We have taken a genetic approach to understanding the mechanisms that control vascular patterning in the leaves of higher plants. Here we present the identification and characterization of the lop1 mutant of Arabidopsis which is defective in basipetal transport of IAA. Mutant leaf midveins show disoriented axial growth, and bifurcation into twin veins that are frequently rotated out of the normal dorsal/ventral axis of the leaf. Mutant plants also display abnormal patterns of cell expansion in the midrib cortex and in the epidermis of the elongation zone of lateral roots. Lateral roots show abnormal curvature during initiation, sometimes encircling the primary root prior to growth in a normal downward direction. Mutant seedlings have normal levels of free IAA, and appear normal in auxin perception, suggesting that transport is the primary lesion. The abnormalities in vascular development, lateral root initiation and patterns of cell expansion observed in the lop] mutant are consistent with a basic disruption in basipetal transport of IAA.

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