Figure 6.

Images of GUS expression in N. benthamiana transformed with NbHIPP26 predicted promoter sequences fused to the uidA gene. A to C, GUS staining in mature plants (A), mature leaves (B), and roots (C) was detected throughout the vasculature of the mature plant but was not present in young sink leaves (A). Expression was particularly strong in the roots and hypocotyl, and in leaves, GUS staining was more evident in older leaves. Expression in leaf veins appeared to follow the sink-source transition: GUS staining was evident in tip (source) veins, while the basal (sink) portion was clearer (B). D and E, When studied in more detail in leaf veins, GUS was localized specifically to cells within the vascular bundle of all vein classes in mature leaves (arrows point to major vein classes and arrowheads to minor veins). At low magnification, there appeared to be little or no GUS staining in ground tissues between the veins, and intact leaf petioles did not show staining. Once cut, however, GUS was visible in the vascular bundles of petioles. D and E show images of intact, mature N. benthamiana leaves photographed using a combination of bottom and top lighting. F and G, GUS staining in transverse sections through major veins. The vascular bundle (F) is delineated with a dotted black line, and phloem regions are outlined with red dotted lines. Some GUS staining was detected in cells surrounding the vascular bundles, but much less than inside the vascular system. G shows an enlargement of the phloem bundle marked in F with a red arrow. Blue GUS staining can be seen clearly in xylem parenchyma (XP) and phloem (P) cells but is generally absent from metaxylem vessels (X), and the bundle sheath (BS) also is shown. Bars = 2 mm in A and D, 500 µm in B and E, 50 µm in C and F, and 20 µm in G. H, Relative NbHIPP26 expression in different tissues measured by RT-qPCR. The histogram bars show mean values ± sd (n = 3, two independent experiments).