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First published online 6 June 2007
doi: 10.1242/dev.006759

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AMP1 and MP antagonistically regulate embryo and meristem development in Arabidopsis

Danielle P. Vidaurre, Sara Ploense^*, Naden T. Krogan and Thomas Berleth

University of Toronto, Department of Cell and Systems Biology, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada.

View larger version (72K): [in this window] [in a new window]   Fig. 1. Arabidopsis embryo development. Quadrant (A,F), octant (B,G), dermatogen (C,H) globular (D,E,I,J), and postglobular (K-T) stages of phenotypically normal (A-E,K-O) and amp1-13 (F-J,P-T) embryos. Staging of amp1 mutants was based on comparison with phenotypically normal (amp1/+ or +/+) embryos from the same silique. Abnormalities in preglobular amp1 mutant embryos are restricted to basal-cell derivatives, whereas the arrangement of apical derivatives (brackets in G-I) remains unaltered. Abnormal suspensor cell divisions (F) lead to a massive three-dimensional cell arrangement (I,J), clearly recognizable as part of the embryo proper by the triangular stage (L,Q). Note the presence of a basally extended epidermal layer (I) and the presence of additional cell tiers in the embryo proper in amp1 mutants (numbered in I). Note the emergence of cotyledon primordia from basal positions and of an oversized SAM at triangular (P), heart (Q-S) and torpedo (T) stage in amp1 mutants. (U) Quantification of cell tier numbers (as illustrated in I) for wild-type and amp1 globular-stage embryos (n values between 42 and 51; error bars indicate s.e.m.). (V,W) Formation of a second embryo (outlined cells) and of twin-seedlings from single seeds in amp1-13 mutant seeds. Arrows in W point to two separate roots. (X-AA) Bent-cotyledon-stage embryos of wild-type (X), amp1-9 (Y), amp1-9 mpG92 (Z) and mpG92 (AA) genotype. The embryo in Z represents a largely normalized individual from a spectrum of embryonic phenotypes. All images except W are cleared whole-mounts viewed with DIC optics. ut, upper tier; lt, lower tier. Scale bars: 20 µm in A-J,V; 50 µm in K-T,X-AA.

View larger version (66K): [in this window] [in a new window]   Fig. 2. Postembryonic interaction of AMP1 and MP in Arabidopsis. (A-D) Scanning electron micrographs showing relative sizes of SAMs of indicated genotypes at 3 days after germination (DAG). (E-L) Cleared whole-mount preparations viewed with DIC optics. Expression of stem cell marker pCLV3::GUS in the SAM of the indicated genotypes at 3 DAG (E-H) and 5 DAG (I-L). Note that expression of pCLV3::GUS extends beyond the most-central domain in the amp1 background. Numerical values of shoot meristem sizes are shown in Table 1A. (M) Discrete domains of pCLV3::GUS expression (arrowheads) in amp1 are associated with the formation of multiple meristems. (N) Formation of multiple SAMs (arrowheads) on expanded apices in amp1 mutants, which are clearly not associated with leaf axils. (O) Left to right: wild type, amp1-9, amp1-9 mpG92 and mpG92. Note that amp1 mp double mutants are smaller than amp1 mutants and have restored flower formation, which is defective in mp mutants. (P,Q) Gene dosage-specific phenotypes of amp1 mp double mutants. Note that the restoration of flower formation is far more complete in a double mutant comprising a strong amp1 and a weak mp allele (amp1-9 mpG92 in P) than in combination with a strong mp allele (amp1-1 mpBS1354 in Q). (R-U) Flower phenotypes. Normal flower organs in amp1 mutants (amp1-9 in S), inflorescence with few, highly reduced flowers in mpG92 mutants (U) and intermediate, fertile flowers in amp1-9 mpG92 mutants (T). (V-Y) Scanning electron micrographs of inflorescences indicating that no size abnormalities were observed in amp1 mutants. (Z-AC) Light-grown seedlings at 7 DAG. Note the advanced stage of the lateral root system in amp1 mutants. (AD-AG) Dark-grown seedlings at 14 DAG. Adventitious roots formed in amp1-9 and amp1-9 mpG92 mutants (arrows point to hypocotyl-root junction). Scale bars: 100 µm in A-D,V-Y; 50 µm in E-M; 1 mm in R-T; 5 mm in Z-AC; 2 mm in N,AD-AG.

View larger version (38K): [in this window] [in a new window]   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).

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