Characterization of the grape powdery mildew genetic resistance loci in Muscadinia rotundifolia Trayshed

Muscadinia rotundifolia cv. Trayshed is a valuable source of resistance to grape powdery mildew (PM). It carries two PM resistance (R) loci, Run1.2 on chromosome 12 and Run2.2 on chromosome 18. This study identified the nucleotide-binding leucine-rich repeat (NLR) genes composing each R locus and their associated defense mechanisms. Evaluation of PM disease development showed that introgression of both loci confers resistance to PM in a V. vinifera background, but with varying speed and intensity of the response. To better understand the effect of NLR composition on PM resistance, both haplotypes of each R locus were reconstructed and the gene models within each haplotype were manually refined. We found that the number and classes of NLR genes differed between Run1.2 and Run2.2 loci and between the haplotypes of each R locus. In addition, NLR genes composing Run1.2b or Run2.2 loci exhibited different levels of gene expression, pointing to candidate NLR genes responsible for PM resistance in Trayshed. Finally, a transcriptomic analysis that included six additional R loci showed differences in the defense mechanisms associated with Run1.2b and Run2.2 in response to PM and at constitutive level. Altogether, our results reveal that Trayshed's R loci are composed of distinct NLRs that trigger different plant defense mechanisms in response to PM and at constitutive level, which would explain the variation of pathogen restriction between the two loci.

The Tomato Interspecific NB-LRR Gene Arsenal and Its Impact on Breeding Strategies

Tomato (Solanum lycopersicum L.) is a model system for studying the molecular basis of resistance in plants. The investigation of evolutionary dynamics of tomato resistance (R)-loci provides unique opportunities for identifying factors that promote or constrain genome evolution. Nucleotide-binding domain and leucine-rich repeat (NB-LRR) receptors belong to one of the most plastic and diversified families. The vast amount of genomic data available for Solanaceae and wild tomato relatives provides unprecedented insights into the patterns and mechanisms of evolution of NB-LRR genes. Comparative analysis remarked a reshuffling of R-islands on chromosomes and a high degree of adaptive diversification in key R-loci induced by species-specific pathogen pressure. Unveiling NB-LRR natural variation in tomato and in other Solanaceae species offers the opportunity to effectively exploit genetic diversity in genomic-driven breeding programs with the aim of identifying and introducing new resistances in tomato cultivars. Within this motivating context, we reviewed the repertoire of NB-LRR genes available for tomato improvement with a special focus on signatures of adaptive processes. This issue is still relevant and not thoroughly investigated. We believe that the discovery of mechanisms involved in the generation of a gene with new resistance functions will bring great benefits to future breeding strategies.

In-silico identification and differential expression of putative disease resistance-related genes within the collinear region of Brassica napus blackleg resistance locus LepR2’ in Brassica oleracea

Blackleg disease, caused by Leptosphaeria maculans, greatly affects the production of cabbage (Brassica oleracea). However, definitive R-gene(s) are yet to be identified in this crop. In contrast, a number of R-loci have been identified in A- or B-genome crops. Identification of few resistant cabbage genotypes indicates the presence of R-genes in this C-genome crop. High ancestral synteny between Brassica genomes suggests that the collinear regions of known A- or B-genome R-loci may also contain functional R-genes in the C-genome. Strong resistance was observed in the cotyledons of cabbage inbred line SCNU-98 against two L. maculans isolates, 03–02 s and 00–100 s. We investigated the collinear region of the Brassica napus blackleg resistance locus LepR2’ in B. oleracea since both isolates of L. maculans contain corresponding avirulence genes. The locus was collinear to a 5.8 Mbp genomic segment of B. oleracea chromosome C09 containing 13 genes that have putative disease resistance-related domains. High expression of genes Bo9g117290 and Bo9g111510 against isolate 00–100 s, and high expression of genes Bo9g126150 and Bo9g111490 against both isolates in the resistant-line SCNU-98 indicate their putative roles in blackleg resistance, which remained to be functionally verified. This work enhances our understanding of R-gene-mediated resistance to blackleg in cabbage.

