Eight independent transgenic lines of Arabidopsis thaliana were generated carrying homozygous insertions of the cauliflower mosaic virus 35S RNA promoter driving antisense MT2a ORF or the heat shock-protein 81-1 promoter driving sense MT2a ORF. Six transgenic lines showed lower levels of MT2a mRNA accumulation than the wild type plants and among them, three lines produced abnormal tissue. masses at the base of floral stalks at low frequencies. Such structures had never been observed in transgenic plants with wild type levels of MT2a transcript accumulation or in the wild type plants. Moreover, we isolated a line from ethyl methanesulfonate (EMS)-mutagenized 5,000 M2 plants that exhibited abnormal tissue masses at the base of floral stalks at low frequencies. The phenotype was inherited at least for four successive generations. This line was found to accumulate MT2a mRNA at a reduced level in the aerial parts. A MT2a promoter::beta-glucuronidase (GUS) gene was introduced into the mutant line and it was found that GUS activity was reduced in the mutant plants, suggesting that the mutation(s) in the line downregulated the MT2a promoter in trans. These results suggest that the reduction in MT2a mRNA accumulation is the cause of the abnormal tissue masses observed. MT2a is likely to be involved in the regulation of tissue formation in A. thaliana and possible mechanisms underlying the effects were discussed.