Medina-Ortega Karla J, Walker Gregory P
Department of Entomology, University of California, Riverside, CA, 92521, USA.
Plant Cell Environ. 2015 Jun;38(6):1167-77. doi: 10.1111/pce.12470. Epub 2014 Nov 28.
Phloem sieve elements have shut-off mechanisms that prevent loss of nutrient-rich phloem sap when the phloem is damaged. Some phloem proteins such as the proteins that form forisomes in legume sieve elements are one such mechanism and in response to damage, they instantly form occlusions that stop the flow of sap. It has long been hypothesized that one function of phloem proteins is defence against phloem sap-feeding insects such as aphids. This study provides the first experimental evidence that aphid feeding can induce phloem protein occlusion and that the aphid-induced occlusions inhibit phloem sap ingestion. The great majority of phloem penetrations in Vicia faba by the generalist aphids Myzus persicae and Macrosiphum euphorbiae triggered forisome occlusion and the aphids eventually withdrew their stylets without ingesting phloem sap. This contrasts starkly with a previous study on the legume-specialist aphid, Acyrthosiphon pisum, where penetration of faba bean sieve elements did not trigger forisome occlusion and the aphids readily ingested phloem sap. Next, forisome occlusion was demonstrated to be the cause of failed phloem ingestion attempts by M. persicae: when occlusion was inhibited by the calcium channel blocker lanthanum, M. persicae readily ingested faba bean phloem sap.
韧皮部筛管分子具有关闭机制，可在韧皮部受损时防止富含营养的韧皮部汁液流失。一些韧皮部蛋白质，如在豆科植物筛管分子中形成成簇体的蛋白质，就是这样一种机制，在受到损伤时，它们会立即形成阻塞物，阻止汁液流动。长期以来，人们一直推测韧皮部蛋白质的一个功能是抵御以韧皮部汁液为食的昆虫，如蚜虫。这项研究提供了首个实验证据，表明蚜虫取食可诱导韧皮部蛋白质阻塞，且蚜虫诱导的阻塞会抑制韧皮部汁液摄取。多食性蚜虫桃蚜和茄无网蚜对蚕豆韧皮部的绝大多数穿刺都引发了成簇体阻塞，蚜虫最终撤回口针，未摄取韧皮部汁液。这与之前对豆科专食性蚜虫豌豆蚜的一项研究形成鲜明对比，在该研究中，蚕豆筛管分子的穿刺并未引发成簇体阻塞，蚜虫很容易摄取韧皮部汁液。接下来，研究证明成簇体阻塞是桃蚜摄取韧皮部失败的原因：当钙通道阻滞剂镧抑制阻塞时，桃蚜很容易摄取蚕豆韧皮部汁液。
