Mohammad Reza Atighi

Phytohormones play essential roles in plant–nematode interactions through complex crosstalk. Although these hormones often accumulate in nematode-resistant plants, the roles of abscisic acid (ABA) and ethylene (ET) in rice (Oryza sativa) resistance to the root-knot nematode (RKN) Meloidogyne graminicola (Mg) remain unclear, particularly regarding concentration dependency and underlying mechanisms. Using exogenous hormone gradient treatments, we show that only high concentrations of ABA (200 µM) and ET-releasing compound ethephon (Eth, 500 µM) induce systemic nematode resistance. High-concentration ET triggers endogenous systemic accumulation of ET and jasmonic acid (JA), accompanied by transient suppression followed by delayed accumulation of ABA, and induces JA-, ABA-, and salicylic acid (SA)-associated transcriptional responses. Exogenous ABA leads to endogenous ABA and SA accumulation and increased expression of related genes, while it suppresses ET biosynthesis gene expression and levels, highlighting a negative feedback effect of ABA on ET. Both hormones converge on a common mitogen-activated protein kinase 5 (OsMPK5)-dependent transcriptional and translational module. Low doses of ABA (50 µM) failed to activate this module and induced plant susceptibility, highlighting a threshold requirement for immune activation. Offspring of rice plants treated bi-weekly with high doses of ABA or ET were less susceptible to nematodes. This intergenerational acquired resistance was also OsMPK5-dependent. Our findings reveal concentration-dependent systemic effects of ABA and ET, whereby high-dose ABA and ET converge on OsMPK5 to reprogram translation and defense gene expression, underpinning both immediate and heritable resistance to root-knot nematodes.

Intergenerational acquired resistance (IAR) occurs when offspring inherit enhanced defenses or tolerance to stresses that have been encountered by the parent plants. This study confirms that infection of rice by the root-knot nematode Meloidogyne graminicola leads to IAR in the offspring. Genome-wide and targeted gene expression analyses demonstrated that offspring of nematode-infected plants are better prepared to fight against such attack through ‘spring loading’ of hormone-related plant defense genes. These genes are suppressed under basal conditions in IAR plants but show a more dramatic induction upon nematode attack. ChIP-sequencing of the offspring of IAR versus naive plants indicated that histone modifications are involved in the spring-loaded expression pattern. We detected, enrichment of H3K4me3 on defense-related genes and H3K27me3 on development-related genes in the roots of IAR plants. Detailed bioinformatic analyses indicated significant epigenetic changes to the abscisic acid, ethylene, and Mitogen Activated Protein Kinase (MAPK) signalling pathways in the offspring of nematode-infected plants. A rice line with reduced activity of MPK5 was found to be deficient in defense spring loading and lacked the IAR phenotype. Our results demonstrate that histone modifications act as carriers of stress memory for infection by root-knot nematodes in rice and identify MPK5 as one of the drivers for inherited resistance.

Summary: A population of a sphaerularioid nematode resembling the free-living mycetophagous stage of three genera, Deladenus , Hexatylus and Rubzovinema , was recovered from a soil sample collected from the Shur Su (Salty Water) spring region in the city of Maragheh, East Azarbaijan province, Iran. It was typologically compared with the free-living stage of all known species under the three aforementioned genera and was assigned to the genus Deladenus as a new member. The new species was described and illustrated based upon morphological, morphometric and molecular characteristics of free-living females. It is mainly characterised by having a pharyngo-intestinal junction anterior to the nerve ring, just behind the subventral gland orifices, and a secretory-excretory pore between the pharyngo-intestinal junction and hemizonid. Deladenus maraghensis n. sp. is further characterised by a medium body length of females (741-887 μ m), having six lines in the lateral fields, short stylet (6.5-7.0 μ m), vulva at 90.4-92.0%, post-vulval uterine sac (PUS) absent, small lateral vulval flaps present and a 32-38 μ m long conical tail with a rounded tip. The morphological comparisons of the new species with relevant species are discussed. In molecular phylogenetic analyses, sequences of small subunit (SSU) and D2-D3 expansion segments of large subunit (LSU) ribosomal DNA were used and the inferred topologies are discussed.

A population of Deladenus, representing a new species, was recovered from a rhizospheric soil sample of a walnut tree collected in the forests of the city of Ramian, northeastern Iran. D. ramianensis sp. nov. is characterized by a moderately long body (872–1034 µm), a long distance from anterior end to dorsal pharyngeal gland end (255–288 µm), pharyngo-intestinal junction at nerve ring level, small lateral vulval flaps, absence of postvulval uterine sac (PUS) and tail short, conical, tapering uniformly towards the sharp tip. Additionally, it has a small-sized stylet (8.5 µm) with three knobs, a secretory-excretory pore (S-E pore) located posterior to nerve ring and behind hemizonid and seven lines in the lateral fields. The new species was compared with mycetophagous generation of all species of the genera Hexatylus, Deladenus and Rubzovinema. Based upon the position of the hemizonid, S-E pore, seven lines in the lateral fields, no pharyngeal chamber, no PUS and conical tail with the sharp tip, it is comparable to 12 species of the genus Deladenus, namely D. aridus, D. bonabensis, D. brevis, D. cocophilus, D. durus, D. parvus, D. pakistanensis, D. posteroporus, D. albizicus, D. apopkaetus, D. persicus and D. processus. In the molecular phylogenetic analyses using small and large subunits of ribosomal DNA (SSU and LSU D2-D3 rDNA) sequences, the newly generated SSU sequence of the new species formed a maximally supported clade with the SSU sequence of an unidentified tylenchid species and D. bonabensis. In the LSU phylogeny, the newly generated sequence of the new species formed a clade with the D2-D3 sequences of a sphaerularioid sp. and D. bonabensis.

Summary: Plectonchus zondagi is redescribed based on a population found in northern Iran. New morphological characterisations based on light and scanning electron microscopy are provided. Also, molecular analyses based on near full-length of small subunit and partial large subunit (SSU and LSU D2-D3) rDNA are provided. The recovered population of this species is characterised by having lips fused in pairs, amphidial openings posterior to frontal end, stoma with large cheilo-gymnostom, pharynx with scarcely swollen metacorpus, female reproductive system prodelphic, absence of post-vulval uterine sac, male with angular spicules and sigmoid gubernaculum and tail in both sexes conoid with acute tip bearing a refringent cuticular ring between the attached and detached parts of the tail cuticle. Phylogenetic analyses based on SSU and D2-D3 fragments of LSU rDNA showed that P. zondagi was nested within the family Brevibuccidae; however, additional sequences from valid species of the genus are required to confirm its classification within this family, as morphological data suggest.