Daniel Kunk

Background: Daily cycles in environmental conditions shape the biology of plants and their insect herbivores. Because aphids feed nearly continuously from the phloem to sustain rapid population growth, they maintain prolonged, direct contact with their hosts, which exposes them to plant-derived daily rhythms in addition to those arising from the environment. We investigated whole-body transcriptional rhythmicity in the bird cherry-oat aphid (Rhopalosiphum padi) under light-dark (LD) and constant darkness (DD) conditions while aphids fed on either wheat or an artificial diet sachet, sampling transcript expression every 4 h over a 48 h window. Results: Only a small fraction of the transcriptome in R. padi displayed 24 h rhythms, and this fraction declined further as light and host cues were removed. Unexpectedly, more transcripts were rhythmic in DD than in LD on wheat. This trend was not present in diet-fed aphids and was not consistently detected across rhythmic detection tools or significance thresholds. Transcripts that were rhythmic in aphids feeding on wheat hosts had higher amplitudes compared to those feeding on diet. Peak expression (i.e., phase) for rhythmic transcripts clustered near dawn and pre-dusk (zeitgeber time, ZT0 and ZT12), except for transcripts detected as rhythmic in darkness-only on wheat, which peaked approximately 4 h later. Rhythmic transcripts detected in this study were functionally enriched for processes relevant to plant-aphid interactions, including carbohydrate metabolism and detoxification. Subsequent promoter analysis found E-box motifs in both rhythmic and non-rhythmic transcripts, and de novo promoter motif discovery on rhythmic transcripts identified recurring Mad/SMAD- and PAR-bZIP-like motifs. Conclusions: Whole-body transcriptional rhythmicity in R. padi is limited in scope and appears strongly dependent on environmental and host-derived cues. The candidate transcripts and regulatory pathways identified in this study provide clear targets for tissue-specific, functionally minded follow-up studies to investigate the extent and regulatory mechanisms of endogenous physiological and behavioral rhythms in R. padi.

Wheat is a staple crop and one of the most widely consumed grains globally. Wheat yields can experience significant losses due to the damaging effects of herbivore infestation. However, little is known about the effect aphids have on the natural diurnal rhythms in plants. Our time-series transcriptomics and metabolomics study reveal intriguing molecular changes occurring in plant diurnal rhythmicity upon aphid infestation. Under control conditions, 15,366 out of the 66,559 genes in the tetraploid wheat cultivar Svevo, representing approximately 25% of the transcriptome, exhibited diurnal rhythmicity. Upon aphid infestation, 5,682 genes lost their rhythmicity, while additional 5,203 genes began to exhibit diurnal rhythmicity. The aphid-induced rhythmic genes were enriched in GO terms associated with plant defense, such as protein phosphorylation and cellular response to ABA and were enriched with motifs of the WRKY transcription factor families. Conversely, the genes that lost rhythmicity due to aphid infestation were enriched with motifs of the TCP and ERF transcription factor families. While the core circadian clock genes maintain their rhythmicity during infestation, we observed that approximately 60% of rhythmic genes experience disruptions in their rhythms during aphid infestation. These changes can influence both the plant’s growth and development processes as well as defense responses. Furthermore, analysis of rhythmic metabolite composition revealed that several monoterpenoids gained rhythmic activity under infestation, while saccharides retained their rhythmic patterns. Our findings highlight the ability of insect infestation to disrupt the natural diurnal cycles in plants, expanding our knowledge of the complex interactions between plants and insects.

Diurnal rhythms influence insect behavior, physiology, and metabolism, optimizing their performance by adapting to daily changes in the environment. While their impact on agricultural pests has been briefly explored, our understanding of how these rhythms drive adaptative responses in pest biology and influence host colonization remains elusive. Here, we show that a notorious global aphid pest, Rhopalosiphum padi , exhibits distinct diurnal patterns in feeding behavior, with elevated honeydew excretion at night and extended phloem salivation during early nighttime. Temporal aphid transcriptome profiling reveals four diurnally rhythmic clusters, two of which peak at night, exhibiting enrichment in carbohydrate and amino acid metabolism. Beyond the established role in manipulating host responses and allowing sustained feeding, our study reveals novel evidence of cyclical fluctuations in salivary effector expression in an insect species. Silencing key effector genes, peaking in expression during the increased nighttime salivation, results in a more pronounced reduction in aphid excretion activity on host plants during the night compared to the day, a phenomenon not observed on artificial diets. A better understanding of aphid diurnal rhythms and their roles in shaping aphid performance provides a promising avenue to refine and optimize pest management, granting a strategic advantage for minimizing crop damage.