The influence of growth substances upon sulphydryl compounds 



DISCUSSION 



In a re-examination of the possibiHty that auxin may form thio-esters with 

 CoA, the experiments reported here cast some doubt on the possibility. The 

 disappearance of sulphydryl groups in the presence of auxins may not be due 

 to the formation of thio-esters. Direct examination of the amount of ester 

 formation in the presence and absence of naphthalene acetic acid failed to 

 show any increase in ester formation due to the auxin. For the present, then, 

 we do not feel that the theory of Leopold and Guernsey (1953) is adequate. 



Looking further into the possibility that growth regulators may alter 

 sulphydryl reactivity, it has been found that TIBA, coumarin, and 2:4- 

 dichloroanisole may react non-enzymatically with the sulphydryls of cysteine, 

 a reactivity which may account in part for their activity as growth substances. 



With regard to the action of iodoacetate, Thimann and Bonner (1948) 

 interpreted the inhibition of auxin-induced growth by iodoacetate as indicat- 

 ing that there may be a growth enzyme requiring a free sulphydryl group for 

 its activity, and that iodoacetate may thus be blocking auxin action in a 

 fairly specific way. That iodoacetate may compete with auxin in some fairly- 

 close way is suggested further by the report of Reinhold (1954) that iodo- 

 acetate competes with indoleacetic acid for entrance into pea stem sections. 

 Both of these instances, however, may be somewhat clouded by the fact that 

 iodoacetate is affecting respiration itself, which may then alter the indole- 

 acetic acid function. 



Turning next to TIBA, this compound was early suggested to have pro- 

 perties antagonistic to auxin (Galston, 1947) and has since been shown to 

 reverse the auxin inhibition of root growth by auxin (Minarik et al., 1951), 

 and to be reversed in its effects on abscission by auxin (Weintraub et al., 1952). 

 Its synergistic action with auxin in growth led Thimann and Bonner (1948) 

 to suggest that it was sufficiently similar to auxins that it might be competing 

 with auxins for the same sites. With respect to its spontaneous reactivity 

 toward sulphydryl groups, TIBA shows strong reactivity where the auxins 

 show very little. 



The ability of coumarin to react with sulphydryl groups is in direct agree- 

 ment with the early suggestion of Veldstra and Havinga (1945) that lactones 

 of this sort may be sulphydryl inactivators andauxin antagonists. The ability 

 of BAL (1 :2-dithiopropanol) to prevent the coumarin inhibition of growth 

 (Thimann and Bonner, 1949) is further support for their suggestion. The 

 synergism of coumarin and auxin in growth and in seed germination 

 (Thimann and Bonner, 1949; Mayer and Evanari, 1951) has opened the 

 rather tenuous possibility that coumarin, too, may compete with auxins for 

 the same site of action. Some new and interesting implications concerning 

 coumarin have come recently from the work of Libbert (1955) who provides 

 evidence that the lactones may be intimately involved in the ability of 

 auxins to inhibit growth, particularly in relation to apical dominance. The 

 work of Hemberg (1949, 1950) suggests too that growth inhibitors may be 

 responsible for the bud inhibition involved in dormancy, and their inhibitions 

 may be naturally relieved by the biosynthesis of glutathione. Von Guttenberg 

 and Meinle (1954) provide some indirect evidence that the dormancy 

 factors may be lactone type substances. 



V 281 



