韧皮部取食的昼夜节律是蚜虫渗透压调节的潜在机制

植物和昆虫通常利用昼夜节律来调节他们的生长发育活动，从而提高适合度。植物的光合作用、养分含量、代谢物运输、叶片运动、气孔运动、某些基因的表达等均受到昼夜节律的影响。同样的，昼夜节律也影响昆虫运动、蜕皮、交配和产卵等生命活动。

蚜虫是一种重要的农业害虫，以富含蔗糖的韧皮部汁液为唯一的营养来源，以往的研究表明：蚜虫能够依赖一系列生理机制和行为调节克服韧皮部高糖饮食的渗透胁迫，缓解脱水症状，有利于维持自身的营养摄取。比如，蚜虫能够迅速将蔗糖水解为果糖和葡萄糖，其中果糖被吸收，而葡萄糖则被肠道蔗糖酶聚合成低聚糖；不仅如此，从肠道远端到近端的水循环有助于蚜虫的渗透调节，并且水循环能够稀释肠道内容物，导致大量蜜露的产生和排泄。不仅仅通过生理代谢和适应，蚜虫还会利用取食行为调控降低渗透压胁迫，比如间隙性的取食含水量高的木质部汁液，降低渗透势缓解脱水的胁迫。

植物的韧皮部汁液成分存在昼夜节律变化，夜间取食可能对蚜虫渗透调节产生重要影响。本研究以禾谷缢管蚜Rhopalosiphum padi为研究对象，利用刺吸电位仪（EPG）分析蚜虫昼夜取食行为的变化，并在白天和夜间测量蚜虫的含水量，明确蚜虫取食行为与体内含水量的关系。结果表明，蚜虫昼夜取食行为差异不大，仅在韧皮部取食分泌唾液的时间参数上夜间要显著长于白天。同时，蚜虫白天的含水量比夜间有所降低。

进一步定量了生物钟基因（TIM）和三个渗透调节基因（SUC1、ST4、AQP1），发现夜间蚜虫肠道内糖水解和水分运输减少，表明韧皮部汁液在夜间相对稀释，含糖量低。因此推测蚜虫能够利用韧皮部汁液在夜间蔗糖浓度较低这一特点，通过夜间取食保持含水量，维持渗透压平衡，该研究揭示了蚜虫克服韧皮部汁液高渗透胁迫的一种潜在机制。

图1 蚜虫渗透调节各种机制的概念模型

Aphids possess several mechanisms to counter the high osmotic pressure generated by sugars in the plant phloem sap. Anatomically, the arrangement of the midgut (MG) and hindgut (HG) allows for the rapid transfer of water between the proximal and distal regions. This transfer of water and the equilibration of the osmotic pressure between the hemolymph and the gut is facilitated by the gene aquaporin 1 (AQP1). In the gut lumen, a sucrase-transglucosidase coded by the gene sucrase 1 (SUC1) breaks down sucrose to glucose and fructose. SUC1 combines glucose monosaccharides to generate oligosaccharides that are excreted as honeydew. Fructose in the gut lumen is rapidly transported to the hemolymph by the gene sugar transporter 4 (ST4). Additionally, aphids ingest xylem sap which has a diluted composition relative to phloem sap to replenish their water balance. The empty and filled arrows indicate relative amount of honey dew produced or the level of gene expression observed during the day (empty arrow) and night (filled arrow).

原文信息：

Nalam, V., Isaacs, T., Moh, S., Kansman, J., Finke, D., Albrecht, T. and Nachappa, P. (2021) Diurnal feeding as a potential mechanism of osmoregulation in aphids. Insect Science, 28(2), 521–532.