Biochemical and genetic studies of two Heterodera avenae resistance genes transferred from Aegilops ventricosa to wheat

Redox signalling in plant-nematode interactions: Insights into molecular crosstalk and defense mechanisms

Plant-parasitic nematodes, specifically cyst nematodes (CNs) and root-knot nematodes (RKNs), pose significant threats to global agriculture, leading to substantial crop losses. Both CNs and RKNs induce permanent feeding sites in the root of their host plants, which then serve as their only source of nutrients throughout their lifecycle. Plants deploy reactive oxygen species (ROS) as a primary defense mechanism against nematode invasion. Notably, both CNs and RKNs have evolved sophisticated strategies to manipulate the host's redox environment to their advantage, with each employing distinct tactics to combat ROS. In this review, we have focused on the role of ROS and its scavenging network in interactions between host plants and CNs and RKNs. Overall, this review emphasizes the complex interplay between plant defense mechanism, redox signalling and nematode survival tactics, suggesting potential avenues for developing innovative nematode management strategies in agriculture.

Identification of genomic regions associated with cereal cyst nematode (Heterodera avenae Woll.) resistance in spring and winter wheat

Cereal cyst nematode (CCN) is a major threat to cereal crop production globally including wheat (Triticum aestivum L.). In the present study, single-locus and multi-locus models of Genome-Wide Association Study (GWAS) were used to find marker trait associations (MTAs) against CCN (Heterodera avenae) in wheat. In total, 180 wheat accessions (100 spring and 80 winter types) were screened against H. avenae in two independent years (2018/2019 "Environment 1" and 2019/2020 "Environment 2") under controlled conditions. A set of 12,908 SNP markers were used to perform the GWAS. Altogether, 11 significant MTAs, with threshold value of -log10 (p-values) ≥ 3.0, were detected using 180 wheat accessions under combined environment (CE). A novel MTA (wsnp_Ex_c53387_56641291) was detected under all environments (E1, E2 and CE) and considered to be stable MTA. Among the identified 11 MTAs, eight were novel and three were co-localized with previously known genes/QTLs/MTAs. In total, 13 putative candidate genes showing differential expression in roots, and known to be involved in plant defense mechanisms were reported. These MTAs could help us to identify resistance alleles from new sources, which could be used to identify wheat varieties with enhanced CCN resistance.

Resistance to Heterodera filipjevi and H. avenae in Winter Wheat is Conferred by Different QTL

The coexistence of cereal cyst nematode (CCN) species Heterodera avenae and H. filipjevi, often involving multiple pathotypes, is a limiting factor for wheat production in China. Some of the known genes for resistance to CCN are not effective against both nematode species, hence complicating breeding efforts to develop CCN-resistant wheat cultivars. Here, we demonstrate that the CCN resistance in wheat cultivar Madsen to both Heterodera spp. is controlled by different genetic loci, both of which originated from Aegilops ventricosa. A new quantitative trait locus (QTL), QCre-ma7D, was identified and localized in a 3.77-Mb genomic region on chromosome arm 7DL, which confers resistance to H. filipjevi. QCre-ma2A on chromosome arm 2AS corresponds to CCN resistance gene Cre5 and confers resistance to H. avenae. This QTL is a new locus on chromosome arm 7DL and is designated Cre9. Three Kompetitive allele-specific PCR markers (BS00150072, BS00021745, and BS00154302) were developed for molecular marker-assisted selection of Cre9 and locally adapted wheat lines with resistance to both nematode species were developed. QCre-ma2A on chromosome arm 2AS corresponds to CCN resistance gene Cre5 and confers resistance to H. avenae. The identification of different loci underlying resistance to H. filipjevi and H. avenae and the development of adapted resistant entries will facilitate breeding of wheat cultivars that are resistant to these devastating nematodes in China.

Resistance to Cereal Cyst Nematodes in Wheat and Barley: An Emphasis on Classical and Modern Approaches

Cereal cyst nematodes (CCNs) are among the most important nematode pests that limit production of small grain cereals like wheat and barley. These nematodes alone are estimated to reduce production of crops by 10% globally. This necessitates a huge enhancement of nematode resistance in cereal crops against CCNs. Nematode resistance in wheat and barley in combination with higher grain yields has been a preferential research area for cereal nematologists. This usually involved the targeted genetic exploitations through natural means of classical selection breeding of resistant genotypes and finding quantitative trait luci (QTLs) associated with resistance genes. These improvements were based on available genetic diversity among the crop plants. Recently, genome-wide association studies have widely been exploited to associate nematode resistance or susceptibility with particular regions of the genome. Use of biotechnological tools through the application of various transgenic strategies for enhancement of nematode resistance in various crop plants including wheat and barley had also been an important area of research. These modern approaches primarily include the use of gene silencing, exploitation of nematode effector genes, proteinase inhibitors, chemodisruptive peptides and a combination of one or more of these approaches. Furthermore, the perspective genome editing technologies including CRISPR-Cas9 could also be helpful for improving CCN resistance in wheat and barley. The information provided in this review will be helpful to enhance resistance against CCNs and will attract the attention of the scientific community towards this neglected area.

