Identification of phloem involved in assimilate loading in leaves by the activity of the galactinol synthase promoter

The definition of "minor" veins in leaves is arbitrary and of uncertain bio logical significance. Generally, the term refers to the smallest vein class es in the leaf, believed to function in phloem loading. We found that a gal actinol synthase promoter, cloned from melon (Cucumis melo), directs expres sion of the gusA gene to the smallest veins of mature Arabidopsis and culti vated tobacco (Nicotiana tabacum) leaves. This expression pattern is consis tent with the role of galactinol synthase in sugar synthesis and phloem loa ding in cucurbits. The expression pattern in tobacco is especially notewort hy since galactinol is not synthesized in the leaves of this plant. Also, w e unexpectedly found that expression in tobacco is limited to two of three companion cells in class-V veins, which are the most extensive in the leaf. Thus, the "minor" vein system is defined and regulated at the genetic leve l, and there is heterogeneity of response to this system by different compa nion cells of the same vein.