Figure 2.

SWEET16 and SWEET17 are expressed in xylem cells throughout inflorescence stem development and form heterodimers. A, C, and E, Histochemical analysis of GUS activity in lines expressing SWEET16-GUS fusion proteins driven by the SWEET16 native promoter in sections taken at different positions in the inflorescence stem of 7-week-old plants. B, D, and F, Histochemical analysis of GUS activity in lines expressing SWEET17-GUS fusion proteins driven by the SWEET17 native promoter in sections taken at different positions in the inflorescence stem section of 7-week-old plants. Sections were taken in a stem region where the growth was still rapid (A, B, and insets), in a stem region where elongation growth had finished but where thickening of the secondary cell wall was still ongoing (C, D, and insets), and at the bottom of the stem, a region that corresponds to a mature stem (E, F, and insets). Arrows point to cells showing blue GUS staining in developing xylem cells and arrow heads point to axial parenchyma cells and asterisks indicate xylary parenchyma cells. Lignin is colored pink after phloroglucinol staining. The intensity of the pink color indicates the level of lignification of the xylary vessels. ep, epidermis; co, cortex; iff, interfascicular fibers; ph, phloem; xy, xylem. G, Arabidopsis mesophyll protoplast expressing SWEET17-cCFP^C and SWEET16-nVenus^C interaction revealed by false color yellow, chloroplast auto-fluorescence is in false color red. H, Arabidopsis mesophyll protoplast expressing SWEET16-cCFP^C and SWEET17-nCerulean^C interaction revealed by false color cyan, chloroplast auto-fluorescence is in false color red. Invaginations around the chloroplasts in (G) and (H) indicate that SWEET16 and SWEET17 interact at the vacuolar membrane. Scale bar = 50 µm (A–F) or 25 µm (insets in A–F) or 20 µm (G and H).