Jones, Gary E. : Gary E. Jones, Associate Professor, Genetics and Associate Geneticist, Botany and Plant Sciences, U.C., Riverside.

0 racil appears to be a useful characteristic for 2 such work . Parent cells growingin culture are to this compound , but vari - highly sensitive ants can be found that are resistant to it , and the difference is easily assayed . We have demonstrated that this difference results from an enzyme deficiencyin the vari - ant cells . They lack an enzyme ( uracil phos - ana - phoribosyltransferase ) that convertsthe log into the compound that actually kills the of cells from two different spe - cells . Strains cies of plants ( diploid Haplopappus gracilk and haploid Daturu innoxiu ) resistant to this analog have been isolated and shown to lack the enzyme . We have studied the effects of several commonly used mutagens on these cells but have not yet found one that effec - tively increases the frequency of azauracil - resistant cells in treated populations . Two possible causes for these results must so far tested be considered . First , the agents might not be mutagenic in cells of these spe - Two plants regenerated from same celery callus culture demonstrate somaclonal variation . cies . Or , second , resistanceto azauracilmight left closely resembles the original . Plant on right has smaller , more divided leaves , Plant on not be the result of real mutations in the gene its growth is much slower . Genetic studies will investigate whether variation resulted and from DNA changes or lingering cultural effects . responsible for the structure of the enzyme but instead might arise from nongenetic causes . In the latter case , mutagenic agents aberrant cells within plants . Approximately would not be expected to affect the frequency 70 percent of plants regeneratedfrom cell cul - of resistant cells . Although we cannot yet Cell mutagens tures of a commercial celery variety and prove that stable azauracil resistance has a grown in three field locations were visibly genetic cause , several characteristics of the normal , while the remaining 30 percent Gary E . Jones resistant cells indicate that this is the case . showed striking differencesin growth rate or Among these traits is the complete lack of habit , leaf shape , color , or flowering be - phosphoribosyltransferase activity af - uracil havior . None of the plants exhibited charac - of plant so - Realizing the full potential ter cells have been cultured for more than two to teristics that could be considered superior matic cell genetic techniques will depend on years without exposure to the analog . the original type . Experiments to assess the development of methods for isolating a wide One feature of our studies showsthat such relative physiological and genetic contribu - variety of cultured cell strains with character - work must be interpreted with care . In many tions to this variation are in progress . of cells in the ori - istics different from those experiments , we demonstrated that several Further results with celery suggest that ginal cultures . To isolate such variant cell potentiallymutagenicagentsappeared at first genotype of the tissue donor , medium consti - strains , techniques well known in microbial to increase the frequency of resistant cells , tution , and culture age are the significantfac - studies will have to be applied to cultured sometimesby as much as 50 - fold . However , tors mediatingsomaclonalvariation , whereas plant cell systems . the vast majority of these resistant popula - differentiated state ( leaf , stem , and the like ) One especiallyimportant method is the ap - tions did not retain the characteristic when and random effects are not important . The plication of mutagens to plant cells to greatly subsequently cultured in medium not con - differences observed among genotypes were increase the frequency of variant strains in loss of the resis - taining the azauracil . Rapid particularly interesting : some lines showed a populations of cells so that they may be more tance strongly indicates that these cells were rapid , progressiveaccumulation of variation easily identifiedand selected . However , work not genetically altered , although we do not ( and loss of ability to regenerate ) , whereas in several laboratories has shown that many know why they initially appeared to be resis - others consistently remained stable ( and able chemicalsthat are potent mutagens in micro - tant . In fact , when careful study of the ini - to regenerate ) . We therefore speculate that a bial systems are only marginally or not at all tially resistant strains was carried out , the fre - of appropriate genotypes and combination effective on cultured plant cells . The search quency of stably resistant cells was not very media may be at least partially effective in for chemicals effective on a wide variety of much increased by the treatment with muta - controlling somaclonal variation . Perhaps it plant cells is therefore an important aspect of gen . Such results demonstrate that detailed will be possible to identify genes responsible plant somatic cell genetics . of resistance to any compound investigation for inhibition or enhancement of variation Testing whether a compound is mutagenic must be performed before conclusionsabout and to transfer them sexually into desired requires that the expression of an easily seen the mutagenic effects of any agent can be backgrounds . Solutions to these problems characteristic be substantially different in claimed , even if stable resistance can be a major block to the applica - will eliminate parent cells than in the variants derived from of a true genetic shown to be the result tion of new molecular and cellular technol - them after treatment with the agent . In our change . ogy in plant breeding and field production . laboratory , we have been attempting to de - GaryE . Jones , Associate Professor , Geneticsand velop such a testing system . Resistance to a Associate Geneticist , Botany and Plant Sciences , ThomasJ . Orton , Assistant Professor , Vegetable U.C . , Riverside . Crops , U.C . , Davis . nucleic acid precursor analog called 6 - azau - CALIFORNIA AGRICULTURE , AUGUST 1982 21