The study, conducted in a controlled greenhouse environment, utilized four plant species from the family Amaranthaceae: two native to China, Alternanthera sessilis and Achyranthes bidentata, and two alien species introduced to the region, Celosia argentea and Amaranthus retroflexus. The experimental design crossed two levels of soil nutrient heterogeneity, homogeneous versus heterogeneous, with the presence or absence of a native generalist herbivore, the moth Spodoptera litura. Plants were grown either alone or in competitive pairs of one native and one alien species. In the heterogeneous soil treatment, nutrients were concentrated in small, discrete patches, simulating the fine-scale spatial variation commonly found in natural soils.

Contrary to the common expectation that plants benefit from nutrient heterogeneity by precisely targeting rich patches, the results indicated an overall negative effect of soil nutrient heterogeneity on the total biomass of both the native Alternanthera sessilis and the alien Celosia argentea. This suggests that the energetic costs of searching for and exploiting small, discrete nutrient patches may outweigh the benefits gained from increased nutrient uptake. The fine-scale nature of the heterogeneity created in the experiment, with patches only one centimeter in diameter, likely imposed a high metabolic burden on the plants, leading to reduced whole-plant growth compared to plants grown in homogeneous soils where nutrients were evenly distributed.

However, post-hoc comparisons uncovered species-specific patterns in how herbivory modified these responses. For the native plant Alternanthera sessilis, the lowest total biomass was observed in heterogeneous soils under herbivory. This plant exhibited clear root foraging behavior in the absence of herbivores, allocating more roots to nutrient-enriched patches. When herbivores were present, this foraging capacity appeared to be constrained, resulting in a dual penalty of reduced nutrient uptake and aboveground damage. In contrast, the alien plant Celosia argentea showed significant biomass differences between homogeneous and heterogeneous soils only in the absence of herbivores. Under herbivore pressure, the suppressive effect of soil heterogeneity on C. argentea was diminished, suggesting a different belowground strategy or greater tolerance to herbivore damage.

The belowground biomass analyses provided further insight into these contrasting responses. In the absence of herbivores, Alternanthera sessilis produced significantly greater root biomass in nutrient-enriched patches compared to low-nutrient patches, confirming its foraging behavior. However, under herbivory, this patch-specific root allocation was no longer significant, indicating that the plant could not maintain its foraging precision when under attack. Celosia argentea, on the other hand, did not show significant differences in root biomass between patch types under either herbivory condition. This lack of strong root foraging response may have limited the costs associated with exploiting fine-scale heterogeneity, allowing the alien plant to maintain growth even when herbivores were present.

These findings highlight the role of co-evolutionary history in shaping plant-herbivore-soil interactions. Native plants like Alternanthera sessilis may be more susceptible to the combined stresses of herbivory and the energetic demands of foraging in heterogeneous soils due to long-term co-evolution with native herbivores. Alien plants, such as Celosia argentea, may benefit from enemy release or greater physiological tolerance, allowing them to cope with herbivore damage while still navigating the challenges of patchy soils. The study suggests that local native generalist herbivores can alter the expression of soil nutrient heterogeneity effects in a species-specific manner, potentially influencing the competitive balance between native and alien plants.

Competition between native and alien plants also played a significant role in the experimental outcomes. When grown together, the aboveground biomass of both native plants was reduced compared to when they were grown alone, while the alien plants were unaffected by competition with natives. This asymmetry in competitive effects is consistent with previous studies showing that alien plants often possess traits that allow them to outcompete native species, such as rapid growth rates or allelopathic capabilities. However, the study found no evidence that competition modified the effects of soil nutrient heterogeneity or herbivory, suggesting that these factors operate independently of interspecific competition in this context.

The magnitude of the effects varied considerably across species and treatments. Competition-induced reductions in aboveground growth were substantial for the native plants, with Alternanthera sessilis experiencing a 49.6 percent reduction and Achyranthes bidentata a 62.2 percent reduction. In contrast, heterogeneity-induced reductions were smaller, around 31 to 33 percent for the affected species. Herbivory had a more variable impact, reducing growth in Alternanthera sessilis by 14.3 percent but actually increasing growth in Achyranthes bidentata by 25.1 percent. These results underscore the complexity of plant responses to multiple environmental stressors and the importance of considering species identity when predicting ecological outcomes.

