In many plant species, including maize, stable variants have appeared after a cycle of tissue culture. However, few of these somaclonal variants have been obtained at defined loci which are amenable to molecular analysis that would shed some light on the mechanisms responsible for somaclonal variation. The maize alcohol dehydrogenase genes have been studied at a molecular level, and this report describes the isolation and preliminary characterisation of a tissue-culture-derived Adh1 mutant.

Plants were regenerated from cultures that were initiated from immature embryos carrying both the Fast allele (from A188) and the Slow allele (from Berkeley Slow) of Adh1. Shoots were generated from cultures that were maintained on a modified MSmedium (C. E. Green and R. L. Phillips, Crop Sci. 15:417-421, 1975) containing between 1.0 mg/l and 2.0 mg/l of 2,4-D. Roots developed from the shoots upon transfer to medium without growth regulators. The roots had a good activity of both ADH1 and ADH2 without a specific induction treatment, and extracts from the roots were run on lithium borate starch gels which were stained for ADH activity (A. D. Hanson and A. H. D. Brown, Biochemical Genetics 22:495-515, 1984). A total of 385 individual regenerant (SC1) plants deriving from 122 embryos have so far been tested in this way. The screening has revealed one ADH1 electrophoretic variant which was first detected by the absence of a band corresponding to the ADH1 Fast-Slow heterodimer. The regenerated plant was self-pollinated. The progeny segregate for the normal ADH1-Fast and an unexpected ADH1 isozyme which runs slightly slower than the homodimer of the Adh1-U725 mutant obtained by R. J. Ferl, S. R. Dlouhy and D. Schwartz (MGG 169:7-12, 1979). This new variant is called Adh1 -Usv. The heterodimer (F/Usv) has the same electrophoretic mobility as the ADH1-Slow homodimer. The Usv mutant has full ADH1 activity as judged by the enzyme reaction on starch gel, and the segregation among the seed progeny conforms to Mendelian expectations:
 

Number of seeds tested	F/F	F/Usv	Usv/Usv
71	17	39	15
		X2(2) (1:2: 1) = 0. 8

No further variants have been detected as yet; however, the screening programme is being continued to find additional Adh mutants, both for enzyme activity and for electrophoretic mobility.

Richard Brettell and Maria Jeppesen

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