Complex characters of plants such as starch and sugar content of seeds, fruits, tubers and roots are controlled by multiple genetic and environmental factors. Understanding their molecular basis will facilitate diagnosis and combination of superior alleles in crop improvement programs (''precision breeding''). Association genetics based on candidate genes is one approach toward this goal. Tetraploid potato varieties and breeding clones related by descent were evaluated for 2 years for chip quality before and after cold storage, tuber starch content, yield and starch yield. Chip quality is inversely correlated with tuber sugar content. A total of 36 loci on 11 potato chromosomes were evaluated for natural DNA variation in 243 individuals. These loci included microsatellites and genes coding for enzymes that function in carbohydrate metabolism or transport (candidate loci). The markers were used to analyze population structure and were tested for association with the tuber quality traits. Highly significant and robust associations of markers with 1-4 traits were identified. Most frequent were associations with chip quality and tuber starch content. Alleles increasing tuber starch content improved chip quality and vice versa. With two exceptions, the most significant and robust associations (q \ 0.01) were observed with DNA variants in genes encoding enzymes that function in starch and sugar metabolism or transport. Comparing linkage and linkage disequilibrium between loci provided evidence for the existence of large haplotype blocks in the breeding materials analyzed. IntroductionMost characters important for crop quality show quantitative phenotypic variation, due to the fact that they are controlled by natural DNA variation at multiple loci and by environmental factors. Knowing the molecular basis of the genetic components of this variation will facilitate the selection of improved cultivars with DNA-based markers, which are diagnostic for superior alleles of the underlying genes.Communicated by J. Yu. Electronic supplementary materialThe online version of this article
Proteins from both the inner and outer envelope membranes are engaged in the recognition and translocation of precursor proteins into chloroplasts. A 110 kDa protein of the chloroplastic inner envelope membrane was identified as a component of the protein import apparatus by two methods. First, this protein was part of a 600 kDa complex generated by cross‐linking of precursors trapped in the translocation process. Second, solubilization with detergents of chloroplasts containing trapped precursors resulted in the identification of a complex containing both radiolabeled precursor and IEP110. Trypsin treatment of intact purified chloroplasts was used to study the topology of IEP110. The protease treatment left the inner membrane intact while simultaneously degrading domains of inner envelope proteins exposed to the intermembrane space. About 90 kDa of IEP110 was proteolitically removed, indicating that large portions protrude into the intermembrane space. Hydropathy analysis of the protein sequence deduced from the isolated cDNA clone in addition to Western blot analysis using an antiserum of IEP110 specific to the N‐terminal 20 kDa, suggests that the N‐terminus serves to anchor the protein in the membrane. We speculate that IEP110 could be involved in the formation of translocation contact sites due to its specific topology.
Transport of precursor proteins across the chloroplastic envelope membranes requires the interaction of protein translocons localized in both the outer and inner envelope membranes. Analysis by blue native gel electrophoresis revealed that the translocon of the inner envelope membranes consisted of at least six proteins with molecular weights of 36, 45, 52, 60, 100 and 110 kDa, respectively. Tic110 and ClpC, identified as components of the protein import apparatus of the inner envelope membrane, were prominent constituents of this complex. The amino acid sequence of the 52 kDa protein, deduced from the cDNA, contains a predicted Rieske-type iron-sulfur cluster and a mononuclear iron-binding site. Diethylpyrocarbonate, a Rieske-type protein-modifying reagent, inhibits the translocation of precursor protein across the inner envelope membrane, whereas binding of the precursor to the outer envelope membrane is still possible. In another independent experimental approach, the 52 kDa protein could be copurified with a trapped precursor protein in association with the chloroplast protein translocon subunits Toc86, Toc75, Toc34 and Tic110. Together, these results strongly suggest that the 52 kDa protein, named Tic55 due to its calculated molecular weight, is a member of the chloroplastic inner envelope protein translocon.
The oomycete Phytophthora infestans causes late blight, the most relevant disease of potato (Solanum tuberosum) worldwide. Field resistance to late blight is a complex trait. When potatoes are cultivated under long day conditions in temperate climates, this resistance is correlated with late plant maturity, an undesirable characteristic. Identification of natural gene variation underlying late blight resistance not compromised by late maturity will facilitate the selection of resistant cultivars and give new insight in the mechanisms controlling quantitative pathogen resistance. We tested 24 candidate loci for association with field resistance to late blight and plant maturity in a population of 184 tetraploid potato individuals. The individuals were genotyped for 230 single nucleotide polymorphisms (SNPs) and 166 microsatellite alleles. For association analysis we used a mixed model, taking into account population structure, kinship, allele substitution and interaction effects of the marker alleles at a locus with four allele doses. Nine SNPs were associated with maturity corrected resistance (P , 0.001), which collectively explained 50% of the genetic variance of this trait. A major association was found at the StAOS2 locus encoding allene oxide synthase 2, a key enzyme in the biosynthesis of jasmonates, plant hormones that function in defense signaling. This finding supports StAOS2 as being one of the factors controlling natural variation of pathogen resistance.
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