Biochemical genetics of resistance to MGBG in tobacco: Mutants that alter SAM decarboxylase or polyamine ratios, and floral morphology

Summary

Previously we reported the isolation of Nicotiana tabacum cell lines resistant to methylglyoxal-bis(guanylhydrazone) (MGBG) an inhibitor of the polyamine synthetic enzyme s-adenosylmethionine decarboxylase (SAMde). Here we report that these mutants fall into several distinct classes on the basis of their biochemical characteristics. At least two lines, Mgr12 and Mgr16, have a SAMdc that displays increased resistance to MGBG in in vitro enzyme assays, suggesting that these two alleles have altered the SAMdc enzyme itself, and thus they may serve to identify the SAMdc structural gene. Other cell lines have elevated levels of some of the polyamines or polyamine-conjugates: Mgr11 and Mgr14 have high levels of putrescine and high levels of activity of putrescine synthesizing enzymes; Mgr23 and Mgr3 have elevated spermidine and spermidine-conjugate levels, with Mgr23 also having elevated putrescine levels. Mgr12 and Mgr3 have been analyzed genetically through F₁ crosses with wild-type tobacco, and subsequently by a backcross of an F₁ plant to wild type; however, the total number of seeds obtained in each cross was very small. The results of the genetic analysis are consistent with Mgr12 and Mgr3 being nuclear dominant traits. The floral abnormalities previously reported as associated with these mutations display linkage with the MGBG resistances. At least for Mgr12 we have thus obtained evidence as to the precise nature of the mutation, an altered SAMdc, and demonstrated that this is likely to be genetic cause of the altered flower phenotype.