Figure 4.

OsHKT2;1 gene expression in the cortex and endodermis of K⁺-starved roots and in leaf vascular bundles. Transgenic rice plants expressing GUS or GFP reporter genes under the control of a 1.6 kb OsHKT2;1 promoter were grown in 1 mM CaSO₄ solution in the presence of hygromycin. (A, B) Strong GUS staining was detected in main roots (A) but not in root tip regions (B). (C) A dark-field microscopic image of sections derived from GUS-stained main roots. Sections of GUS-stained (red staining) main roots showed strong signals in cortical and endodermal cells. (D, E) Dark-field microscopic images of sections derived from GUS-stained leaves. The sections showed strong GUS signals (red stain) in vascular bundle regions. In (E) is an enlarged image of a section of the region shown in (D). Note that the reddish staining in (C–E) shows GUS signals, as these were obtained using dark-field microscopy. (F) A 3D reconstruction image of GFP fluorescence derived from K⁺-starved roots of OsHKT2;1 promoter-GFP plants, showing strong GFP fluorescence in root cortex cells. (G) A 3D reconstruction image of propidium iodide fluorescence derived from K⁺-starved roots of the same plant shown in (F). (H) Combined images of GFP and propidium iodide fluorescence shown in (F) and (G). (I) An enlarged image of K⁺-starved roots shown in (H). (J, K) Combined images of GFP and propidium iodide fluorescence derived from 10-day-old OsHKT2;1 promoter-GFP plants grown in 1 mM CaSO₄ solution supplemented with either 30 mM K⁺ (J) or 30 mM Na⁺ (K). (L) GFP fluorescence from three different conditions were quantified and normalized relative to propidium iodide fluorescence. Fluorescence intensities of GFP and propidium iodide in (J) were measured using three independent plants for each ionic condition (±s.d.).