Maintenance of genome stability is essential for the accurate propagation of genetic information and cell growth and survival. Organisms have therefore developed efficient strategies to prevent DNA lesions and rearrangements. Much of the information concerning these strategies has been obtained through the study of bacterial and nuclear genomes. Comparatively, little is known about how organelle genomes maintain a stable structure. Here, we report that the plastid-localized Whirly ssDNA-binding proteins are required for plastid genome stability in Arabidopsis. We show that a double KO of the genes AtWhy1 and AtWhy3 leads to the appearance of plants with variegated green/white/yellow leaves, symptomatic of nonfunctional chloroplasts. This variegation is maternally inherited, indicating defects in the plastid genome. Indeed, in all variegated lines examined, reorganized regions of plastid DNA are amplified as circular and/or head-tail concatemers. All amplified regions are delimited by short direct repeats of 10 -18 bp, strongly suggesting that these regions result from illegitimate recombination between repeated sequences. This type of recombination occurs frequently in plants lacking both Whirlies, to a lesser extent in single KO plants and rarely in WT individuals. Maize mutants for the ZmWhy1 Whirly protein also show an increase in the frequency of illegitimate recombination. We propose a model where Whirlies contribute to plastid genome stability by protecting against illegitimate repeat-mediated recombination.genome maintenance ͉ microhomology ͉ recombination

Elicitor-induced activation of the potato pathogenesis-related gene PR-10a requires a 30-bp promoter sequence termed the ERE (elicitor response element) that is bound by the nuclear factor PBF-2 ( PR-10a binding factor 2). In this study, PBF-2 has been purified to near homogeneity from elicited tubers through a combination of anion-exchange and DNA affinity chromatography. Evidence demonstrates that inactive PBF-2 is stored in the nuclei of fresh tubers and becomes available for binding to the ERE upon elicitation. A protein with an apparent molecular mass of 24 kD (p24) is a DNA binding component of PBF-2. A cDNA encoding p24 has been cloned and encodes a novel protein with a potential transcriptional activation domain that could also act as a single-stranded DNA binding domain. Both PBF-2 and the cDNA-encoded protein bind with high affinity to the single-stranded form of the ERE in a sequence-specific manner. The inverted repeat sequence of the ERE, TGACAnnnnTGTCA, is critical for binding of this factor in vitro and for PR-10a expression in vivo, supporting the role of PBF-2 as a transcriptional regulator. INTRODUCTIONPlants defend themselves against fungal pathogens by a variety of mechanisms, including preexisting physical barriers and inducible defenses (Lamb et al., 1989). Attack by an avirulent strain of pathogen results in a rapid localized necrosis at the site of infection (termed the hypersensitive response), which contributes to pathogen limitation (Keen, 1992). With a few exceptions, deployment of the inducible defenses requires massive gene induction (Lamb et al., 1989). Despite the importance of transcriptional activation during the plant defense response, very little is known about the players involved and the exact mechanisms that lead to defense gene induction.PR (pathogenesis-related) genes are among the best characterized genes induced by pathogens. Heterogeneous in structure and function, PR genes are subdivided into 11 groups (Van Loon et al., 1994). Although the function of certain PR proteins is unknown, some display in vitro antifungal properties (Schlumbaum et al., 1986;Vigers et al., 1991;Ponstein et al., 1994;Niderman et al., 1995). Genes of the PR-10 group are present in numerous dicots (Somssich et al., 1988;Breiteneder et al., 1989;Matton and Brisson, 1989;Walter et al., 1990) and monocots (Warner et al., 1992;Moons et al., 1997). Evidence is accumulating that some PR-10 proteins might possess ribonuclease activity (Moiseyev et al., 1994;Bufe et al., 1996;Swoboda et al., 1996). More recently, structural and sequential homology between the PR-10 proteins and a group of latex proteins has been described (Osmark et al., 1998). Genes of the PR-10 group encode small, primarily acidic intracellular proteins with molecular masses ranging from 15 to 18 kD and have been shown to be transcriptionally regulated (Linthorst, 1991).In only two cases have cis elements and their trans -acting factors been characterized in the promoters of PR-10 genes. These studies revealed that the processes of tr...

Genetic mapping of plants traditionally involves the analysis of large segregating populations. However, not all individuals in a population contribute equal amounts of genetic information. It is thus possible to achieve rough mapping using a subset of the most informative individuals in the population. We have designed a minimal Brassica napus mapping population of 23 doubled-haploid plants and have tested this method using this population in the mapping of disease resistance gene homologues in B. napus. Several groups have identified such homologues in soybean and potato by amplifying sequences corresponding to conserved nucleotide-binding sites from known resistance genes. However, the sequence conservation in the leucine-rich repeat domain that is present in most of the disease resistance genes isolated has not been exploited via the polymerase chain reaction (PCR). We present the genetic mapping of Brassica napus DNA sequences amplified with primers corresponding to both the nucleotide-binding site and the leucine rich-repeat domain of the Arabidopsis thaliana RPS2 gene. We also describe a method for the quick mapping of resistance gene homologues using the polymerase chain reaction.Key words: Brassica napus, disease resistance genes, minimal mapping population, RFLP markers.

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