Phytonematodes as a limiting biotic factor of agricultural production in the world (review)

Purpose. To analyze domestic and foreign scientific literature on the species composition and harmfulness of the world's most dangerous parasitic species of phytonematodes in crops. Results. Today, according to the available literature, the most dangerous species of phytonematodes include: gall nematode (Meloidogyne spp.), cyst-forming nematode (Heterodera spp. and Globodera spp.), root lesion nematode (Pratylenchus spp.), banana drill nematode (Radoholus similis), stem nematode (Ditylenchus dipsaci), pine stem nematode (Bursaphelenchus xylophilus), reniform nematode (Rotylenchulus reniformis), xiphinema index (Xiphinema index), false head nematode (Nacobbus aberrans), and rice leaf nematode (Aphelenchoides besseyi). Conclusions. The results of research on the prevalence and harmfulness of parasitic nematode species in crops convince us of the need for a more detailed study of this group of microorganisms. Due to the rapid development of molecular genetic methods in the last decade, scientists have been able to expand and improve their knowledge of identifying species, races and pathotypes of phytonematodes, their biological and environmental characteristics, and most importantly, to discover and understand extremely complex mechanisms of parasite and host plants. Nematologists are confident that further research in these and other areas in the future will create a basis for developing a new strategy for long-term and environmentally safe control of these dangerous plant parasites.

Selected genotypes with the genetic background of Vitis aestivalis and Vitis labrusca are resistant to Xiphinema index

The ectoparasitic nematode Xiphinema index transmits grapevine fanleaf virus (GFLV) during feeding on grapevine roots, causing fanleaf degeneration in the plant. Hence, resistance breeding is a key to develop novel rootstocks to overcome such threats. In the past years, various grapevine species were screened, and a few candidates with partial resistance were identified. Yet, they were hardly sufficient for viticulture due to many agronomical defects. To develop reliably resistant rootstocks applicable in viticulture, multiple Vitis spp. genotypes were analyzed using root inoculation with nematodes in glass vials as an early and easy evaluation test. Resistance levels were evaluated 35 days after inoculation based on nematode reproduction factors focusing on juveniles and eggs. Infection of grapevines with GFLV was analyzed after inoculation with viruliferous X. index. With this fast screening system, putative candidates with resistances against X. index have been identified for future breeding programs. Particularly, genotypes with the genetic background of V. aestivalis and V. labrusca were found to be nematode resistant.

Evidence of sexual reproduction events in the dagger nematode Xiphinema index in grapevine resistance experiments under controlled conditions

The dagger nematode Xiphinema index has a major economic impact due to its transmission of Grapevine fanleaf virus (GFLV) to grapevines. This vector nematode, which was introduced into Western countries from the Middle East together with the domesticated grapevine, mostly reproduces by meiotic parthenogenesis, but microsatellite multilocus genotype (MLG) analysis has revealed the occurrence of rare sexual reproduction events in field conditions. In a previous six-year study under controlled conditions, we evaluated the durability of resistance (R) to X. index in accessions derived from a muscadine R source and reference accessions. In this previous study, we used an equal-proportion mixture of four lines (from Spain, Italy, Greece and Iran) representative of X. index diversity as the inoculum, and we collected random samples in three-, four-, five- and six-year-old vines. Here, we genotyped the individuals from these samples using MLG technique, and we analyzed the changes in line frequency and the occurrence of sexual reproduction events between lines over time. The nematode lines differed in aggressiveness and hybrids between lines were detected at a low, but apparently increasing rate. Hybridization events were recovered in all accessions, regardless of resistance status and propagation type. Finally, our data provide the first evidence of sexual reproduction in the nematode X. index under controlled conditions.