Cereal Cyst Nematodes: A Complex and Destructive Group of Heterodera Species

Small grain cereals have served as the basis for staple foods, beverages, and animal feed for thousands of years. Wheat, barley, oats, rye, triticale, rice, and others are rich in calories, proteins, carbohydrates, vitamins, and minerals. These cereals supply 20% of the calories consumed by people worldwide and are therefore a primary source of energy for humans and play a vital role in global food and nutrition security. Global production of small grains increased linearly from 1960 to 2005, and then began to decline. Further decline in production is projected to continue through 2050 while global demand for these grains is projected to increase by 1% per annum. Currently, wheat, barley, and oat production exceeds consumption in developed countries, while in developing countries the consumption rate is higher than production. An increasing demand for meat and livestock products is likely to compound the demand for cereals in developing countries. Current production levels and trends will not be sufficient to fulfill the projected global demand generated by increased populations. For wheat, global production will need to be increased by 60% to fulfill the estimated demand in 2050. Until recently, global wheat production increased mostly in response to development of improved cultivars and farming practices and technologies. Production is now limited by biotic and abiotic constraints, including diseases, nematodes, insect pests, weeds, and climate. Among these constraints, plant-parasitic nematodes alone are estimated to reduce production of all world crops by 10%. Cereal cyst nematodes (CCNs) are among the most important nematode pests that limit production of small grain cereals. Heavily invaded young plants are stunted and their lower leaves are often chlorotic, forming pale green patches in the field. Mature plants are also stunted, have a reduced number of tillers, and the roots are shallow and have a "bushy-knotted" appearance. CCNs comprise a number of closely-related species and are found in most regions where cereals are produced.

Characterization of Resistance to the Cereal Cyst Nematode in the Soft White Winter Wheat 'Madsen'

The cereal cyst nematode (CCN) has a significant negative impact on production of wheat in China. The presence of pathotypes of both Heterodera avenae and H. filipjevi makes it necessary to identify genetic resources with a wide spectrum of resistance. Results of this study confirmed that the soft white winter wheat 'Madsen' was resistant to many different populations of both H. filipjevi and H. avenae in both naturally infested fields and artificial inoculation tests in China. Fewer juvenile nematodes penetrated roots of Madsen than susceptible 'Wenmai 19' in the early stages of the interaction between the nematodes and plant. Testing wheat cultivars in the pedigree of Madsen demonstrated that the CCN resistance of Madsen was inherited from 'VPM1' via the line 'VPM1/Moisson 951'. Presence of a 2NS chromosome segment from Aegilops ventricosa was detected in Madsen using a Vrga1D-specific marker. However, it appears that gene Pm4b for resistance to powdery mildew (caused by Blumeria graminis f. sp. tritici) was not transferred from VPM1 into Madsen because these cultivars had different reaction patterns against 20 B. graminis f. sp. tritici isolates from China. Madsen serves as an effective source of host resistance from damage caused by CCN.

Analysis of ascorbate peroxidase genes expressed in resistant and susceptible wheat lines infected by the cereal cyst nematode, Heterodera avenae

Changes in ascorbate peroxidase (APX) enzyme activity in response to nematode (Heterodera avenae) attack were studied in roots of three hexaploid wheat lines carrying Cre2, Cre5, or Cre7 nematode resistance genes and the susceptible Triticum aestivum cv. Anza. A spectrophotometric analysis was carried out with root extracts of infected plants 4, 7, 11, and 14 days after nematode inoculation using uninfected plant as control. APX induction in infected resistant genotypes was similar and higher than in the susceptible control. The introgression wheat/Aegilops ventricosa H-93-8 line, carrying the Cre2 gene, and its parental line H-10-15 as susceptible control were used to analyze whether this increase of activity was correlated with the induction of APX gene expression. Genes encoding cytosolic forms of APX were induced in roots of both lines in response to nematode infection. This induction took place both earlier and with greater intensity in the resistant line than in the susceptible one, and it was also higher in the root area at the site of nematode attachment.

Analysis of class III peroxidase genes expressed in roots of resistant and susceptible wheat lines infected by Heterodera avenae

The response of resistant wheat-Aegilops ventricosa introgression line H-93-8 and its susceptible parent, Triticum aestivum H-10-15, to Ha71 Spanish population of Heterodera avenae was studied to determine the changes in peroxidase gene expression during incompatible and compatible wheat-nematode interactions. Twenty peroxidase genes were characterized from both 211 expressed sequence tags and 259 genomic DNA clones. Alignment of deduced amino acid sequences and phylogenetic clustering with peroxidases from other plant species showed that these enzymes fall into seven different groups (designated TaPrx108 to TaPrx114) which represent peroxidases secreted to the apoplast by a putative N-terminal peptide signal. TaPrx111, TaPrx112, and TaPrx113 were induced by nematode infection in both genotypes but with differing magnitude and timing. TaPrx112 and TaPrx113 groups increased more in resistant than in susceptible infected lines. In addition, in situ hybridization analyses of genes belonging to TaPrx111, TaPrx112, and TaPrx113 groups revealed a more intense signal in cells close to the vascular cylinder and parenchyma vascular cells of resistant than susceptible wheat when challenged by nematodes. These data seem to suggest that wheat apoplastic peroxidases, because of their different expression in quantity and timing, play different roles in the plant response to nematode infection.

