Aphid Gel Saliva: Sheath Structure, Protein Composition and Secretory Dependence on Stylet-Tip Milieu

In order to separate and analyze saliva types secreted during stylet propagation and feeding, aphids were fed on artificial diets. Gel saliva was deposited as chains of droplets onto Parafilm membranes covering the diets into which watery saliva was secreted. Saliva compounds collected from the diet fluid were separated by SDS-PAGE, while non-soluble gel saliva deposits were processed in a novel manner prior to protein separation by SDS-PAGE. Soluble (watery saliva) and non-soluble (gel saliva) protein fractions were significantly different. To test the effect of the stylet milieu on saliva secretion, aphids were fed on various diets. Hardening of gel saliva is strongly oxygen-dependent, probably owing to formation of sulfide bridges by oxidation of sulphydryl groups. Surface texture of gel saliva deposits is less pronounced under low-oxygen conditions and disappears in dithiothreitol containing diet. Using diets mimicking sieve-element sap and cell-wall fluid respectively showed that the soluble protein fraction was almost exclusively secreted in sieve elements while non-soluble fraction was preferentially secreted at cell wall conditions. This indicates that aphids are able to adapt salivary secretion in dependence of the stylet milieu.为分离并分析蚜虫在口针（stylet）推进与取食过程中分泌的唾液类型，研究人员将蚜虫饲养于人工饲料之上。胶状唾液以液滴链的形式沉积于覆盖饲料的帕拉膜（Parafilm）表面，水性唾液同样分泌至该膜上。从饲料液中收集的唾液组分通过十二烷基硫酸钠-聚丙烯酰胺凝胶电泳（SDS-PAGE）进行分离；而不溶性胶状唾液沉积物则需先经全新方式处理，再通过SDS-PAGE完成蛋白质分离。可溶性（水性唾液）与不溶性（胶状唾液）蛋白质组分存在显著差异。为探究口针微环境对唾液分泌的影响，研究人员使用多种不同饲料饲养蚜虫。胶状唾液的固化过程强烈依赖氧气，这可能源于巯基（sulfhydryl groups）氧化形成硫桥（sulfide bridges）。低氧条件下，胶状唾液沉积物的表面纹理会显著减弱；而在添加二硫苏糖醇（dithiothreitol）的饲料中，该纹理则完全消失。分别使用模拟筛管汁液和细胞壁流体的饲料开展实验，结果显示：可溶性蛋白质组分几乎仅在筛管环境中分泌，而不溶性蛋白质组分则优先在细胞壁环境中分泌。该结果表明，蚜虫能够根据口针微环境调整唾液的分泌模式。