In the phloem cap region of Arabidopsis plants, sulfur-rich cells (S-cells) accumulate >100 mM glucosinolates (GLS), but are biosynthetically inactive. The source and route of S-cell-bound GLS remain elusive. In this study, using single-cell sampling and scanning electron microscopy with energy-dispersive X-ray analysis we show that two GLS importers, NPF2.10/GTR1 and NPF2.11/GTR2, are critical for GLS accumulation in S-cells, although they are not localized in the S-cells. Comparison of GLS levels in S-cells in multiple combinations of homo- and heterografts of gtr1 gtr2, biosynthetic null mutant and wild-type plants indicate that S-cells accumulate GLS via symplasmic connections either directly from neighboring biosynthetic cells or indirectly to non-neighboring cells expressing GTR1/2. Distinct sources and transport routes exist for different types of GLS, and vary depending on the position of S-cells in the inflorescence stem. Based on these findings, we propose a model illustrating the GLS transport routes either directly from biosynthetic cells or via GTR-mediated import from apoplastic space radially into a symplasmic domain, wherein the S-cells are the ultimate sink. Similarly, we observed accumulation of the cyanogenic glucoside defensive compounds in high-turgor cells in the phloem cap of Lotus japonicus, suggesting that storage of defensive compounds in high-turgor cells may be a general mechanism for chemical protection of the phloem cap.

在拟南芥植物的韧皮部帽部区域，富硫细胞（S细胞）积聚> 100 mM的芥子油苷（GLS），但在生物合成上无活性。与S细胞结合的GLS的来源和途径仍然难以捉摸。在这项研究中，使用单细胞采样和扫描电子显微镜以及能量色散X射线分析，我们发现两个GLS进口商NPF2.10 / GTR1和NPF2.11 / GTR2对S细胞中GLS的积累至关重要，尽管它们未定位在S细胞中。Gtr1 gtr2同种和异种移植物多种组合中S细胞中GLS水平的比较，生物合成无效突变和野生型植物表明，S细胞通过同质连接直接从邻近的生物合成细胞或间接与表达GTR1 / 2的非邻近细胞积累GLS。不同类型的GLS存在不同的来源和运输途径，并且取决于S细胞在花序茎中的位置。基于这些发现，我们提出了一个模型，该模型说明了直接从生物合成细胞或通过GTR介导的从质外体空间径向进入同质域的GLS转运途径，其中S细胞是最终的汇聚区。同样，我们观察到了日本莲的韧皮部帽盖中的高膨胀细胞中氰基防御糖苷化合物的积累。，表明防御化合物在高强度细胞中的储存可能是韧皮部帽化学保护的一般机制。