Suppression of the Arabidopsis cinnamoyl-CoA reductase 1-6 intronic T-DNA mutation by epigenetic modification.

Arabidopsis (Arabidopsis thaliana) transfer DNA (T-DNA) insertion collections are popular resources for fundamental plant research. Cinnamoyl-CoA reductase 1 (CCR1) catalyzes an essential step in the biosynthesis of the cell wall polymer lignin. Accordingly, the intronic T-DNA insertion mutant ccr1-6 has reduced lignin levels and shows a stunted growth phenotype. Here, we report restoration of the ccr1-6 mutant phenotype and CCR1 expression levels after a genetic cross with a UDP-glucosyltransferase 72e1 (ugt72e1),-e2,-e3 T-DNA mutant. We discovered that the phenotypic recovery was not dependent on the UGT72E family loss of function but due to an epigenetic phenomenon called trans T-DNA suppression. Via trans T-DNA suppression, the gene function of an intronic T-DNA mutant was restored after the introduction of an additional T-DNA sharing identical sequences, leading to heterochromatinization and splicing out of the T-DNA-containing intron. Consequently, the suppressed ccr1-6 allele was named epiccr1-6. Long-read sequencing revealed that epiccr1-6, not ccr1-6, carries dense cytosine methylation over the full length of the T-DNA. We showed that the SAIL T-DNA in the UGT72E3 locus could trigger the trans T-DNA suppression of the GABI-Kat T-DNA in the CCR1 locus. Furthermore, we scanned the literature for other potential cases of trans T-DNA suppression in Arabidopsis and found that 22% of the publications matching our query report on double or higher-order T-DNA mutants that meet the minimal requirements for trans T-DNA suppression. These combined observations indicate that intronic T-DNA mutants need to be used with caution since methylation of intronic T-DNA might derepress gene expression and can thereby confound results.

A comprehensive genomic scan reveals gene dosage balance impacts on quantitative traits in Populus trees.

Gene dosage variation and the associated changes in gene expression influence a wide variety of traits, ranging from cancer in humans to yield in plants. It is also expected to affect important traits of ecological and agronomic importance in forest trees, but this variation has not been systematically characterized or exploited. Here we performed a comprehensive scan of the Populus genome for dosage-sensitive loci affecting quantitative trait variation for spring and fall phenology and biomass production. The study population was a large collection of clonally propagated F1 hybrid lines of Populus that saturate the genome 10-fold with deletions and insertions (indels) of known sizes and positions. As a group, the phenotypic means of the indel lines consistently differed from control nonindel lines, with an overall negative effect of both insertions and deletions on all biomass-related traits but more diverse effects and an overall wider phenotypic distribution of the indel lines for the phenology-related traits. We also investigated the correlation between gene dosage at specific chromosomal locations and phenotype, to identify dosage quantitative trait loci (dQTL). Such dQTL were detected for most phenotypes examined, but stronger effect dQTL were identified for the phenology-related traits than for the biomass traits. Our genome-wide screen for dosage sensitivity in a higher eukaryote demonstrates the importance of global genomic balance and the impact of dosage on life history traits.

Scab resistance in 'Geneva' apple is conditioned by a resistance gene cluster with complex genetic control.

Apple scab, caused by the fungal pathogen Venturia inaequalis, is one of the most severe diseases of apple worldwide. It is the most studied plant-pathogen interaction involving a woody species using modern genetic, genomic, proteomic and bioinformatic approaches in both species. Although 'Geneva' apple was recognized long ago as a potential source of resistance to scab, this resistance has not been characterized previously. Differential interactions between various monoconidial isolates of V. inaequalis and six segregating F1 and F2 populations indicate the presence of at least five loci governing the resistance in 'Geneva'. The 17 chromosomes of apple were screened using genotyping-by-sequencing, as well as single marker mapping, to position loci controlling the V. inaequalis resistance on linkage group 4. Next, we fine mapped a 5-cM region containing five loci conferring both dominant and recessive scab resistance to the distal end of the linkage group. This region corresponds to 2.2 Mbp (from 20.3 to 22.5 Mbp) on the physical map of 'Golden Delicious' containing nine candidate nucleotide-binding site leucine-rich repeat (NBS-LRR) resistance genes. This study increases our understanding of the complex genetic basis of apple scab resistance conferred by 'Geneva', as well as the gene-for-gene (GfG) relationships between the effector genes in the pathogen and resistance genes in the host.

Gene expression profiling by cDNA-AFLP reveals potential candidate genes for partial resistance of 'Président Roulin' against Venturia inaequalis.

BACKGROUND: Scab, caused by the fungus Venturia inaequalis, is one of the most important diseases of cultivated apple. While a few scab resistance genes (R genes) governing qualitative resistance have been isolated and characterized, the biological roles of genes governing quantitative resistance, supposed to be more durable, are still unknown. This study aims to investigate the molecular mechanisms involved in the partial resistance of the old Belgian apple cultivar 'Président Roulin' against V. inaequalis. RESULTS: A global gene expression analysis was conducted in 'Président Roulin' (partially resistant) and in 'Gala' (susceptible) challenged by V. inaequalis by using the cDNA-AFLP method (cDNA-Amplified Fragment Length Polymorphism). Transcriptome analysis revealed significant modulation (up- or down-regulation) of 281 out of approximately 20,500 transcript derived fragments (TDFs) in 'Président Roulin' 48 hours after inoculation. Sequence annotation revealed similarities to several genes encoding for proteins belonging to the NBS-LRR and LRR-RLK classes of plant R genes and to other defense-related proteins. Differentially expressed genes were sorted into functional categories according to their gene ontology annotation and this expression signature was compared to published apple cDNA libraries by Gene Enrichment Analysis. The first comparison was made with two cDNA libraries from Malus x domestica uninfected leaves, and revealed in both libraries a signature of enhanced expression in 'Président Roulin' of genes involved in response to stress and photosynthesis. In the second comparison, the pathogen-responsive TDFs from the partially resistant cultivar were compared to the cDNA library from inoculated leaves of Rvi6 (HcrVf2)-transformed 'Gala' lines (complete disease resistance) and revealed both common physiological events, and notably differences in the regulation of defense response, the regulation of hydrolase activity, and response to DNA damage. TDFs were in silico mapped on the 'Golden Delicious' apple reference genome and significant co-localizations with major scab R genes, but not with quantitative trait loci (QTLs) for scab resistance nor resistance gene analogues (RGAs) were found. CONCLUSIONS: This study highlights possible candidate genes that may play a role in the partial scab resistance mechanisms of 'Président Roulin' and increase our understanding of the molecular mechanisms involved in the partial resistance against apple scab.