Transgene-triggered, epigenetically regulated ectopic expression of a flower homeotic gene pMADS3 in Petunia

pMADS3 is a class C floral homeotic gene of Petunia that is specifically expressed in stamens and carpels of developing flowers. We previously reported that introduction of a part of the pMADS3 genomic sequence silenced endogenous pMADS3 (sil-pMADS3) in transgenic Petunia hybrida. Here we report that introduction of the same sequence triggers ectopic expression of endogenous pMADS3 in sepals, petals and leaves (ect-pMADS3), accompanied by homeotic conversion of the floral organs and altered leaf morphology similar to that of an Arabidopsis curly leaf mutant. The occurrence of the ect-pMADS3 phenotype depended on the presence of pMADS3 intron 2 in the transgenes. Occasionally, sil-pMADS3 and ect-pMADS3 phenotypes somatically interconverted. Some T1 progeny inherited their parent's pMADS3 expression pattern, while others switched from sil-pMADS3 to ect-pMADS3 and vice versa. Both phenotypes occasionally occurred even after the transgenes were segregated away. RT-PCR analyses of ectopically expressed pMADS3 transcripts indicated that two pMADS3 alleles were often differently regulated. Furthermore, reciprocal crosses with untransformed Petunia indicated that pMADS3 alleles other than the one ectopically expressed in T0 plants were sometimes expressed ectopically in T1 plants: the paramutation-like transmission of epigenetic regulation between alleles. We detected in the transformants aberrant transcripts, including sense and antisense pMADS3 intron 2 sequences of heterogeneous molecular sizes, irrespective of the pMADS3 phenotypes. We speculate on possible molecular mechanisms underlying these epigenetic phenomena.