Sublethal fluazaindolizine doses inhibit development of the cyst nematode Heterodera schachtii during sedentary parasitism.

BACKGROUND: Fluazaindolizine is a new compound for the control of plant-parasitic nematodes (PPNs) with an unknown and novel mode-of-action. This compound is very effective against important PPNs. However, investigations elucidating the impact of sublethal fluazaindolizine doses on early nematode virulence and plant-nematode interaction parameters are lacking. RESULTS: The effect of direct exposure of Heterodera schachtii juveniles to 50 ppm fluazaindolizine was negligible. Infection assays revealed a 57% reduction in adult females at 1.25 ppm and a 46% reduction in offspring at 40 ppm when juveniles were soaked in the compound for 48 h and subsequently inoculated onto Arabidopsis thaliana. Pre-incubation of A. thaliana roots with fluazaindolizine was not effective against H. schachtii. Conversely, supplementing the plant growth medium with fluazaindolizine led to a significant reduction of adults (-35%), females (-75%) and female size at 1.25 ppm and nearly completely inhibited nematode parasitism at 5 ppm. The impact of fluazaindolizine on A. thaliana was dependent on plant age, compound concentration and duration of contact. Very low sublethal fluazaindolizine concentrations, 5 or 10 ppm, did not interfere with nematode mobility, host finding, penetration, and induction of the feeding site, but specifically inhibited sedentary nematode development inside the root in a concentration-dependent manner. CONCLUSION: Fluazaindolizine does not have direct toxicity against PPN infective juveniles, but has a clear effect on nematodes during sedentary development. The formation of females and the development of offspring are strongly reduced. It will be interesting to identify the underlying mechanism in the future. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Single-cell damage elicits regional, nematode-restricting ethylene responses in roots.

Plants are exposed to cellular damage by mechanical stresses, herbivore feeding, or invading microbes. Primary wound responses are communicated to neighboring and distal tissues by mobile signals. In leaves, crushing of large cell populations activates a long-distance signal, causing jasmonate production in distal organs. This is mediated by a cation channel-mediated depolarization wave and is associated with cytosolic Ca2+ transient currents. Here, we report that much more restricted, single-cell wounding in roots by laser ablation elicits non-systemic, regional surface potential changes, calcium waves, and reactive oxygen species (ROS) production. Surprisingly, laser ablation does not induce a robust jasmonate response, but regionally activates ethylene production and ethylene-response markers. This ethylene activation depends on calcium channel activities distinct from those in leaves, as well as a specific set of NADPH oxidases. Intriguingly, nematode attack elicits very similar responses, including membrane depolarization and regional upregulation of ethylene markers. Moreover, ethylene signaling antagonizes nematode feeding, delaying initial syncytial-phase establishment. Regional signals caused by single-cell wounding thus appear to constitute a relevant root immune response against small invaders.

Re-targeting of a plant defense protease by a cyst nematode effector.

Plants mount defense responses during pathogen attacks, and robust host defense suppression by pathogen effector proteins is essential for infection success. 4E02 is an effector of the sugar beet cyst nematode Heterodera schachtii. Arabidopsis thaliana lines expressing the effector-coding sequence showed altered expression levels of defense response genes, as well as higher susceptibility to both the biotroph H. schachtii and the necrotroph Botrytis cinerea, indicating a potential suppression of defenses by 4E02. Yeast two-hybrid analyses showed that 4E02 targets A. thaliana vacuolar papain-like cysteine protease (PLCP) 'Responsive to Dehydration 21A' (RD21A), which has been shown to function in the plant defense response. Activity-based protein profiling analyses documented that the in planta presence of 4E02 does not impede enzymatic activity of RD21A. Instead, 4E02 mediates a re-localization of this protease from the vacuole to the nucleus and cytoplasm, which is likely to prevent the protease from performing its defense function and at the same time, brings it in contact with novel substrates. Yeast two-hybrid analyses showed that RD21A interacts with multiple host proteins including enzymes involved in defense responses as well as carbohydrate metabolism. In support of a role in carbohydrate metabolism of RD21A after its effector-mediated re-localization, we observed cell wall compositional changes in 4E02 expressing A. thaliana lines. Collectively, our study shows that 4E02 removes RD21A from its defense-inducing pathway and repurposes this enzyme by targeting the active protease to different cell compartments.