Grapevine Resistance to the Nematode Xiphinema index Is Durable in Muscadine-Derived Plants Obtained from Hardwood Cuttings but Not from In Vitro

Breeding for varieties carrying natural resistance (R) against plant-parasitic nematodes is a promising alternative to nematicide ban. In perennial crops, the long plant−nematode interaction increases the risk for R breaking and R durability is a real challenge. In grapevine, the nematode Xiphinema index has a high economic impact by transmitting Grapevine fanleaf virus (GFLV) and, to delay GFLV transmission, rootstocks resistant to this vector are being selected, using Muscadinia rotundifolia in particular as an R source. To optimize in fine this strategy, the durability has been studied under controlled conditions in F1 and BC1 muscadine-derived resistant accessions previously obtained from either hardwood-cutting or in vitro propagation. After inoculation with a mix, in equal proportions, of four lines representative of the X. index diversity, multiplication on plants has been monitored 3 to 6 years. The nematode reproduction factor remained lower than 1 in resistant plants obtained from hardwood cuttings while it increased at values far beyond 1 in resistant plants of in vitro origin. Data for nematode numbers per gram of roots mostly paralleled those obtained for the reproduction factor. The effect of the propagation type on resistance over years was also evaluated for the ratio female/juvenile and the frequency of males. Altogether our results illustrate that the muscadine-derived resistance based on hardwood cuttings is durable. By contrast, in resistant and reference accessions obtained from in vitro, our data suggest that the increased nematode multiplication might be mainly due to the modification of root architecture consecutive to this propagation method.

Characterization of the Biogenic Volatile Organic Compounds (BVOCs) and Analysis of the PR1 Molecular Marker in Vitis vinifera L. Inoculated with the Nematode Xiphinema index

Upon pathogen attack, plants very quickly undergo rather complex physico-chemical changes, such as the production of new chemicals or alterations in membrane and cell wall properties, to reduce disease damages. An underestimated threat is represented by root parasitic nematodes. In Vitis vinifera L., the nematode Xiphinema index is the unique vector of Grapevine fanleaf virus, responsible for fanleaf degeneration, one of the most widespread and economically damaging diseases worldwide. The aim of this study was to investigate changes in the emission of biogenic volatile organic compounds (BVOCs) in grapevines attacked by X. index. BVOCs play a role in plant defensive mechanisms and are synthetized in response to biotic damages. In our study, the BVOC profile was altered by the nematode feeding process. We found a decrease in β-ocimene and limonene monoterpene emissions, as well as an increase in α-farnesene and α-bergamotene sesquiterpene emissions in nematode-treated plants. Moreover, we evaluated the PR1 gene expression. The transcript level of PR1 gene was higher in the nematode-wounded roots, while in the leaf tissues it showed a lower expression compared to control grapevines.

Characterization of genetic determinants of the resistance to phylloxera, Daktulosphaira vitifoliae, and the dagger nematode Xiphinema index from muscadine background

Background: Muscadine (Muscadinia rotundifolia) is known as a resistance source to many pests and diseases in grapevine. The genetics of its resistance to two major grapevine pests, the phylloxera D. vitifoliae and the dagger nematode X. index, vector of the Grapevine fanleaf virus (GFLV) was investigated in a backcross progeny between the F1 resistant hybrid material VRH8771 (Muscadinia-Vitis) derived from the muscadine R source ‘NC184-4’ and V. vinifera cv. ‘Cabernet-Sauvignon’ (CS). Results: In this pseudo-testcross, parental maps were constructed using simple-sequence repeats markers and single nucleotide polymorphism markers from a GBS approach. For the VRH8771 map, 2271 SNP and 135 SSR markers were assembled, resulting in 19 linkage groups (LG) and an average distance between markers of 0.98 cM. Phylloxera resistance was assessed by monitoring root nodosity number in an in planta experiment and larval development in a root in vitro assay. Nematode resistance was studied using 10-12 month long tests for the selection of durable resistance and rating criteria based on nematode reproduction factor and gall index. A major QTL for phylloxera larval development, explaining more than 70% of the total variance and co-localizing with a QTL for nodosity number, was identified on LG 7. Additional QTLs were detected on LG 3 and LG 10, depending on the in planta or in vitro experiments, suggesting that various loci may influence or modulate nodosity formation and larval development. Using a Bulk Segregant Analysis approach and a proportion test, markers clustered in three regions on LG 9, LG 10 and LG 18 were shown to be associated to the nematode resistant phenotype. QTL analysis confirmed the results, although a LOD-score below the significant threshold value was obtained for the QTL on LG 18. Conclusions: Based on a high-resolution linkage map and a segregating grapevine backcross progeny, the first QTLs for resistance to Daktulosphaira vitifoliae and to Xiphinema index were identified from a muscadine source. All together these results open the way to the development of marker-assisted selection in grapevine rootstock breeding programs based on muscadine derived resistance to phylloxera and to X. index in order to delay GFLV transmission.

