Department of Life Sciences, Imperial College London, London, SW7 2AZ, UK.
Plant Cell Environ. 2017 Nov;40(11):2780-2789. doi: 10.1111/pce.13053. Epub 2017 Sep 27.
Cucurbits have long been known to possess two types of phloem: fascicular (FP) within vascular bundles and extrafascicular phloem (EFP) surrounding vascular bundles and scattered through the cortex. Recently, their divergent composition was revealed, with FP having high sugar content consistent with conventional phloem, but EFP having much lower sugar levels and a very different proteome. However, the evolutionary advantages of possessing both FP and EFP have remained unclear. Here, we present four lines of quantitative evidence that together support the hypothesis that FP represents a typical phloem and is an attractive diet for aphids, whereas aphids avoid feeding on EFP. First, aphid stylet track endings were more abundant near the abaxial FP element of minor veins, suggesting a feeding preference for FP over EFP. Second, sugar profiles from stylet exudates were wholly consistent with FP origins, further supporting preference for FP and avoidance of EFP. Third, supplementation of EFP exudate into artificial diets confirmed an aversion to EFP in choice experiments. Finally, EFP exudate had negative effects on aphid performance. On the basis of aphids' inability to thrive on EFP, we conclude that EFP is atypical and perhaps should not be classed as a phloem system.
位于维管束内的束状韧皮部（FP）和环绕维管束并散布在皮层中的束间韧皮部（EFP）。最近，它们的不同组成被揭示出来，FP 具有与传统韧皮部一致的高糖含量，但 EFP 的糖含量要低得多，蛋白质组也非常不同。然而，拥有 FP 和 EFP 的进化优势仍然不清楚。在这里，我们提出了四条定量证据，共同支持以下假设：FP 代表典型的韧皮部，是蚜虫的理想食物，而蚜虫则避免取食 EFP。首先，蚜虫口针轨迹的终点在小叶的下表皮 FP 元素附近更为丰富，这表明蚜虫对 FP 的取食偏好高于 EFP。其次，从口针分泌物中提取的糖谱与 FP 的起源完全一致，进一步支持了对 FP 的偏好和对 EFP 的回避。第三，在选择实验中，将 EFP 渗出物添加到人工饲料中证实了对 EFP 的回避。最后，EFP 渗出物对蚜虫的表现有负面影响。基于蚜虫无法在 EFP 上茁壮成长，我们得出结论，EFP 是异常的，也许不应该被归类为韧皮部系统。