Parasitic nematodes manipulate plant development to establish feeding sites.

Cyst and root-knot nematodes, the two economically most important groups of plant parasitic nematodes, induce neoplastic feeding sites in the roots of their host plants. The formation of feeding sites is accompanied by large-scale transcriptomic, metabolomic, and structural changes in host plants. However, the mechanisms that lead to such remarkable changes have remained poorly understood until recently. Now, genomic and genetic analyses have greatly enhanced our understanding of all aspects of plant-nematode interaction. Here, we review some of the recent advances in understanding cyst and root-knot nematode parasitism. In particular, we highlight new findings on the role of plant hormones and small RNAs in nematode feeding site formation and function. Finally, we touch on our emerging understanding of the function of nematode-associated secretions.

Starch as a sugar reservoir for nematode-induced syncytia.

The plant parasitic nematode Heterodera schachtii invades the roots of Arabidopsis thaliana to induce nematode feeding structures in the central cylinder. During nematode development, the parasites feed exclusively from these structures. Thus, high sugar import and specific sugar processing of the affected plant cells is crucial for nematode development. In the present work, we found starch accumulation in nematode feeding structures and therefore studied the expression genes involved in the starch metabolic pathway. The importance of starch synthesis was further shown using the Atss1 mutant line. As it is rather surprising to find starch accumulation in cells characterised by a high nutrient loss, we speculate that starch serves as long- and short-term carbohydrate storage to compensate the staggering feeding behaviour of the parasites.

Expansins are among plant cell wall modifying agents specifically expressed during development of nematode-induced syncytia.

Cyst nematodes are economically important pests. As obligatory biotrophic endoparasites they invade host roots and induce formation of syncytia, structures that serve them as the only source of nutrients. During syncytium development, extensive cell wall modifications take place. Cell wall dissolution occurs during cell wall opening formation, cell walls expand during hypertrophy of syncytial elements and local cell wall synthesis leads to the thickening of syncytial cell wall and the formation of cell wall ingrowths. Numerous studies revealed that nematodes change expression of plant genes encoding cell wall modifying proteins including expansins. Expansins poses unique abilities to induce cell wall extension in acidic pH. Recently, we demonstrated that two alpha-expansin genes LeEXPA4 and LeEXPA5 are upregulated in tomato roots infected with potato cyst nematode (Globodera rostochiensis). In this addendum, we present the most recent results concerning involvement of plant cell wall modifying genes in syncytium development and discuss possible practical applications of this knowledge for developing plants with resistance against nematodes.

Expanding nematode-induced syncytia: the role of expansins.

Parasitism of the cyst nematode Heterodera schachtii is characterised by the formation of syncytial feeding structures within the host root. Syncytia are formed by the fusion of root cells accompanied by local cell wall degradation, fusion of protoplasts and hypertrophy. Expansins are cell wall-loosening proteins involved in growth and cell wall disassembly. In this study we analysed whether members of the expansin gene family are specifically and developmentally regulated during syncytium formation in roots of Arabidopsis thaliana. By screening of a cDNA library of syncytia for expansin transcripts several AtEXPAs and one AtEXPB were found. In a semi-quantitative RT-PCR and a Genechip analysis AtEXPA3, AtEXPA6, AtEXPA8, AtEXPA10 and AtEXPA16 were found to be up-regulated specifically in syncytia but not to be transcribed in surrounding root tissue. Histological analyses including analysis of promoter::GUS lines and in situ RT-PCR supported the specific expression pattern. Among the specifically expressed genes, AtEXPA3 and AtEXPA16 turned out to be of special interest as they are shoot-specific in un-infected plants. We conclude that a number of expansins take part in cell growth and cell wall disassembly during syncytium formation.