318 /. Bonner 



wall chemistry. Such alterations have been sought since the beginnings 

 of auxinology. Only in recent years, however, and after the advent 

 of appropriate methodology have they been foiuid. 



"With such methodology it has been shown that the application 

 of lAA to Avena coleoptile sections results in an increased rate of 

 pectic synthesis by the tissue. That lAA influences pectic metabolism 

 was first detected by the use of C^^ inethyl-labeled methionine. The 

 methyl gioup of methionine serves as donor of the methyl ester groups 

 of pectic substances (Ordin et al., 21; Sato, 23). The application of 

 lAA to coleoptile sections in the presence of C^^ methyl-labeled me- 

 thionine (or of other methyl donors, as formaldehyde), increases the 

 rate of appearance of labeled pectic methyl ester groups (Ordin et 

 al., 21, 22; Jansen et al., 16). It was subsequently found that the in- 

 creased rate of pectic methyl ester formation in the presence of lAA 

 is paralleled by increased rate of polygalacturonic acid formation (Al- 

 bersheim and Bonner, 1). lAA therefore increases the rate of forma- 

 tion of pectic material. The effect of lAA upon pectic synthesis pos- 

 sesses all of the earmarks of an authentic auxin-controlled reaction, 

 with characteristics similar to those of lAA-induced gro^v•th. Thus the 

 effect of lAA upon rate of pectic synthesis is a rapid one, manifest 

 within 15 to 30 min. after application of the auxin. It is inhibited by 

 antiauxins such as 2,4,6-trichlorophenoxyacetic acid. It takes place 

 only under aerobic conditions (Cleland, 10). It is inhibited, as is 

 growth, by ethionine. It occurs in sections which are restrained from 

 growing by the presence of isotonic mannitol solution, a condition 

 under which auxin-induced cell wall plasticization continues to oc- 

 cur. It woidd appear therefore that lAA-induced cell wall plasticiza- 

 tion and lAA-induced alteration in pectin synthesis are similar and 

 possibly identical reactions, although there is as yet no rigorous proof 

 that this is so. 



Further analysis of the effects of lAA on pectic metabolism re- 

 quires a knowledge that the pectic material of Avena coleoptile sec- 

 tions includes three principal forms (Jansen et ah, 16). The first is the 

 so-called hot water soluble pectin (extracted from the cell wall by 

 hot water). This fraction is, as summarized in Table 3, highly esteri- 

 fied. A second pectic fraction is the cold water soluble, 70 per cent al- 

 cohol precipitatable material. This fraction, which constitutes ap- 

 proximately 5 per cent of the total pectic substance, is presumably 

 removed from the cell wall during the preparation of the latter but 

 can be recovered from the cold water washings by alcohol precipita- 

 tion. The third pectic fraction — that which remains in tiie wall 

 after cold and hot water washing— is known as the residual pectin. 



