GSD1 is expressed in companion cells demonstrated by promoter analysis and immunolocalization. A to H, Histochemical localization of GSD1 promoter-GUS activity in pGSD1-L-GUS transgenic rice. GUS expression in the root (A), cross sectioned stem (B), leaf blade (C), young spikelet (D), matured spikelet (E), and immature seed at 10 DAF with the inset for cross section (F). Cross sections of rice culm (G) and longitudinal sections of rice culm (H). I to K, Immunolocalization by GSD1-specific antibodies. GSD1 is localized in phloem companion cells in the wild type (I) and in the gsd1-D mutant (K). Detection in the wild type by preimmune IgG (J). L, GUS staining in culm of the pGSD1-S-GUS transgenic rice. CC, Companion cell; CV, commissural vein; DV, dorsal vascular bundle; LV, large vein; MV, midvein; Ph, phloem; SE, sieve element; SV, small vein; Xy, xylem. Bars = 500 μm in A to F; 100 μm in the F inset; and 50 μm in G to L.