Fig. 3. Vascular systems in cotyledons of wild-type and mutant Arabidopsis seedlings. (A,B) Vascular phenotype categories: `straight', referring to the formation of a single unbranched midvein; `branched', a complete midvein plus some secondary venation; `circles', complete midvein and at least one second-order vein has formed a complete loop. Frequencies of vascular phenotypic categories (shown in B) in various genotypes. Note the higher frequencies of more-complete vascular systems in amp1 mp double mutants as compared with mp mutants. One cotyledon per seedling was evaluated. (C) Scheme of AMP1 interaction with MP. As illustrated for cell fate acquisition in the leaf primordium, but applicable also to other locations, AMP1 functions as a universal negative regulator of meristematic activity (promoter of cell differentation). Locally interfering with AMP1 function, MP maintains cells in a procambial state by preventing the acquisition of mesophyll characteristics (green) along lines of elevated auxin levels and MP expression (yellow). (D) RT-PCR of AMP1 transcript abundance in wild-type and mpG92 seedlings at 7 days after germination (DAG). ACT7 was used as an internal control as described (Hardtke et al., 2004).