substrate 



231 



