Defence and nutrition synergistically contribute to the distinct tolerance of rice subspecies to the stem borer, Chilo suppressalis.

Damage caused by the rice striped stem borer (SSB), Chilo suppressalis (Walker) (Lepidoptera: Pyralidae), is much more severe on indica/xian rice than on japonica/geng rice (Oryza sativa) which matches pest outbreak data in cropping regions of China. The mechanistic basis of this difference among rice subspecies remains unclear. Using transcriptomic, metabolomic and genetic analyses in combination with insect bioassay experiments, we showed that japonica and indica rice utilise different defence responses to repel SSB, and that SSB exploited plant nutrition deficiencies in different ways in the subspecies. The more resistant japonica rice induced patterns of accumulation of methyl jasmonate (MeJA-part of a defensive pathway) and vitamin B1 (VB1-a nutrition pathway) distinct from indica cultivars. Using gene-edited rice plants and SSB bioassays, we found that MeJA and VB1 jointly affected the performance of SSB by disrupting juvenile hormone levels. In addition, genetic variants of key biosynthesis genes in the MeJA and VB1 pathways (OsJMT and OsTH1, respectively) differed between japonica and indica rice and contributed to performance differences; in indica rice, SSB avoided the MeJA defence pathway and hijacked the VB1 nutrition-related pathway to promote development. The findings highlight important genetic and mechanistic differences between rice subspecies affecting SSB damage which could be exploited in plant breeding for resistance.

Caterpillar-Induced Rice Volatile (E)-ß-Farnesene Impairs the Development and Survival of Chilo suppressalis Larvae by Disrupting Insect Hormone Balance.

Significant research progress has recently been made on establishing the roles of tps46 in rice defense. (E)-ß-farnesene (Eßf) is a major product of tps46 activity but its physiological functions and potential mechanisms against Chilo suppressalis have not yet been clarified. In the present study, C. suppressalis larvae were artificially fed a diet containing 0.8 g/kg Eßf and the physiological performance of the larvae was evaluated. In response to Eßf treatment, the average 2nd instar duration significantly increased from 4.78 d to 6.31 d while that of the 3rd instar significantly increased from 5.70 d to 8.00 d compared with the control. There were no significant differences between the control and Eßf-fed 4th and 5th instars in terms of their durations. The mortalities of the 2nd and 3rd Eßf-fed instars were 21.00-fold and 6.39-fold higher, respectively, than that of the control. A comparative transcriptome analysis revealed that multiple differentially expressed genes are involved in insect hormone biosynthesis. An insect hormone assay on the 3rd instars disclosed that Eßf disrupted the balance between the juvenile hormone and ecdysteroid levels. Eßf treatment increased the juvenile hormones titers but not those of the ecdysteroids. The qPCR results were consistent with those of the RNA-Seq. The foregoing findings suggested that Eßf impairs development and survival in C. suppressalis larvae by disrupting their hormone balance. Moreover, Eßf altered the pathways associated with carbohydrate and xenobiotic metabolism as well as those related to cofactors and vitamins in C. suppressalis larvae. The discoveries of this study may contribute to the development and implementation of an integrated control system for C. suppressalis infestations in rice.

Vitellogenin from planthopper oral secretion acts as a novel effector to impair plant defenses.

Vitellogenin (Vg) is a well-known nutritious protein involved in reproduction in nearly all oviparous animals, including insects. Recently, Vg has been detected in saliva proteomes of several piercing-sucking herbivorous arthropods, including the small brown planthopper (Laodelphax striatellus, SBPH). Its function, however, remains unexplored. We investigated the molecular mechanism underlying SBPH orally secreted Vg-mediated manipulation of plant-insect interaction by RNA interference, phytohormone and H2 O2 profiling, protein-protein interaction studies and herbivore bioassays. A C-terminal polypeptide of Vg (VgC) in SBPH, when secreted into rice plants, acted as a novel effector to attenuate host rice defenses, which in turn improved insect feeding performance. Silencing Vg reduced insect feeding and survival on rice. Vg-silenced SBPH nymphs consistently elicited higher H2 O2 production, a well-established defense mechanism in rice, whereas expression of VgC in planta significantly hindered hydrogen peroxide (H2 O2 ) accumulation and promoted insect performance. VgC interacted directly with the rice transcription factor OsWRKY71, a protein which is involved in induction of H2 O2 accumulation and plant resistance to SBPH. These findings indicate a novel effector function of Vg: when secreted into host rice plants, this protein effectively weakened H2 O2 -mediated plant defense through its association with a plant immunity regulator.

Wolbachia supplement biotin and riboflavin to enhance reproduction in planthoppers.

