smith: directed mutation 429 



that 2-aminopurine, or at least that part with which the tritium 

 label remains, is incorporated into the chromosomes and cytoplasm. 

 It has been reported recently (68) that only a small quantity of 

 unchanged 2-aminopurine is incorporated into DNA and RNA of 

 E. coli, and that a large proportion is transformed into adenine and 

 guanine. 



Striking differences in growth of Arabidopsis thaliana can be 

 induced by incorporation of 2-aminopurine and/or 5-iododeoxyu- 

 ridine in the medium on which this plant can be cultured. Threshold 

 concentrations which barely permit the plants to mature are 0.0008 

 M 2-aminopurine, 0.00005 M 5-iododeoxyuridine, and a combina- 

 tion of 0.0004 M of 2-aminopurine with 0.00005 M of 5-iododeoxy- 

 uridine. There has not yet been time to test for induced mutations 

 in the progeny of plants so treated. 



In a symposium on "Mutation and Plant Breeding" it is appro- 

 priate to make reference to the use of directed mutation. This has 

 been an objective of plant breeders since artificial induction of muta- 

 tions became possible. Today we are able, by a choice of mutagenic 

 agents, to exercise some selectivity in differentially inducing chromo- 

 some breaks compared to gene mutations. This accomplishment is 

 recent and there are many ramifications to explore in theory and to 

 exploit in practice. Applications can be made in spite of our lack of 

 understanding of the phenomena involved. 



An ultimate objective would seem to be able to induce one 

 particular mutation to the exclusion of all others. However, a more 

 legitimate objective on the basis of present working hypotheses about 

 the structures that store and transmit genetic information, i.e., se- 

 quences of a four (or fewer) word code, is limited control of the 

 spectra of mutations rather than a complete "all-or-none" direction 

 of specific mutations. As we have seen, there is limited evidence that 

 with higher plants this can be accomplished in broad spectra experi- 

 ments by using a variety of mutagens. The theoretical basis for 

 specificity in forward mutations among gene loci remains obscure, 

 and it will be difficult to devise definite experiments of sufficient 

 resolving power with higher plants. The best material for experi- 

 mentation would appear at present to be Neurospora where refined 

 tests can be applied to a plant with a hereditary apparatus of appar- 

 ently the same level of organization as green plants. Experiments 



