Table_6_Transmission of a Protease-Secreting Bacterial Symbiont Among Pea Aphids via Host Plants.xlsx

Aphids are economically important pest insects that damage plants by phloem feeding and the transmission of plant viruses. Their ability to feed exclusively on nutritionally poor phloem sap is dependent on the obligatory symbiotic bacterium Buchnera aphidicola, but additional facultative symbionts may also be present, a common example of which is Serratia symbiotica. Many Serratia species secrete extracellular enzymes, so we hypothesised that S. symbiotica may produce proteases that help aphids to feed on plants. Molecular analysis, including fluorescence in situ hybridization (FISH), revealed that S. symbiotica colonises the gut, salivary glands and mouthparts (including the stylet) of the pea aphid Acyrthosiphon pisum, providing a mechanism to transfer the symbiont into host plants. S. symbiotica was also detected in plant tissues wounded by the penetrating stylet and was transferred to naïve aphids feeding on plants containing this symbiont. The maintenance of S. symbiotica by repeated transmission via plants may explain the high frequency of this symbiont in aphid populations. Proteomic analysis of the supernatant from a related but cultivable S. symbiotica strain cultured in liquid medium revealed the presence of known and novel proteases including metalloproteases. The corresponding transcripts encoding these S. symbiotica enzymes were detected in A. pisum and in plants carrying the symbiont, although the mRNA was much more abundant in the aphids. Our data suggest that enzymes from S. symbiotica may facilitate the digestion of plant proteins, thereby helping to suppress plant defense, and that the symbionts are important mediators of aphid–plant interactions.

蚜虫（Aphids）是具有重要经济意义的农业害虫，它们通过韧皮部取食以及传播植物病毒对寄主植物造成危害。蚜虫仅能以营养匮乏的韧皮部汁液为食的特性，依赖于专性共生细菌蚜虫布赫纳菌（Buchnera aphidicola），但同时也可能携带其他兼性共生菌，其中最为常见的一例是粘质沙雷氏菌（Serratia symbiotica）。多数沙雷氏菌属物种可分泌胞外酶，基于此我们提出假说：粘质沙雷氏菌或许能够产生蛋白酶，辅助蚜虫完成植物取食过程。包括荧光原位杂交（fluorescence in situ hybridization, FISH）在内的分子分析结果显示，粘质沙雷氏菌可定殖于豌豆蚜（Acyrthosiphon pisum）的肠道、唾液腺以及口器（含口针）中，这为该共生菌向寄主植物体内转移提供了可行途径。研究同时在被蚜虫穿刺口器造成损伤的植物组织中检测到了粘质沙雷氏菌，且该共生菌可传递至以携带该共生菌的植物为食的未接触过该菌的蚜虫体内。通过植物进行反复传播以维持粘质沙雷氏菌种群的方式，或可解释该共生菌在蚜虫种群中的高检出频率。对一株可培养的近缘粘质沙雷氏菌菌株在液体培养基中培养后的上清液开展蛋白质组学分析，结果检出了包括金属蛋白酶在内的多种已知蛋白酶与新型蛋白酶。编码这些粘质沙雷氏菌酶的对应转录本，在豌豆蚜体内以及携带该共生菌的植物组织中均有检测到，不过蚜虫体内的信使RNA（mRNA）丰度显著更高。本研究数据表明，粘质沙雷氏菌产生的酶或许能够促进植物蛋白的消化，进而帮助抑制植物防御反应，而该共生菌是蚜虫与植物互作过程中的重要介导因子。