The study concludes that soil nutrient heterogeneity and native generalist herbivores jointly shape the performance of native and alien plants in a species-specific manner. The negative effects of fine-scale nutrient heterogeneity on plant biomass can be modulated by herbivory, with native and alien plants responding differently due to variations in their foraging strategies and tolerance to herbivore damage. These findings have important implications for understanding plant invasions and ecosystem dynamics, as they suggest that the impact of soil heterogeneity on alien plant success may be contingent on the presence of native herbivores. Further research is needed to explore how these interactions scale up to natural ecosystems and how they may be influenced by other environmental factors.

As climate change and land-use alterations continue to modify soil nutrient patterns and herbivore communities, understanding these intricate interactions will be crucial for predicting and managing plant community composition. The study provides valuable insights into the mechanisms underlying plant responses to patchy soils and herbivory, highlighting the need for integrated approaches that consider belowground and aboveground processes simultaneously. By revealing the species-specific nature of these responses, the research opens new avenues for investigating the role of soil-herbivore-plant interactions in shaping biodiversity and ecosystem function.

Subject of Research: Effects of herbivory on plant responses to soil nutrient heterogeneity

Article Title: Contrasting effects of herbivory on responses of a native and an alien plant to soil nutrient heterogeneity

Article References: Ou-Yang, K., Na, X.-Y., Wu, Y.-N., Huang, L., Xue, W., & Yu, F.-H. (2026). Contrasting effects of herbivory on responses of a native and an alien plant to soil nutrient heterogeneity. NeoBiota, 106, 91-105. https://doi.org/10.3897/neobiota.106.180549

Image Credits: AI Generated

DOI: 10.3897/neobiota.106.180549

Keywords: soil nutrient heterogeneity, herbivory, alien plants, native plants, root foraging, plant competition, Spodoptera litura, ecosystem dynamics, NeoBiota, plant performance, Contrasting, effects

The research team, led by scientists at Nanjing Agricultural University together with collaborators in Kenya, focused on IR75q.2, a member of the IR75 clade, in the cotton leafworm Spodoptera litura and the beet armyworm Spodoptera exigua. Both species are major agricultural pests with broad host ranges and formidable dispersal abilities, making them priority targets for novel pest management strategies. IR75q.2 had previously been characterized in a third species, the fall armyworm Spodoptera frugiperda, where it was shown to mediate avoidance of nonanoic acid. The central question of the new work was whether this ligand recognition profile, and the structural features underlying it, have been preserved across related moth lineages or whether evolutionary divergence has reshaped them.

To answer that question, the researchers assembled a multi-pronged analytical pipeline that combined phylogenetics, expression profiling, electrophysiology, structural modeling, molecular docking, and site-directed mutagenesis. Phylogenetic analysis of IR75 amino acid sequences from three Spodoptera species and two additional noctuid moths, Agrotis segetum and Helicoverpa armigera, revealed that the IR75 receptors form a highly conserved clade. Motif analysis using the MEME program identified ten conserved motifs arranged in nearly identical order across the sequences, with statistical significance values so extreme that the probability of chance occurrence fell below ten to the power of minus 170. This remarkable sequence conservation suggested that members of the IR75 clade share structural features tied to ligand recognition.

Transcriptome data added a functional dimension to this conservation story. Among all IR75 genes in the two Spodoptera species, IR75q.2 exhibited the highest expression in the antennae of both sexes, with no significant differences between males and females. Because antennae are the primary olfactory organs, this expression pattern pointed to an important role in odor detection. Sequence comparisons showed that the SlitIR75q.2 and SexiIR75q.2 proteins share 90.65 percent amino acid identity, both genes contain eleven exons, and both proteins possess three transmembrane domains. Evolutionary analysis using the PAML package revealed that while selective pressures vary across branches, all omega values were below one, indicating strong purifying selection has preserved the function of these orthologs over evolutionary time.

The functional core of the study came from experiments using the Xenopus oocyte expression system with two-electrode voltage clamp recording. The team co-expressed each IR75q.2 ortholog with its obligatory co-receptor IR8a and tested 59 odorant compounds in three mixtures. Both receptors responded primarily to just six compounds: three medium-chain fatty acids, namely octanoic acid, nonanoic acid, and decanoic acid, and their corresponding aldehydes, octanal, nonanal, and decanal. Nonanoic acid was by far the strongest ligand for both receptors, eliciting peak currents of 491.61 nanoamperes in SlitIR75q.2 and 347.70 nanoamperes in SexiIR75q.2. Dose-response assays yielded half-maximal effective concentrations of 7.596 times ten to the minus four molar for the cotton leafworm receptor and 4.052 times ten to the minus four molar for the beet armyworm receptor, confirming nearly identical sensitivities.

