Functional Evaluation of Proteins in Watery and Gel Saliva of Aphids

Abstract

Gel and watery saliva are regarded as key players in aphid–pIant interactions. The salivary composition seems to be influenced by the variable environment encountered by the stylet tip. Milieu sensing has been postulated to provide information needed for proper stylet navigation and for the required switches between gel and watery saliva secretion during stylet progress. Both the chemical and physical factors involved in sensing of the stylet’s environment are discussed. To investigate the salivary proteome, proteins were collected from dissected gland extracts or artificial diets in a range of studies. We discuss the advantages and disadvantages of either collection method. Several proteins were identified by functional assays or by use of proteomic tools, while most of their functions still remain unknown. These studies disclosed the presence of at least two proteins carrying numerous sulfhydryl groups that may act as the structural backbone of the salivary sheath. Furthermore, cell-wall degrading proteins such a pectinases, pectin methylesterases, polygalacturonases, and cellulases as well as diverse Ca2+-binding proteins (e.g., regucalcin, ARMET proteins) were detected. Suppression of the plant defense may be a common goal of salivary proteins. Salivary proteases are likely involved in the breakdown of sieve-element proteins to invalidate plant defense or to increase the availability of organic N compounds. Salivary polyphenoloxidases, peroxidases and oxidoreductases were suggested to detoxify, e.g., plant phenols. During the last years, an increasing number of salivary proteins have been categorized under the term ‘effector’. Effectors may act in the suppression (C002 or MIF cytokine) or the induction (e.g., Mp10 or Mp 42) of plant defense, respectively. A remarkable component of watery saliva seems the protein GroEL that originates from Buchnera aphidicola, the obligate symbiont of aphids and probably reflects an excretory product that induces plant defense responses. Furthermore, chitin fragments in the saliva may trigger defense reactions (e.g., callose deposition). The functions of identified proteins and protein classes are discussed with regard to physical and chemical characteristics of apoplasmic and symplasmic plant compartments.

Salivary DNase II from Laodelphax striatellus acts as an effector that suppresses plant defence

“…In a previous study, the presence of salivary DNase II has been reported in the white-backed planthopper (Huang et al, 2018a;Miao et al, 2018), brown planthopper Huang et al, 2018a), and SBPH (Huang et al, 2018a). Although the composition of the saliva has been reported in > 10 Aphididae species, no salivary DNase II was found in these phloem-feeding aphids (van Bel & Will, 2016), which deserved further investigation. DNase II is an endonuclease that acts on phosphodiester linkages of DNA (Evans & Aguilera, 2003).…”

Insect transmission of plant viruses: Multilayered interactions optimize viral propagation

“…In a study by Will et al (2007), an unknown factor in aphid saliva was found to prevent sieve tube clogging; it is tempting to speculate that this or other factors might also participate in the inoculation of viruses. Saliva components are also assumed to modify PTI, ETI and other defense responses, but hardly any candidates are known (reviewed by Kaloshian & Walling, 2016;van Bel & Will, 2016) and their possible role in facilitating virus transmission is even less explored. On a promising note, there are ongoing efforts to analyze the roles of aphid saliva in plant defense mitigation (i.e., Jaouannet et al, 2014;Kettles & Kaloshian, 2016;Mugford et al, 2016;Rodriguez et al, 2017) and we are hopeful that specific gaps in knowledge might soon be filled.…”

The secreted salivary proteome of Asian citrus psyllid Diaphorina citri

“…Plants possess many complex mechanisms to suppress herbivory, such as the production of phenols, alkaloids and proteins, as well as the initiation of induced plant defensive pathways involving ethylene, salicylic and jasmonic acids, and other plant hormones (Goggin, 2007;Elzinga & Jander, 2013). Evidence is accumulating that insect saliva proteins may be integral in the recognition and initiation of plant responses (Hogenhout & Bos, 2011;van Bel & Will, 2016). The results of proteome analyses of salivary components are reported in several insect orders, including Lepidoptera (Afshar et al, 2013;Rivera-Vega et al, 2017) and Diptera (Valenzuela et al, 2003;Assumpção et al, Correspondence: Nabil Killiny, Department of Plant Pathology, Citrus Research and Education Center, University of Florida, 700 Experiment Station Road, Lake Alfred, Florida 33850, U.S.A.…”

“…For most phloem-feeding hemipteran insects, watery and gel saliva are considered to be critical during stylet penetration and ingestion (van Bel & Will, 2016). During stylet penetration into the host tissues, gel saliva is involved in the coagulation and formation of salivary sheath, which protects, stabilizes and lubricates the stylet.…”

“…During stylet penetration into the host tissues, gel saliva is involved in the coagulation and formation of salivary sheath, which protects, stabilizes and lubricates the stylet. The watery saliva, which is either continuously or periodically secreted into the phloem sieve elements, can suppress plant defence responses (Elzinga & Jander, 2013;Will et al, 2013;van Bel & Will, 2016). The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) is the most important insect pest of citrus because it effectively vectors the huanglongbing (HLB) pathogen Candidatus Liberibacter asiaticus (CLas).…”