Fig. 4. BRL1 and BRL3 are expressed in the vascular tissues. BRL expression was assessed by GUS staining in 7-day-old seedlings, in embryos and in expanded leaves. (A) pBRL1::GUS is expressed in the columella root cap cells and in mature vascular tissues. (B) pBRL3::GUS is expressed in the roots in the two protophloem cell files in the elongation zone. (C) pBRI1:GUS is ubiquitously expressed in roots, especially in the elongation zone. (D) pBRL1::GUS is expressed in the mid-vein and lower part of the cotyledons. (E) pBRL3::GUS is preferentially expressed in the secondary veins at the tip of cotyledons. (F) pBRI1::GUS expression in the petioles and the lower part of cotyledons and leaves. (G) pBRL1::GUS expression in leaves is predominantly in the mid-vein. (H) pBRL3::GUS expression in leaves is predominantly in the secondary and tertiary veins and reduced in the mid-vein. (I) Strong expression of pBRI1::GUS in the petioles. (J) GUS staining in the embryos of pBRL1::GUS transgenic seeds. (K) Transverse section of the root tip of seedlings shows that pBRL1::GUS expression is confined to the vascular cylinder. en, endodermis; pe, pericycle; st, stele. (L) The root tip of a five-day-old pBRL1::GUS transgenic seedling showing that, after germination, BRL1 expression appears associated with the stele. (M) Transverse section at the base of the inflorescence stem in mature plants shows pBRL1::GUS expression in the procambial cells in the vascular bundles. (N) Detail of a leaf expressing pBRL3::GUS shows higher expression in the secondary and tertiary veins compared to the mid-vein. (O) Close-up showing the leaf vasculature stained with pBRL3::GUS reveals specific expression in the phloem sieve elements. (P) The root tip of a 5-day-old pBRL3::GUS transgenic seedling showing that, after germination, BRL3 expression appears associated with the protophloem. (Scale bars: 100 µm in A,B,C; 10 µm in J; 25 µm in K,L,M,N,P.