Francesco Salamini

The Barley Mlo Gene: A Novel Control Element of Plant Pathogen Resistance

Mutation-induced recessive alleles (mlo) of the barley Mlo locus confer a leaf lesion phenotype and broad spectrum resistance to the fungal pathogen, Erysiphe graminis f. sp. hordei. The gene has been isolated using a positional cloning approach. Analysis of 11 mutagen-induced mlo alleles revealed mutations leading in each case to alterations of the deduced Mlo wild-type amino acid sequence. Susceptible intragenic recombinants, isolated from mlo heteroallelic crosses, show restored Mlo wild-type sequences. The deduced 60 kDa protein is predicted to be membrane-anchored by at least six membrane-spanning helices. The findings are compatible with a dual negative control function of the Mlo protein in leaf cell death and in the onset of pathogen defense; absence of Mlo primes the responsiveness for the onset of multiple defense functions.

A PCR-based approach for isolating pathogen resistance genes from potato with potential for wide application in plants

Plant genes for pathogen resistance can be used to engineer disease resistant crops. Oligonucleotides were designed from sequence motifs conserved between resistance genes of tobacco and Arabidopsis thaliana and used as PCR primers in potato DNA. Amplification products were obtained that were homologous to known resistance genes and linked without recombination with the nematode resistance locus Gro1 and the Phytophthora infestans resistance locus R7 of potato. Map positions of PCR–derived potato gene fragments were also correlated with resistance loci of the related tomato and tobacco genomes. Our results indicate that plant resistance genes that are effective against nematodes, fungi.viruses and bacteria may be isolated based on common sequence motifs and PCR methodology.

The high-quality draft genome of peach (Prunus persica) identifies unique patterns of genetic diversity, domestication and genome evolution

Rosaceae is the most important fruit-producing clade, and its key commercially relevant genera (Fragaria, Rosa, Rubus and Prunus) show broadly diverse growth habits, fruit types and compact diploid genomes. Peach, a diploid Prunus species, is one of the best genetically characterized deciduous trees. Here we describe the high-quality genome sequence of peach obtained from a completely homozygous genotype. We obtained a complete chromosome-scale assembly using Sanger whole-genome shotgun methods. We predicted 27,852 protein-coding genes, as well as noncoding RNAs. We investigated the path of peach domestication through whole-genome resequencing of 14 Prunus accessions. The analyses suggest major genetic bottlenecks that have substantially shaped peach genome diversity. Furthermore, comparative analyses showed that peach has not undergone recent whole-genome duplication, and even though the ancestral triplicated blocks in peach are fragmentary compared to those in grape, all seven paleosets of paralogs from the putative paleoancestor are detectable.

Genetics and geography of wild cereal domestication in the near east

About 12,000 years ago, humans began the transition from hunter-gathering to a sedentary, agriculture-based society. From its origins in the Near East, farming expanded throughout Europe, Asia and Africa, together with various domesticated plants and animals. Where, how and why agriculture originated is still debated. But newer findings, on the basis of genome-wide measures of genetic similarity, have traced the origins of some domesticated cereals to wild populations of naturally occurring grasses that persist in the Near East. A better understanding of the genetic differences between wild grasses and domesticated crops adds important facets to the continuing debate on the origin of Western agriculture and the societies to which it gave rise.

Combined mapping of AFLP and RFLP markers in barley

AFLP marker technology allows efficient DNA fingerprinting and the analysis of large numbers of polymorphic restriction fragments on polyacrylamide gels. Using the doubled haploids from the F1 of the cross Proctor × Nudinka, 118 AFLP markers were mapped onto a barley (Hordeum vulgare L.) RFLP map, also including five microsatellite and four protein marker loci. The AFLP markers mapped to all parts of the barley chromosomes and filled in the gaps on barley chromosomes 2L, 4L and 6 in which no RFLP loci had been mapped. Interestingly, the AFLP markers seldom interrupted RFLP clusters, but grouped next to them. The combined map covers 1873 cM, with a total of 282 markers. The merging of AFLP and RFLP markers increased the total map length; 402 cM were added to the map at the tips of chromosomes or in regions corresponding to earlier gaps. Another 375 cM resulted from mapping AFLP markers near to RFLP clusters or in between non-clustered RFLP markers.

RFLP maps of potato and their alignment with the homoeologous tomato genome

SummaryAn RFLP linkage map of the potato is presented which comprises 304 loci derived from 230 DNA probes and one morphological marker (tuber skin color). The self-incompatibility locus of potato was mapped to chromosome I, which is homoeologous to tomato chromosome I. By mapping chromosome-specific tomato RFLP markers in potato and, vice versa, potato markers in tomato, the different potato and tomato RFLP maps were aligned to each other and the similarity of the potato and tomato genome was confirmed. The numbers given to the 12 potato chromosomes are now in accordance with the established tomato nomenclature. Comparisons between potato RFLP maps derived from different genetic backgrounds revealed conservation of marker order but differences in chromosome and total map length. In particular, significant reduction of map length was observed in interspecific compared to intraspecific crosses. The distribution of regions with distorted segregation ratios in the genome was analyzed for four potato parents. The most prominent distortion of recombination was found to be caused by the self-incompatibility locus.

