The GC1 promoter mediates strong reporter expression in guard cells of wild-type Arabidopsis seedlings, too many mouths mutant and also in tobacco. A. A two-week-old pGC1::GUS transgenic seedling. B. Different stages of guard cells exhibited different levels of GUS expression. C. Upper part of the hypocotyl. D. Young leaf and petiole. E. Leaf edge and petiole. F. & G. pGC1::YC3.60 was mainly expressed in mature guard cells, very weak in young or immature guard cells (white arrows in (f) & (g)). H. & I.pGC1::YC3.60 was expressed in guard cells on the hypocotyl. J. & K.pGC1::YC3.60 was expressed in guard cells on the sepal. L. & M. pGC1 mediated GUS (L) and GFP (M) reporter expression in clustered stomata in too many mouths. N. pGC1 mediated strong reporter gene expression in tobacco guard cells.