A considerable amount of attention has been given to both the bio chemistry and physiology of auxin destruction in plants since the review of this topic by Gordon (54) in 1954 ; the present review deals largely with the progress in this field since Gordon's review. The greatest amount of reCent work has concerned the oxygen-consuming enzymes which destroy indole-3-acetic acid (IAA), and most of the discussion will necessarily be devoted to this aspect of the subject. An effort will also be made to survey other destruction mechanisms which are known, both for IAA and for other auxins. Finally, the relation between these processes and the physi ology of plants will be considered.
CHEMISTRY OF IAA DESTRUCTIONPhotooxidation.-Brauner (17) followed riboflavin-sensitized photooxi dation of IAA simultaneously with (a) a H elianthus curvature test, (b) the Salkowski color reaction, (c) the change in pH, which accompanies decarboxyla tion, and (d) the Hopkins-Cole color reaction for the indole nucleus. He found that the changes in different assays did not run parallel, and interpreted the results as indicating that IAA photodestruction was occurring in three steps, the first converting IAA to a physiologically in active, Salkowski-positive acid, the second involving decarboxylation and loss of Salkowski reactivity, and the third forming a product of weak Hopkins-Cole reactivity. This conclusion might be partly questioned, be cause the bioassay showed evidence of being affected by riboflavin, and by illumination of the solutions, in such a way as to make calibration with pure IAA unreliable. However, other evidence also indicates the existence of II. sequence of reactions in photooxidation.Fischer (40) reported the detection, by chromatography and electro phoresis, of indole-3-carboxaldehyde (hereafter called indolealdehyde ) as a production of both ultraviolet and of riboflavin-sensitized photooxidation of IAA. From products of the latter reaction it was isolated by chromatog raphy and identified chemically. Another substance "E," having an RF between that of IAA and that of indolealdehyde, was also detected with the nitrite reagent (similar to the Salkowski reaction); it was isolated and reported to be identical with indole-3-glycolic acid. Fischer (40) ac-'The survey of literature pertaining to this review was concluded in October, 1957.• The following abbreviations will be used: DPNH, diphosphopyridine nucleo tide (reduced form); IAA, indole-3-acetic acid; IAN, indole-3-acetonitrile; 2,4-D, 2,4-dichlorophenoxyacetic acid. 81 Annu. Rev. Plant. Physiol. 1958.9:81-118. Downloaded from www.annualreviews.org by NORTH CAROLINA STATE UNIVERSITY on 09/25/12. For personal use only. Quick links to online content Further ANNUAL REVIEWS