RFLP analysis and linkage mapping in Solanum tuberosum

SummaryA morphologically and agronomically heterogeneous collection of 38 diploid potato lines was analysed for restriction fragment length polymorphisms (RFLPs) with 168 potato probes, including random genomic and cDNA sequences as well as characterized potato genes of known function. The use of four cutter restriction enzymes and a fragment separation range from 250 to 2,000 bases on denaturing polyacrylamide gels allowed the detection of RFLPs of a few nucleotides. With this system, 90% of all probes tested showed useful polymorphism, and 95% of those were polymorphic with two or all three enzymes used. On the average, 80% of the probes were informative in all pairwise comparisons of the 38 lines with a minimum of 49% and a maximum of 95%. The percentage of heterozygosity was determined relative to each other for each line and indicated that direct segregation analysis in F1 populations should be feasible for most combinations. From a backcross involving one pair of the 38 lines, a RFLP linkage map with 141 loci was constructed, covering 690 cMorgan of the Solanum tuberosum genome.

Molecular cloning of abscisic acid-modulated genes which are induced during desiccation of the resurrection plant Craterostigma plantagineum.

Leaves of the resurrection plant Craterostigma plantagineum Hochst, can be desiccated up to 1% relative water content and are still viable after rehydration. To clone genes related to this extreme desiccation tolerance, an in-vitro system was first developed which allows the induction of the same resurrection response in callus tissue upon treatment with abscisic acid (ABA). Several proteins and in-vitro-synthesized polypeptides were then identified which can be induced both in desiccation-tolerant, naturally dried leaves and in ABA-treated calli surviving after rehydration. Complementary-DNA clones corresponding to mRNAs expressed only in desiccation-tolerant tissues were obtained and classified into several gene families. In hybrid-selected translation experiments, representative cDNA clones were associated with water stress and ABA-inducible polypeptides abundantly expressed in dried leaves and ABA-treated calli. The expression pattern of several of these abundant transcripts was analyzed in RNA-hybridization experiments. Upon stress or ABA treatment the transcription levels increased rapidly, but they declined after relief from the stress state. This, together with data on genomic copy numbers indicated that a set of abundantly expressed genes are involved in the desiccation process of resurrection plants. Data on endogenous ABA contents before and after stress applications and on the physiological effects of exogenous ABA treatments indicate that in Craterostigma plantagineum the induction of an extreme desiccation tolerance is mediated by this plant hormone.

The mlo resistance alleles to powdery mildew infection in barley trigger a developmentally controlled defence mimic phenotype

Recessive mlo resistance alleles of the Mlo locus in barley control a non race-specific resistance response to infection by the obligate biotrophic fungus Erysiphe graminis f.sp. hordei. All the mlo alleles analysed stop fungal growth at the same developmental stage within a subcellularly restricted, highly localized cell wall apposition directly beneath the site of abortive fungal penetration. We report that near-isogenic lines carrying the alleles mlo1, mlo3 or mlo5 undergo dramatic spontaneous formation of cell wall appositions, not only in the absence of the fungal pathogen but also in sterile grown plants. A comparative study of spontaneous and infection-triggered cell wall appositions reveals a high degree of similarity with respect to structure, chemical composition and distinct localization within plant tissue. We show that the rate of spontaneous apposition formation is dependent on the genetic background of the plant and that its onset is under developmental control. Furthermore, spontaneous formation of wall appositions is specifically triggered by mlo alleles, since it is unaffected in the presence of the race-specific resistance allele Mlg. We propose a model for the function of the Mlo locus that suggests that both Mlo and mlo alleles control qualitatively the same apposition-based resistance mechanism, which, in the presence of the wild-type Mlo allele, is merely less efficient to provide protection against the currently common races of E. graminis f.sp. hordei.

A prediction of the size and evolutionary origin of the proteome of chloroplasts of Arabidopsis

cells are descended from an original cyanobacterial endosymbiont 1 (Fig. 1). In the course of their development from a free-living cyanobacterial ancestor into highly specialized organelles, chloroplasts have lost their autonomy. A key factor in this process of adaptation was the loss of genetic material resulting from gene transfer to the cell nucleus . This is why the vast majority of proteins in present-day chloroplasts are encoded by the nucleus and require N-terminal presequences (transit peptides) that target them to the chloroplast . Previous estimates of the total number of different proteins resident in the chloroplast (the proteome) vary from 600 to 5000 (Refs 4,5). The reason for this broad range is that the known size of the proteome in the cyanobacterium Synechocystis (~3200 proteins) 6,7 cannot serve as a model for the plant chloroplast. First, genes of the endosymbiont that coded for functions, which were already present in the eukaryotic host or were irrelevant for the evolving organelle, had a high probability of being lost during evolution. Second, unlike free-living cyanobacteria, chloroplasts had to import a set of new proteins to adapt their metabolism to the new conditions and establish a functional organelle–nucleus interaction. Thus, several important questions arise: • Do chloroplasts need more or fewer functions (and proteins) than free-living cyanobacteria? • How similar are the proteomes of chloroplasts and cyanobacteria? • How many non-cyanobacterial proteins reside in the chloroplast? Here, we describe our attempts to provide answers to these questions.