CELL WALL DYNAMICS 55 



of the biosynthetic reaction. The relation between 3-indoleacetic 

 acid (TAA) as a hormone and lignin synthesis is of paramount 

 importance. 



The effects of IAA are, however, far from simple and unidirec- 

 tional. Thus, IAA induces synthesis of the enzyme peroxidase 

 (as part of the IAA-oxidase system) and can stimulate an abnormal 

 pattern of vascularization in pine wood — formation of highly 

 lignified tracheids. IAA also stimulates xylem regeneration and 

 differentiation in the Coleus stem. On the other hand, in experi- 

 mental systems where peroxidase is abundant and not subject to 

 increase, the effect of IAA is that of a powerful inhibitor of lignin 

 synthesis. This inhibitory property relates to the peroxidative 

 reaction itself and is a sensitive reflection of the newly discovered 

 antioxidant character of IAA. 



How may these apparently conflicting views be reconciled? 

 One answer to this question depends upon a broader view of the 

 relation between IAA, peroxidase, growth, and lignification. 

 Thus, it has been proposed that the IAA-induced formation of 

 peroxidase would in itself lead to increased oxidizing power. The 

 relatively non-specific character of peroxidase would in turn 

 lead to increased and indiscriminate oxidation of essential meta- 

 bolites, enzymes, and structural elements of the protoplast. For 

 example, catalytically important protein tyrosyl and sulfhydryl 

 groups, free phenols, ascorbic acid and a host of cellular constitu- 

 ents would be oxidized to the detriment of their biological functions. 

 Such changes accompanying lignin formation would lead altogether 

 to cellular dysfunction and senescence, perhaps eventually to 

 the dead, fully lignified cell. As an antioxidant and growth promoter. 

 IAA may suppress temporarily these potentially heightened destruc- 

 tive processes, thus extending the period of active growth. Eventually, 

 the peroxidizing power of the tissues must destroy the IAA itself 

 (as part of IAA-oxidase), removing the constraint upon wide- 

 spectrum oxidation. The net effect would then be one of enhanced 

 lignification displaced in time. This concept is supported by the 

 variety of growth promoters which as antioxidants inhibit lignin 

 synthesis in model systems and the growth-inhibiting oxidants 

 which hasten formation of the lignin polymer: 