To probe the structural basis of this shared tuning, the researchers predicted three-dimensional structures of both receptors using AlphaFold 3.0 and docked nonanoic acid into the binding pockets using AutoDock Vina. The predicted binding energies were minus 6.04 kilocalories per mole for SlitIR75q.2 and minus 4.93 kilocalories per mole for SexiIR75q.2. In both receptors, a phenylalanine and an arginine residue within the S1 domain of the ligand-binding domain were predicted to form hydrogen bonds with the ligand: Phe285 and Arg290 in SlitIR75q.2, and Phe284 and Arg289 in SexiIR75q.2. Ten amino acid residues within four angstroms of nonanoic acid were conserved between the two receptors, underscoring the preservation of the binding pocket.

Site-directed mutagenesis put these structural predictions to a rigorous test. Substituting either Phe285 or Arg290 with alanine in SlitIR75q.2 completely abolished the response to nonanoic acid, preventing any estimate of sensitivity. In SexiIR75q.2, single alanine substitutions of Phe284 or Arg289 did not eliminate responses but increased the half-maximal effective concentration roughly four to five-fold, from 1.38 times ten to the minus four molar in the intact receptor to 6.3 and 6.2 times ten to the minus four molar in the mutants. These results establish Phe and Arg as conserved key residues for nonanoic acid recognition and suggest that subtle changes in binding-pocket chemistry can modulate ligand sensitivity across species. Docking of the remaining five ligands showed the same hydrogen-bonding anchors plus conserved hydrophobic contacts involving residues such as Ile230/229, Trp257/256, and Pro282/282.

The physiological findings translated into behavior for at least one of the two species. Electroantennogram recordings demonstrated that adults of both Spodoptera litura and Spodoptera exigua responded in a dose-dependent manner to nonanoic acid and the other five ligands, confirming that the antennal sensory system detects these compounds. In Y-tube olfactometer assays, nonanoic acid significantly repelled S. litura adults at doses of 50 and 100 micrograms. Behavioral testing of S. exigua proved unreliable because the beet armyworm adults showed extremely low locomotor activity in the olfactometer, but the electrophysiological evidence indicates the compound is detected by this species as well. Together, the behavioral and physiological data align with the earlier finding in the fall armyworm, reinforcing the idea that nonanoic acid serves as a conserved aversive cue across Spodoptera moths.

The broader evolutionary implications of the study are considerable. The ligands detected by IR75q.2 overlap partly with those sensed by certain odorant receptors, and nonanal in particular is known to attract females of S. frugiperda and S. litura through a conserved odorant receptor lineage. The authors propose that odorant receptors and ionotropic receptors do not simply duplicate one another but instead provide complementary information: the former tuned largely to neutral plant volatiles, the latter to acids, amines, and other polar ecological cues. This parallel coding may help moths discriminate chemically related compounds with different behavioral valences and broaden their detectable chemical space. At the same time, comparisons with the related IR75q.1 group, which prefers the aversive compound octanoic acid, suggest that subfunctionalization after gene duplication has shaped the ligand preferences of acid-sensing IRs in Lepidoptera. By pinpointing conserved binding residues and demonstrating functional conservation across two major pests, the study provides a molecular framework both for understanding how olfactory systems evolve and for designing targeted behavioral interventions, such as repellent formulations built around nonanoic acid analogs, that could exploit this preserved sensory channel in the fight against some of the world’s most destructive crop pests.

Subject of Research: Functional conservation of the ionotropic receptor IR75q.2 in detecting volatile acids and aldehydes across two Spodoptera moth species

Article Title: Functional conservation of IR75q.2 in the recognition of volatile acids and aldehydes in two Spodoptera species

Article References: Guo, J.-M., Wang, J.-X., He, Y., Luan, X.-P., Wei, Z.-Q., Liu, X.-L., Obiero, G. F., Yan, Q., Dong, S.-L., & Zhang, J. (2026). Functional conservation of IR75q.2 in the recognition of volatile acids and aldehydes in two Spodoptera species. Crop Health, 4(1), Article 20. https://doi.org/10.1007/s44297-026-00082-7

Image Credits: AI Generated

DOI: 10.1007/s44297-026-00082-7

Keywords: ionotropic receptors, IR75q.2, Spodoptera litura, Spodoptera exigua, nonanoic acid, insect olfaction, chemosensory receptors, Xenopus oocyte expression, molecular docking, site-directed mutagenesis, purifying selection, pest management