Riptortus pedestris (Fabricius) is a major agricultural pest feeding on soybean pods and seeds. The large populations occur during seed maturity stages from pod filling to harvest. Its infestation results in shriveled and dimpled seeds while vegetative structures (leaflet and stem) remain green, known as “Stay Green” syndrome. Additional evidence also demonstrates that soybean pods and seeds are required for Riptortus pedestris development. However, the feeding behavior strategies employed by this stink bug to feed on soybean plants are still not clear. In the present study, the feeding behaviors of R. pedestris on soybean plants were recorded by electropenetrography (EPG), and a waveform library was created for this species. A total of five phases of waveforms—nonprobing, pathway (Rp1), xylem sap ingestion (Rp2), salivation and ingestion (Rp3), and interruption (Rp4)—were identified. Non-probing waveforms Z and NP and pathway (Rp1) were found in all tested plant structures (leaflet, stem, cotyledon, and pods). Waveform Rp2 (xylem sap ingestion, xylem ingestion) was primarily recorded during R. pedestris feeding on leaflets and stems, while Rp3 (salivation/ingestion) was only observed during feeding on cotyledon and pods. Histological examinations confirmed that correlation between Rp2 and stylet tip positioning in the xylem vessel in leaflets and stems. Stylet tips end in the tissues of cotyledon and pods when Rp3 is recorded. Taken together, our results demonstrate that R. pedestris ingests xylem sap from vegetative tissues of soybean (leaflet and stem) via a salivary sheath strategy to obtain water. It mainly acquires nutrients from soybean pods and/or seeds using cell-rupture tactics. This study provided insightful information to understand the field occurrence patterns of “Stay Green” syndrome, which may have important implications for pest control.

No abstract

The mirid bug Apolygus lucorum, a dominant mirid species in northern China, is a notorious polyphagous pest with more than 200 hosts, including several major crops such as cotton and soybean, resulting in massive economic loss. Studies of insect salivary effectors may provide a novel control strategy for A. lucorum. An A. lucorum effector, that is, Al6, that inhibits plant immunity by using glutathione peroxidase to repress reactive oxidase accumulation was previously identified. In this study, we further explored the molecular functions of Al6 associated with feeding behaviour and insect survival on soybean, a major host of A. lucorum, using RNA interference and electrical penetration graph (EPG) techniques. We initially observed the injury symptom of this mirid bug and characterized feeding behaviour on soybean leaves using EPG. Our results revealed that A. lucorum preferred to feed on young plant organs such as tender leaves, shoots and buds. This mirid bug used cell rupture as a feeding strategy to ingest cell contents from plant tissues. Subsequently, we silenced the Al6 gene using RNAi and investigated the feeding behaviour, honeydew excretion, body weight, and survival rates of A. lucorum on soybean after Al6 knockdown. Our results demonstrated that silencing of Al6 significantly reduced feeding duration, amount of honeydew secretion, body weight, and survival rates of A. lucorum. Thus, our findings provide a novel molecular target of plant‐mediated RNAi for the control of A. lucorum.

Summary Apolygus lucorum (Meyer‐Dur; Heteroptera: Miridae) is a major agricultural pest infesting crops, vegetables, and fruit trees. During feeding, A. lucorum secretes a plethora of effectors into its hosts to promote infestation. However, the molecular mechanisms of these effectors manipulating plant immunity are largely unknown. Here, we investigated the molecular mechanism underlying the effector Al106 manipulation of plant–insect interaction by RNA interference, electrical penetration graph, insect and pathogen bioassays, protein–protein interaction studies, and protein ubiquitination experiment. Expression of Al106 in Nicotiana benthamiana inhibits pathogen‐associated molecular pattern‐induced cell death and reactive oxygen species burst, and promotes insect feeding and plant pathogen infection. In addition, peptidyl‐prolyl cis‐trans isomerase (PPIase) activity of Al106 is required for its function to inhibit PTI.Al106 interacts with a plant U‐box (PUB) protein, PUB33, from N. benthamiana and Arabidopsis thaliana. We also demonstrated that PUB33 is a positive regulator of plant immunity. Furthermore, an in vivo assay revealed that Al106 inhibits ubiquitination of NbPUB33 depending on PPIase activity. Our findings revealed that a novel cyclophilin effector may interact with plant PUB33 to suppress plant immunity and facilitate insect feeding in a PPIase activity‐dependent manner.