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Two Effects of Cytokinin on the Auxin Requirement of Tobacco Callus Cultures ¹

^a Institute of Plant Development, University of Wisconsin, Madison, Wisconsin 53706

Cytokinin affects the requirement for auxin of a strain of tobacco callus (Nicotiana tabacum) which is cytokinin-autotrophic when grown on Murashige and Skoog medium with 11.4 µM of indole-3-acetic acid but requires cytokinin 6-(3-methyl-2-butenylamino)purine (i⁶ Ade) when grown on the same medium with <3 µM indole-3-acetic acid. As the exogenous concentration of cytokinin (i⁶ Ade) is increased, the concentration of indole-3-acetic acid required for growth is decreased. A second effect of cytokinin, observed sporadically in cultures with 2.5 µM or 5 µM i⁶ Ade, is the transformation of some of the callus pieces to auxin-autotrophic growth. Strains, both callus-forming and bud-forming tissues, that arise in this manner are not permanently altered in their auxin requirement because subcultures on medium without cytokinin still require exogenous auxin.