摘要
In the phloem cap region o i 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 o lg t r l 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.
