High-copy-number transposable elements comprise the majority of eukaryotic genomes where they are major contributors to gene and genome evolution. However, it remains unclear how a host genome can survive a rapid burst of hundreds or thousands of insertions because such bursts are exceedingly rare in nature and therefore difficult to observe in real time. In a previous study we reported that in a few rice strains the DNA transposon mPing was increasing its copy number by approximately 40 per plant per generation. Here we exploit the completely sequenced rice genome to determine 1,664 insertion sites using high-throughput sequencing of 24 individual rice plants and assess the impact of insertion on the expression of 710 genes by comparative microarray analysis. We find that the vast majority of transposable element insertions either upregulate or have no detectable effect on gene transcription. This modest impact reflects a surprising avoidance of exon insertions by mPing and a preference for insertion into 5' flanking sequences of genes. Furthermore, we document the generation of new regulatory networks by a subset of mPing insertions that render adjacent genes stress inducible. As such, this study provides evidence for models first proposed previously for the involvement of transposable elements and other repetitive sequences in genome restructuring and gene regulation.

Pollen-pistil interactions are crucial for controlling plant mating. For example, S-RNase-based self-incompatibility prevents inbreeding in diverse angiosperm species. S-RNases are thought to function as specific cytotoxins that inhibit pollen that has an S-haplotype that matches one of those in the pistil. Thus, pollen and pistil factors interact to prevent mating between closely related individuals. Other pistil factors, such as HT-B, 4936-factor and the 120 kDa glycoprotein, are also required for pollen rejection but do not contribute to S-haplotype-specificity per se. Here we show that S-RNase is taken up and sorted to a vacuolar compartment in the pollen tubes. Antibodies to the 120 kDa glycoprotein label the compartment membrane. When the pistil does not express HT-B or 4936-factor, S-RNase remains sequestered, unable to cause rejection. Similarly, in wild-type pistils, compatible pollen tubes degrade HT-B and sequester S-RNase. We suggest that S-RNase trafficking and the stability of HT-B are central to S-specific pollen rejection.

Miniature inverted repeat transposable elements (MITEs) are widespread in eukaryotic genomes, where they can attain high copy numbers despite a lack of coding capacity. However, little is known about how they originate and amplify. We performed a genome-wide screen of functional interactions between Stowaway MITEs and potential transposases in the rice genome and identified a transpositionally active MITE that possesses key properties that enhance transposition. Although not directly related to its autonomous element, the MITE has less affinity for the transposase than does the autonomous element but lacks a motif repressing transposition in the autonomous element. The MITE contains internal sequences that enhance transposition. These findings suggest that MITEs achieve high transposition activity by scavenging transposases encoded by distantly related and self-restrained autonomous elements.

An active miniature inverted repeat transposable element (MITE), mPing, was discovered by computer-assisted analysis of rice genome sequence. The mPing element is mobile in rice cell culture and in a few rice strains where it has been amplified to >1,000 copies during recent domestication. However, determination of the transposase source and characterization of the mechanism of transposition have been hampered by the high copy number of mPing and the presence of several candidate autonomous elements in the rice genome. Here, we report that mPing is active in Arabidopsis thaliana, where its transposition is catalyzed by three sources of transposase from rice: the autonomous Ping and Pong elements and by a cDNA derived from a Ping transcript. In addition to transposase, the product of a second element-encoded ORF of unknown function is also required for mPing transposition. Excision of mPing in A. thaliana is usually precise, and transposed copies usually insert into unlinked sites in the genome that are preferentially in or near genes. As such, this will be a valuable assay system for the dissection of MITE transposition and a potentially powerful tagging system for gene discovery in eukaryotes.autonomous element ͉ PIF/Harbinger ͉ transgenic

Insertional mutagenesis of legume genomes such as soybean (Glycine max) should aid in identifying genes responsible for key traits such as nitrogen fixation and seed quality. The relatively low throughput of soybean transformation necessitates the use of a transposon-tagging strategy where a single transformation event will produce many mutations over a number of generations. However, existing transposon-tagging tools being used in legumes are of limited utility because of restricted transposition (Ac/Ds: soybean) or the requirement for tissue culture activation (Tnt1: Medicago truncatula). A recently discovered transposable element from rice (Oryza sativa), mPing, and the genes required for its mobilization, were transferred to soybean to determine if it will be an improvement over the other available transposon-tagging tools. Stable transformation events in soybean were tested for mPing transposition. Analysis of mPing excision at early and late embryo developmental stages revealed increased excision during late development in most transgenic lines, suggesting that transposition is developmentally regulated. Transgenic lines that produced heritable mPing insertions were identified, with the plants from the highest activity line producing at least one new insertion per generation. Analysis of the mPing insertion sites in the soybean genome revealed that features displayed in rice were retained including transposition to unlinked sites and a preference for insertion within 2.5 kb of a gene. Taken together these findings indicate that mPing has the characteristics necessary for an effective transposon-tagging resource.