Deletions within intronic T-DNA lead to reversion of T-DNA mutant phenotypes

Agrobacterium-mediated transformation enables random transfer-DNA (T-DNA) insertion into plant genomes. T-DNA insertion into a gene's exons, introns or untranscribed regions close to the start or stop codon can disrupt gene function. Such T-DNA mutants have been useful for reverse genetics analysis, especially in Arabidopsis thaliana. As T-DNAs are inserted into genomic DNA, they are generally believed to be stably inherited. Here, we report a phenomenon of reversion of intronic T-DNA mutant phenotypes. From a suppressor screen using intronic T-DNA pi4k beta 1,2 double mutant, we recovered intragenic mutants of pi4k beta 1, which suppressed the autoimmunity of the double mutant. These mutants carried deletions in the intronic T-DNAs, resulting in elevated transcription of normal PI4K beta 1. Such reversion of T-DNA insertional mutant phenotype stresses the need for caution when using intronic T-DNA mutants and reiterates the importance of using irreversible null mutant alleles in genetic analyses.