Symbiont-mediated nutritional mutualisms can contribute to the host fitness of insects, especially for those that feed exclusively on nutritionally unbalanced diets. Here, we elucidate the importance of B group vitamins in the association of endosymbiotic bacteria Wolbachia with two plant-sap feeding insects, the small brown planthopper, Laodelphax striatellus (Fallén), and the brown planthopper, Nilaparvata lugens (Stål). Infected planthoppers of both species laid more eggs than uninfected planthoppers, while the experimental transfer of Wolbachia into uninfected lines of one planthopper species rescued this fecundity deficit. The genomic analysis showed that Wolbachia strains from the two planthopper species encoded complete biosynthesis operons for biotin and riboflavin, while a metabolic analysis revealed that Wolbachia-infected planthoppers of both species had higher titers of biotin and riboflavin. Furthermore, experimental supplementation of food with a mixture of biotin and riboflavin recovered the fecundity deficit of Wolbachia-uninfected planthoppers. In addition, comparative genomic analysis suggested that the riboflavin synthesis genes are conserved among Wolbachia supergroups. Biotin operons are rare in Wolbachia, and those described share a recent ancestor that may have been horizontally transferred from Cardinium bacteria. Our research demonstrates a type of mutualism that involves a facultative interaction between Wolbachia and plant-sap feeding insects involving vitamin Bs.

Vertical Transmission of Wolbachia Is Associated With Host Vitellogenin in Laodelphax striatellus.

Wolbachia in host germ lines are essential for their vertical transmission to the next generation. It is unclear how the regulation of host oocyte development influences Wolbachia location and the mechanistic basis of transmission. Here, we investigated whether vitellogenin influences Wolbachia transmission in Laodelphax striatellus. Wolbachia increased in density and spread from the anterior tropharium to developing oocytes as ovaries developed. Microscopic observations indicated that Wolbachia invaded ovarioles from the tropharium of its anterior side rather than the pedicel side. Wolbachia utilized the host Vg transovarial transportation system to enter the ovaries and were transmitted from the tropharium into the developing oocytes through nutritive cords. These observations were supported by knocking down the Vg transcript, in which low Wolbachia titers were detected in ovaries and fewer Wolbachia were transmitted into oocytes. Our findings establish a link between the Vg-related mode of transovarial transmission and efficient maternal transmission of Wolbachia.

Wolbachia-induced loss of male fertility is likely related to branch chain amino acid biosynthesis and iLvE in Laodelphax striatellus.

Wolbachia are endosymbionts that infect many species of arthropods and nematodes. Wolbachia-induced cytoplasmic incompatibility (CI) is the most common phenotype in affected hosts, involving embryonic lethality in crosses between Wolbachia-infected males and uninfected females. The molecular mechanisms underlying this phenomenon are currently unclear. Here we examine the molecular correlates of the Wolbachia infection in Laodelphax striatellus (Fallén), an important rice pest, where embryonic lethality is strong and almost complete. We compared the gene expression of 4-day-old Wolbachia-infected and uninfected L. striatellus testes to identify candidate genes for paternal-effect embryonic lethality induction. Based on microarray analysis, iLvE was the most down-regulated gene; this gene mediates branched-chain amino acid (BCAA) biosynthesis and participates in many processes related to reproductive performance. After knocking down iLvE by RNAi in uninfected male L. striatellus, male fertility was reduced, leading to a decrease in embryo hatching rates, but fertility was rescued in crosses between these males and Wolbachia-infected females. Removal of BCAA in chemically-defined diets of uninfected males also led to a loss of male fertility. Low amino acid nutrition may enhance exposure time of sperm to Wolbachia in the testes to affect adult reproduction in L. striatellus by reducing the number of sperm transferred per mating by males. These results indicate that Wolbachia may decrease male fertility in L. striatellus by acting on iLvE, a key factor of BCAA biosynthesis, and delaying sperm maturation.

The complete mitochondrial genomes of six species of Tetranychus provide insights into the phylogeny and evolution of spider mites.

Many spider mites belonging to the genus Tetranychus are of agronomical importance. With limited morphological characters, Tetranychus mites are usually identified by a combination of morphological characteristics and molecular diagnostics. To clarify their molecular evolution and phylogeny, the mitochondrial genomes of the green and red forms of Tetranychus urticae as well as T. kanzawai, T. ludeni, T. malaysiensis, T. phaselus, T. pueraricola were sequenced and compared. The seven mitochondrial genomes are typical circular molecules of about 13,000 bp encoding and they are composed of the complete set of 37 genes that are usually found in metazoans. The order of the mitochondrial (mt) genes is the same as that in the mt genomes of Panonychus citri and P. ulmi, but very different from that in other Acari. The J-strands of the mitochondrial genomes have high (∼ 84%) A+T contents, negative GC-skews and positive AT-skews. The nucleotide sequence of the cox1 gene, which is commonly used as a taxon barcode and molecular marker, is more highly conserved than the nucleotide sequences of other mitochondrial genes in these seven species. Most tRNA genes in the seven genomes lose the D-arm and/or the T-arm. The functions of these tRNAs need to be evaluated. The mitochondrial genome of T. malaysiensis differs from the other six genomes in having a slightly smaller genome size, a slight difference in codon usage, and a variable loop in place of the T-arm of some tRNAs by a variable loop. A phylogenic analysis shows that T. malaysiensis first split from other Tetranychus species and that the clade of the family Tetranychoidea occupies a basal position in the Trombidiformes. The mt genomes of the green and red forms of T. urticae have limited divergence and short evolutionary distance.