Dwarfing Genes in Zea mays and Relation to Gibberellins 499 



the dwarf habit of growth through the control of the amount of 

 native gibberellins in Zea mays. It is suggested that these genes in- 

 terfere with different steps in a gibberellin pathway leading to a 

 product necessary for the normal growth of Zea mays. 



The normal type growth response of the five mutants to the gib- 

 berellins, combined with their lack of response to other known plant 

 growth regulators, implicates gibberellin as the limiting factor re- 

 sponsible for the dwarf habit of growth. The correlation of presence 

 of gibberellin-like substances in normal plants, and the lowered 

 amounts or absence in the mutants are further evidence for a causal 

 relation between gibberellin and the dwarf habit of growth. The rela- 

 tively small response to added gibberellin found for normal plants 

 would be expected if native gibberellins were less limiting in normal 

 plants than in the dwarf mutants. 



The response of all five mutants to gibberellic acid suggests that 

 GA3 or a compound very similar to it occupies a position in a meta- 

 bolic pathway in Zea mays subsequent to the steps controlled by the 

 five dwarfing genes. While the gibberellins GAi, GA2, and GA3 ex- 

 hibit the order of activity of GA3 > GA^ > GAo, this order is the 

 same for all five mutants and for normal plants. Such similarities in 

 relative activities would exclude any interrelationship between these 

 gibberellins controlled by the five dwarfing genes. The similarity in 

 the relatively high activities of GA3 and BF-II on the four mutants 

 do, ^3, d^, and an^, contrasted with the high activity of GA3 and the 

 low activity of BF-II for the mutant d^, suggests a relationship between 

 these two gibberellins controlled by the d-^ gene; bean factor II would 

 be an intermediate the conversion of which to gibberellic acid is 

 blocked by the d-^ gene. The other four genes could then control steps 

 in the gibberellin pathway prior to compounds having properties 

 similar to GA3 and BF-II. Further support of this interpretation is 

 given by the lack of a differential response of normal seedlings to 

 GA3 and BF-II. 



The difference in auxin level between normal and dwarf seedlings 

 of Zea mays can be attributed to an indirect effect of the dwarfing 

 genes. A lower auxin level would then be the result of the reduced 

 amounts of native gibberellins in the mutants. The lack of response 

 of the dwarf mutants of Zea mays to a number of auxins is in agree- 

 ment with this interpretation. 



The accumulation of inhibitors of gibberellin-induced growth 

 could be an obvious alternate explanation for the dwarf habit of 

 growth in Zea mays. Added gibberellin would then be expected to 

 overcome this inhibition effect, resulting in a normal growth response 

 for the GAg-responding mutants. Such an explanation would require 

 a series of inhibitors, one specific for gibberellic acid, another specific 



