Auxin is required for leaf vein pattern in Arabidopsis

To investigate possible roles of polar auxin transport in vein patterning, cotyledon and leaf vein patterns were compared for plants grown in medium c ontaining polar auxin transport inhibitors (N-1-naphthylphthalnmic acid, 9- hydroxyfluorene-9-carboxylic acid, and 2,3,5-triiodobenzoic acid) and in me dium containing a less well-characterized inhibitor of auxin-mediated proce sses, 2-(p-chlorophynoxy)-2-methylpropionic acid. Cotyledon vein pattern wa s not affected by any inhibitor treatments, although vein morphology was al tered. In contrast, leaf vein pattern was affected by inhibitor treatments. Growth in polar auxin transport inhibitors resulted in leaves that lacked vascular continuity through the petiole and had broad, loosely organized mi dveins, an increased number of secondary veins, and a dense band of misshap en tracheary elements adjacent to the leaf margin. Analysis of leaf vein pa ttern developmental time courses suggested that the primary vein did not de velop in polar auxin transport inhibitor-grown plants, and that the broad m idvein observed in these seedlings resulted from the coalescence of proxima l regions of secondary veins. Possible models for leaf vein patterning that could account for these observations are discussed.