Characterization of genetic determinants of the resistance to phylloxera, Daktulosphaira vitifoliae, and the dagger nematode Xiphinema index from muscadine background

Background: Muscadine (Muscadinia rotundifolia) is known as a resistance source to many pests and diseases in grapevine. The genetics of its resistance to two major grapevine pests, the phylloxera D. vitifoliae and the dagger nematode X. index, vector of the Grapevine fanleaf virus (GFLV) was investigated in a backcross progeny between the F1 resistant hybrid material VRH8771 (Muscadinia-Vitis) derived from the muscadine R source ‘NC184-4’ and V. vinifera cv. ‘Cabernet-Sauvignon’ (CS). Results: In this pseudo-testcross, parental maps were constructed using simple-sequence repeats markers and single nucleotide polymorphism markers from a GBS approach. For the VRH8771 map, 2271 SNP and 135 SSR markers were assembled, resulting in 19 linkage groups (LG) and an average distance between markers of 0.98 cM. Phylloxera resistance was assessed by monitoring root nodosity number in an in planta experiment and larval development in a root in vitro assay. Nematode resistance was studied using 10-12 month long tests for the selection of durable resistance and rating criteria based on nematode reproduction factor and gall index. A major QTL for phylloxera larval development, explaining more than 70% of the total variance and co-localizing with a QTL for nodosity number, was identified on LG 7. Additional QTLs were detected on LG 3 and LG 10, depending on the in planta or in vitro experiments, suggesting that various loci may influence or modulate nodosity formation and larval development. Using a Bulk Segregant Analysis approach and a proportion test, markers clustered in three regions on LG 9, LG 10 and LG 18 were shown to be associated to the nematode resistant phenotype. QTL analysis confirmed the results, although a LOD-score below the significant threshold value was obtained for the QTL on LG 18. Conclusions: Based on a high-resolution linkage map and a segregating grapevine backcross progeny, the first QTLs for resistance to Daktulosphaira vitifoliae and to Xiphinema index were identified from a muscadine source. All together these results open the way to the development of marker-assisted selection in grapevine rootstock breeding programs based on muscadine derived resistance to phylloxera and to X. index in order to delay GFLV transmission.

Detection of Multiple Variants of Grapevine Fanleaf Virus in Single Xiphinema index Nematodes

Grapevine fanleaf virus (GFLV) is responsible for a widespread disease in vineyards worldwide. Its genome is composed of two single-stranded positive-sense RNAs, which both show a high genetic diversity. The virus is transmitted from grapevine to grapevine by the ectoparasitic nematode Xiphinema index. Grapevines in diseased vineyards are often infected by multiple genetic variants of GFLV but no information is available on the molecular composition of virus variants retained in X. index following nematodes feeding on roots. In this work, aviruliferous X. index were fed on three naturally GFLV-infected grapevines for which the virome was characterized by RNAseq. Six RNA-1 and four RNA-2 molecules were assembled segregating into four and three distinct phylogenetic clades of RNA-1 and RNA-2, respectively. After 19 months of rearing, single and pools of 30 X. index tested positive for GFLV. Additionally, either pooled or single X. index carried multiple variants of the two GFLV genomic RNAs. However, the full viral genetic diversity found in the leaves of infected grapevines was not detected in viruliferous nematodes, indicating a genetic bottleneck. Our results provide new insights into the complexity of GFLV populations and the putative role of X. index as reservoirs of virus diversity.