Plasmodesmata-located protein overexpression negatively impacts the manifestation of systemic acquired resistance and the long-distance movement of Defective in Induced Resistance1 in Arabidopsis.

Systemic acquired resistance (SAR) is a plant defence response that provides immunity to distant uninfected leaves after an initial localised infection. The lipid transfer protein (LTP) Defective in Induced Resistance1 (DIR1) is an essential component of SAR that moves from induced to distant leaves following a SAR-inducing local infection. To understand how DIR1 is transported to distant leaves during SAR, we analysed DIR1 movement in transgenic Arabidopsis lines with reduced cell-to-cell movement caused by the overexpression of Plasmodesmata-Located Proteins PDLP1 and PDLP5. These PDLP-overexpressing lines were defective for SAR, and DIR1 antibody signals were not observed in phloem sap-enriched petiole exudates collected from distant leaves. Our data support the idea that cell-to-cell movement of DIR1 through plasmodesmata is important during long-distance SAR signalling in Arabidopsis.

Mu transposase-stimulated illegitimate recombination of Tn3kan- and IS101-containing plasmids.

The transposable bacteriophage Mu and the mobile genetic elements Tn3 and IS101 replicatively transpose to random target sites, produce 5 bp target site duplications, and contain the sequence 5'-PuCGAAAPu-3' starting at bp 21 from their ends. The presence of these shared characteristics, plus the fact that Mu transposase can specifically bind to the termini of Tn3 and IS101 in vitro, suggests that the elements may be evolutionarily conserved and retain some functional capacity to transpose each other's DNA. To examine this proposition, in vivo transposition-mating assays were performed and demonstrated that Mu transposase stimulated the formation of recA-independent recombination products between Tn3kan- or IS101-containing plasmids and a target plasmid (pOX38cam) up to 200-fold. However, when transferred to recA+ hosts, these recA-independent products yielded resolution products suggestive of illegitimate recombination, as similar recombination and resolution products were generated, at reduced frequencies, in the absence of Mu transposase. Thus, Mu transposase may stimulate a host-mediated, recA-independent illegitimate recombination reaction. As adjacent pSC101 sequences, including a formerly unknown but functional IHF site (bp 2238-2251), were required for Mu transposase-stimulated IS101 illegitimate recombination, IHF may be one of the putative host factors involved in these recombination reactions.

The bacteriophage Mu transposase protein can form high-affinity protein-DNA complexes with the ends of transposable elements of the Tn 3 family.

The 37 kb transposable bacteriophage Mu genome encodes a transposase protein which can recognize and bind to a consensus sequence repeated three times at each extremity of its genome. A subset of this consensus sequence (5'-PuCGAAA(A)-3') is found in the ends of many class II prokaryotic transposable elements. These elements, like phage Mu, cause 5 bp duplications at the site of element insertion, and transpose by a cointegrate mechanism. Using the band retardation assay, we have found that crude protein extracts containing overexpressed Mu transposase can form high-affinity protein-DNA complexes with Mu att R and the ends of the class II elements Tn 3 (right) and IS101. No significant protein-DNA complex formation was observed with DNA fragments containing the right end of the element IS102, or a non-specific pBR322 fragment of similar size. These results suggest that the Mu transposase protein can specifically recognize the ends of other class II transposable elements and that these elements may be evolutionarily related.