490 B. O. Phinney 



paper to evaluate certain evidence which leads to an interpretation 

 of the physiological action of the dwarfing genes d^, d^, d.^, dr^, and 

 an I of Zea mays. 



Both auxins and gibberellins-' have been studied in their relation 

 to the growth of several of the dwarf mutants of Zea mays. AVhile the 

 auxin level from certain of the dwarfs can be shown to be less than 

 that from normals, these mutants, as well as other dwarfs of Zea mays, 

 exhibit no growth response to added indole-auxins such as lAA, 

 lAEE, IAN, IBA, or to the auxin NAA, or to kinetin and numerous 

 individual amino acids (5, 7, 9, 14, 15). In contrast, the five mutants 

 f/j, ^2' ^3» ^5» ^nd rt/ij respond by normal growth to microgram 

 amounts of the gibberellin GA3 (8, 9, 13) (Figures 1 and 2). It has been 

 suggested that these GAg-responding mutants might be controlling 

 different steps in a biochemical pathway leading to the production 

 of a naturally occurring gibberellin which is similar to GA3 and 

 necessary for the normal growth of Zea mays (10, 11). This native gib- 

 berellin would then be limiting in the mutants and its absence or 

 presence in limiting amounts responsible for the d^varf habit of 

 growth. Five other mutants, Jia^, na^, dg, pe^, and m,i2, give no growth 

 response, or only a slight growth response in the early seedling 

 stages, to added GA.^. It has been suggested that these five nonre- 

 sponders could have blocks in a pathway stibsequent to a CrAg-like 

 compound; or they could be due to blocks in a biochemical pathway 

 or pathways unrelated to the gibberellins (10, 11). 



If the interpretation of the GAg-responding mutants is correct, 

 it is possible to make certain predictions and subject them to experi- 

 mental tests. Some of the predictions that will be considered are as 

 follows: 



(1) It should be j)ossible to find gibberellins producing differen- 

 tial growth responses for the five GAg-responding mutants. Compounds 

 from different steps in the presumed gibberellin pathway should 

 either be active or inactive for a particular mutant, depending on the 

 position of the mutant block in this pathway. Thus, there should be 

 some gibberellins that produce a normal growth response to four of 

 the five GAg-responding mutants; others that produce a growth re- 



^ Auxin will refer lo any native growth regulator found to be active in the 

 Ax)ena curvature test or the Avena straight growth test. The term gibberellin is 

 used for substances active in the d^, d.,, d^, d^„ or aii^ bioassay of Zea mays. As used 

 here, the term is further restrictecl to substances found to contain a fluorene ring 

 system. The term gibbryrlUn-lilie is used for substances ha\ing i)iological proper- 

 ties similar to the gibberellins but for which the chemical properties are unknown. 

 The abbreviations used for certain growth factors considered in this paper are: 

 lAA, indole-3-acetic acid; IAN, indolc-3-acetonitrile; lAEE, ethvl indole-3-ace- 

 tate; NAA, 2-naphthoxyacetic acid: IRA, 4-(indole-.'?-)-)/-butyric acid: GA„ GAo, 

 GA,, and GA^, gibberellin A,, A.„ A.„ and A,: and BF-II, the gibberellin, bean fac- 

 tor II. 



