Early and multiple Ac transpositions in rice suitable for efficient insertional mutagenesis

A GFP excision assay was developed to monitor the excision of Ac introduced into rice by Agrobacterium -mediated transformation. The presence of a strong double enhancer element of the CaMV 35S promoter adjacent to the Ac promoter induced very early excision, directly after transformation into the plant cell, exemplified by the absence of Ac in the T-DNA loci. Excision fingerprint analysis and characterization of transposition events from related regenerants revealed an inverse correlation between the number of excision events and transposed Ac copies, with single early excisions after transformation generating Ac amplification. New transpositions were generated at a frequency of 15–50% in different lines, yielding genotypes bearing multiple insertions, many of which were inherited in the progeny. The sequence of DNA flanking Ac in three representative lines provided a database of insertion tagged sites suitable for the identification of mutants of sequenced genes that can be examined for phenotypes in a reverse genetics strategy to elucidate gene function. Remarkably, two-thirds of Ac tagged sites showing homology to sequences in public databases were in predicted genes. A clear preference of transposon insertions in genes that are either predicted by protein coding capacity or by similarity to ESTs suggests that the efficiency of recovering knockout mutants of genes could be about three times higher than random. Linked Ac transposition, suitable for targeted tagging, was documented by segregation analysis of a crippled Ac element and by recovery of a set of six insertions in a contiguous sequence of 70 kb from chromosome 6 of rice.