Plant Physiol, April 2003, Vol. 131, pp. 1555-1565

Structural and Functional Vein Maturation in Developing Tobacco Leaves in Relation to AtSUC2 Promoter Activity¹

Cell-Cell Communication Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom (K.M.W., A.G.R., K.J.O.); Department of Plant Biology, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark (H.J.M.); and Lehrstuhl für Molekulare Pflanzenphysiologie, Staudtstr 5 Friedrich-Alexander-Universität Erlangen-Nürnberg, D-91058 Erlangen, Germany (N.S.)

Transgenic tobacco (Nicotiana tabacum) plants expressing green fluorescent protein (GFP) from the AtSUC2 promoter were used to study the function of different vein classes in developing leaves. In sink leaves, unloading capacity occurred acropetally, with the class I (midrib) and class II veins becoming functional in phloem unloading before the maturation of the class III veinal network. In contrast, in developing cotyledons and source leaves, loading capacity occurred in a basipetal direction. There was a strong correlation between loading capacity, as assessed by ¹⁴C Suc uptake and companion cell expression of AtSUC2-GFP. Developing cotyledons were shown to utilize all available vein classes for loading. A second line of transgenic plants was produced in which GFP, expressed from the AtSUC2 promoter, was targeted to the endoplasmic reticulum instead of the cytoplasm. In these AtSUC2-GFP-ER plants, GFP was unable to traffic into the sieve element and was restricted solely to the companion cells of source leaf tissues. Partial shading of leaves undergoing the sink-source transition demonstrated that the activation of the AtSUC2 promoter in tobacco was influenced by light. Functional and structural maturation of the minor veins required light or a product of light. The activation of the AtSUC2 promoter within major veins appears to be regulated differently from that in the minor veins. The relationship between AtSUC2 activation and the activity of endogenous tobacco Suc transporters is discussed.