Cucurbit extrafascicular phloem has strong negative impacts on aphids and is not a preferred feeding site

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. Unlike the sugar‐rich fascicular phloem of cucurbits, the atypical extrafascicular phloem is low in sugars and its function has remained unclear. Here, we used aphids as a major group of phloem feeders to demonstrate quantitatively that extrafascicular phloem is avoided by aphids and it strongly impairs their performance. On this basis, we propose that extrafascicular phloem may be less involved in nutrient distribution but instead may potentially act in herbivore resistance.