Arabidopsis hydathodes contain a dense and heterogeneous epithem for apoplastic fluid release

Hydathodes are specialized leaf tissues that mediate guttation, the release of liquid water droplets from vascular plants. They consist of xylem endings, water pores, and the epithem. The epithem comprises small cells whose cell types and functional properties remain poorly understood. While molecular genetic information on hydathodes has accumulated--particularly in the model plant Arabidopsis thaliana through recent RNA-seq analyses--high-resolution anatomical insights into their spatial organization remain limited. In this study, we aimed to characterize the specialized anatomy of Arabidopsis hydathodes using complementary imaging approaches. A fluorescent dye taken up from roots stained hydathodes, consistent with a connection between hydathodes and the vasculature. Microfocus-X-ray computed tomography revealed that the hydathodes protrude from the abaxial leaf surface and are more densely packed internally than surrounding tissues. Light microscopy showed that the epithem was heterogeneous, with elongated cells proximally and rounded cells distally. We found that the epithem was surrounded by cells (hereafter referred to as "boundary cells") with plastids that resembled those of mesophyll cells. Transmission electron microscopy showed that the organelles in epithem cells were immature, and the apoplast contained electron-dense material. Finally, using transgenic plants expressing a secreted fluorescent marker protein, we detected GFP in guttation droplets, indicating that the droplets can contain apoplastic material. Together, these findings reveal that Arabidopsis hydathodes possess a dense and spatially heterogeneous epithem associated with the release of apoplastic fluid.