Cereal cyst nematode resistance conferred by the Cre7 gene from Aegilops triuncialis and its relationship with Cre genes from Australian wheat cultivars

Cereal cyst nematode (CCN; Heterodera avenae Woll.) is a root pathogen of cereal crops that can cause severe yield losses in wheat (Triticum aestivum). Differential host-nematode interactions occur in wheat cultivars carrying different CCN resistance (Cre) genes. The objective of this study was to determine the CCN resistance conferred by the Cre7 gene from Aegilops triuncialis in a 42-chromosome introgression line and to assess the effects of the Cre1, Cre3, Cre4, and Cre8 genes present in Australian wheat lines on Spanish pathotype Ha71. Inhibition of nematode reproduction was rank-ordered as Cre1 >or = Cre4 > or = Cre7 >> Cre8 > Cre3. Lines carrying Cre1, Cre4, or Cre7 exhibited a significantly higher level of resistance than those carrying Cre8 or Cre3. Allelism tests indicated that Cre7 segregated independently of Cre1 on chromosome 2BL and Cre4 on chromosome 2DL, and these genes could consistently be combined in the same genotype, inducing a more durable resistance. Tests to determine the chromosomal location of Cre7 using addition lines were inconclusive.

Wheat (Triticum aestivum) Is Susceptible to the Parasitic Angiosperm Striga hermonthica, a Major Cereal Pathogen in Africa

ABSTRACT Striga hermonthica is a parasitic weed endemic to sub-Saharan Africa. It most commonly parasitizes sorghum, maize, pearl millet, and upland rice, lowering yields and affecting the welfare of over 100 million people, principally subsistence farmers. Cereal crops with complete resistance to this pathogen have not been reported. In southern and eastern Africa, where Striga spp. are endemic, 5.6 million ha of wheat are cultivated annually. Despite this, there are only isolated field reports of wheat infected with Striga spp. It is not clear whether this is due to resistance in this cereal or to environmental factors. In this article, we examined the ability of root exudates from five cultivars of wheat (Chablis, Cadenza, Hereward, Riband, and Brigadier) to trigger germination of S. hermonthica seed. A study of the development of S. hermonthica on two cultivars of wheat (Hereward and Chablis) and on a range of ancestral relatives of wheat (Triticum and Aegilops spp.) then was conducted. Last, the effect of Striga spp. on host growth and yield was examined using wheat cv. Chablis and compared with that of a highly susceptible sorghum cultivar (CSH-1). Wheat was able to support the germination, attachment, and subsequent development of Striga spp. All wheat cultivars and ancestral species of modern wheat (Triticum and Aegilops spp.) were susceptible to S. hermonthica. In addition, in wheat, infection severely lowered plant height (-24%) and biomass accumulation (-33%); a small parasite biomass elicited a large host response. In conclusion, wheat is highly susceptible to S. hermonthica and, in light of global climate change, this may have implications for wheat-producing areas of Africa.

Root enzyme activities associated with resistance to Heterodera avenae conferred by gene Cre7 in a wheat/Aegilops triuncialis introgression line

The effect of Cereal cyst nematode (Heterodera avenae) infection on the expression of putative root defence-related enzymes, peroxidase (PER), esterase (EST) and superoxide dismutase (SOD), was studied in roots of a wheat/Aegilops triuncialis introgression line TR-3531 carrying the Cre7 resistance gene. We analysed detoxificant isozyme changes within roots of the resistant line and their susceptible parent (H-10-15) as a control, during the early interaction with the pathotype Ha71 of H. avenae. Isoelectrofocusing (IEF) isozyme analysis, four and seven days after infection, revealed that PER, EST and SOD activities increased in the resistant line TR-3531 in comparison with the susceptible control. Moreover, four and seven days after infection, the TR-3531 line showed the expression of new PER isozymes, with pIs of 9.7, 9.0, 8.5, 6.5 and 5.0, and an increased activity of some constitutive isoforms. The intensity of some EST and SOD constitutive bands increased in the resistant line after infection. However, no new isoforms were detected for EST and SOD systems. Nematode-induced enzyme activity was minor (PER) or did not occur (EST and SOD) in the compatible interaction with H-10-15. The enhanced peroxidase and esterase activities may play a role in the lignification of cell walls, which assists in the resistance to penetration by the nematode.