762 PLANT GROWTH 10 



optile sections, it is almost completely inactive on pea sections under the same 

 conditions (Bentley and Bickle, 1952). On other test plants, notably lupine hypo- 

 cotyls and corn coleoptiles, it shows only a low activity which is not proportional 

 to concentration (Thimann, 19543). This behavior has been taken to indicate that 

 it is active only through conversion to indoleacetic acid, a process which would 

 occur rapidly in Avena, slowly or to a strictly limited extent in other plants, and 

 not at all in peas. The conversion in Avena has been proved by both chromato- 

 graphic means and bioassay (Stowe and Thimann, 1954). It remains to be deter- 

 mined whether III has any true auxin activity of its own. 



In addition to the above, indolepyruvic acid, IV, has been reported by chro- 

 matography and bioassay in corn (Stowe and Thimann, 1954). Similar materials 

 are present in soybean and tobacco leaves (Vlitos and Meudt, 1953-4) wheat roots 

 and broad bean seedlings. Unfortunately, the pure substance is quite unstable, 

 being rapidly converted in aqueous solution to a mixture of substances including 

 indoleacetic acid. Whether it has true auxin activity of its own is thus, just as in 

 the case of indoleacetonitrile, difficult to determine. Recently it has been shown 

 that the chromatographic spot given by pure indolepyruvic acid is actually due 

 to its conversion to indoleglycollic acid, V, and other compounds so that the 

 materialinplantsmay really be indoleglycollic acid (Bentley etal., 1956). However, 

 since the biological activity of this substance is extremely low, not over 0.3% of 

 that of indoleacetic acid, it is not certain whether this interpretation can be con- 

 sistent with all the facts. Apart from this, V has not been reported in plants, 

 though it was tentatively identified among the products of the breakdown of in- 

 doleacetic acid by light (Fischer, 1954). Evidence for other naturally occurring 

 auxins, with and without the indole nucleus, has recently been reviewed by 

 Bendey (1958). 



The special significance of indolepyruvic acid would be, of course, as an inter- 

 mediate in the conversion of tryptophan, VI, to auxin. This reaction, as carried 



CH2COCOOH f^^^^^ n— CHCOOH f=^^^^^ n-CHjCHCOGH 



OH 



Indolepyruvic acid Indoleglycollic acid Tryptophan 



IV V VI 



out by bacteria (Stowe, 1955) comprises two steps: (a) transamination to indo- 

 lepyruvic acid, using a-ketoglutaric or other a-keto acids (but not pyruvic acid), 

 together with pyridoxal-5-phosphate as co-enzyme, and (b) oxidation (or con- 

 ceivably dismutation) of the resulting indolepyruvic to indoleacetic acid and COj, 

 a reaction which proceeds spontaneously to some extent and is probably acceler- 

 ated by systems in the plant. The ready conversion of tryptophan to lAA by 

 several bacteria and fungi, by spinach and pineapple leaves (see review of Gordon, 

 1954) and by pollen tubes (Lund, 1956) may proceed in this way or by direct 

 oxidative deamination. However, some higher plants do not form an active auxin 

 from tryptophan very readily. Avena coleoptiles show a delayed growth response 

 to tryptamine, VII, which has been interpreted as due to its conversion to lAA. 