R-Loci Arrangement Versus Downy and Powdery Mildew Resistance Level: A Vitis Hybrid Survey

For the viticulture of the future, it will be an essential prerequisite to manage grapevine diseases with fewer chemical inputs. The development and the deployment of novel mildew resistant varieties are considered one of the most promising strategies towards a sustainable viticulture. In this regard, a collection of 102 accessions derived from crossing Vitis hybrids with V. vinifera varieties was studied. In addition to the true-to-type analysis, an exhaustive genetic characterization was carried out at the 11 reliable mildew resistance (R) loci available in the literature to date. Our findings highlight the pyramiding of R-loci against downy mildew in 15.7% and against powdery mildew in 39.2% of the total accessions. The genetic analysis was coupled with a three-year evaluation of disease symptoms in an untreated field in order to assess the impact of the R-loci arrangement on the disease resistance degree at leaf and bunch level. Overall, our results strongly suggest that R-loci pyramiding does not necessarily mean to increase the overall disease resistance, but it guarantees the presence of further barriers in case of pathogens overcoming the first. Moreover, our survey allows the discovery of new mildew resistance sources useful for novel QTL identifications towards marker-assisted breeding.

Reaction of Trifolium semipilosum to four species of root-knot nematode (Meloidogyne spp)

The clover root-knot nematode, Meloidogyne trifoliophila (Mt), debilitates white clover (Trifolium repens) in New Zealand pastures. Genetic resistance (R) to Mt in white clover is complexly inherited and difficult to utilise in breeding programmes. Single-gene, dominant, and complete R to Mt has been identified at the TRKR locus in T. semipilosum. The current study aimed to characterise the T. semipilosum reaction to three additional root-knot nematode species, viz. M. hapla, M. javanica, and M. incognita and if resistant, to determine if TRKR was the R source. Tests were conducted in peat-based potting mix in a temperature-controlled glasshouse using a subset of Mt-resistant and Mt-susceptible plants from the full-sib T. semipilosum population used to map TRKR. Mt resistant and susceptible white clover plants were challenged with Mt, M. hapla, M. incognita, and M. javanica for comparison. Experiments confirmed that T. semipilosum was able to host the four Meloidogyne species. Some T. semipilosum plants exhibited R to M. javanica and M. incognita whereas all were uniformly susceptible to M. hapla. There were no correlated reactions among the full-sib plants to challenge by the four nematode species, suggesting the genetic factor(s) conferring R to M. javanica and M. incognita are independent of the TRKR locus. Furthermore, the R to Mt is complete, with no galls formed on challenged plants whereas the observed R to M. javanica and M. incognita is apparently partial. White clover plants either resistant or susceptible to Mt were uniformly susceptible to M. hapla and M. javanica. These experiments suggest that T. semipilosum is a source of multiple nematode R loci. We plan to characterise further nematode resistances at loci in Trifolium genomes, and to investigate transfer of R alleles from T. semipilosum to white clover and other economic species.

THE TRANSPOSABLE ELEMENT Ds AFFECTS THE PATTERN OF INTRAGENIC RECOMBINATION AT THE bz AND R LOCI IN MAIZE

ABSTRACT Insertion of the transposable element Ds into either the bz or R locus affects intragenic recombination in various ways. We have examined here one aspect of this problem; namely, the distribution of flanking markers among intragenic recombinations produced by different types of heterozygotes carrying Ds insertion mutations. Heteroallelic combinations of a Ds insertion mutation and a mutation borne on a structurally normal chromosome generate a majority of intragenic recombinants of a crossover type. In contrast to this, most intragenic recombinants obtained from heterozygotes between two different Ds insertion mutations have a parental arrangement of outside markers. Therefore, the resolution of the recombination intermediate would appear to depend on the nature of the mutations in the heterozygote.

DEVELOPMENTAL DIFFERENCES IN ACTION OF R AND B ALLELES IN MAIZE

Alleles of the B and R loci collected from different geographic races of maize differ with respect to concentration, pattern, and tissue specificity of anthocyanin formation. No differences were found between the pigments formed by B action and those formed by R action. The activities of four B alleles and five R alleles when compared in a common genetic background and described in terms of the whole life cycle show differences in the following respects: In a given tissue, for example the aleurone, there are differences in a) rate of activity, b) time of onset of activtiy, c) time that activity ceases. Apparent tissue specificity is shown when the development of a tissue coincides with the period during which the gene is active. True tissue specificity is shown by alleles that have different activities in tissues that develop at the same time. This latter type of specificity was shown only by alleles known to consist of more than one synaptically homologous region, or by those derived in some way from such complex alleles. It is suggested that for most genetic systems there probably exists potential for change in level of action, change in time of onset and termination of activity, and change toward tissue specificity. The type and extent of the change tolerated will depend on the system involved.