TPX1 expression in tomato roots by in situ hybridization. Transversal sections of hydroponically grown tomato roots were probed with digoxigenin-labeled antisense TPX1 RNA. A, Expression of TPX1 (RNA signal was detected as a dark brown or purple color) in the apical zone of the root (last 0.5 cm from the root tip). B, Detail of A showing that the expression is restricted to the endodermis (En) and protoxylem (P). C, Expression of TPX1 (purple) in the medium zone of the root (8–10 cm from the root tip). D, Detail of C showing the expression in the endodermis (En). E, Expression of TPX1 in the basal zone of the root (13–15 cm from the root tip). F, Detail of E revealing expression in the endodermis (En) and the exodermis (Ex).