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First published online 24 January 2007
doi: 10.1242/dev.000497

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Establishing leaf polarity: the role of small RNAs and positional signals in the shoot apex

¹ Cold Spring Harbor Laboratory, One Bungtown Road, Cold Spring Harbor, NY 11724, USA.
² Watson School of Biological Sciences, One Bungtown Road, Cold Spring Harbor, NY 11724, USA.

View larger version (116K): [in this window] [in a new window]   Fig. 1. Adaxial-abaxial leaf architecture. (A) The adaxial side of an Arabidopsis leaf is dark green and trichome rich, whereas the abaxial leaf surface is matte, grey-green and trichome poor. (B) The adaxial and abaxial sides of a maize leaf blade (b) and sheath (s) are separated by the auricle (a) and the ligule (l), an adaxial, epidermal fringe. (C) Transverse section of an Arabidopsis leaf, showing adaxial palisade cells (p), abaxial spongy mesophyll cells (s) and the central midvein (mv). (D) Magnified cross-section of a vascular bundle in an Arabidopsis leaf, showing the spatial relationship between adaxial xylem (x) and abaxial phloem (ph). Images C and D, which were first published by Lin et al. (Lin et al., 2003), are reproduced with permission from the American Society of Plant Biologists.

View larger version (144K): [in this window] [in a new window]   Fig. 2. Meristem architecture. (A) Transverse section of an Arabidopsis vegetative shoot apex, showing the youngest (P1) and increasingly older (P2 and onward) leaf primordia and meristem (M). Notice the spiral phyllotaxis of leaves around the SAM. (B) Longitudinal section of an Arabidopsis meristem, showing the proximal-distal and adaxial-abaxial axes of leaf primordia relative to the SAM. (C) Scanning electron micrograph (SEM) of an Arabidopsis vegetative SAM, showing the spatial relationship of primordia relative to each other and to the meristem (M). (D) Transverse section of a maize SAM, showing successively older (P1-P4) leaf primordia encircling the SAM. (E) Longitudinal section of a maize SAM, showing the proximal-distal and adaxial-abaxial (Ad/Ab) axes of leaf primordia. Notice the alternate phyllotaxis. (F) SEM of a maize SAM with two leaf primordia. Images in C and F kindly provided by C. Kidner and D. Jackson, respectively.

View larger version (81K): [in this window] [in a new window]   Fig. 3. Surgical and genetically induced leaf-polarity defects. (A) Vegetative tomato shoot apex, showing ablation of the L1 layer (black arrowheads), separating the incipient leaf (I1) from the meristem. Leaf primordia of increasing age are shown (P1-P4). (B) Longitudinal section of a tomato apex, showing an incision restricted to the outermost (L1) layer of the SAM (black arrowhead), which separates the P1 primordium from the meristem. (C) SEM of a tomato apex 1 day after the P1 has been surgically separated from the SAM. The P1 develops as a radial, abaxialized primordium (black arrowhead shows the scar from the ablation). By contrast, a primordium that developed in contact with the meristem (I1) is dorsoventrally flattened and develops leaflet pairs (white arrowheads). (D) Transverse sections of Antirrhinum leaves from wild-type (WT) and phan plants. Relative to wild type, the phan leaf is radial and composed of abaxial parenchyma surrounded by abaxial epidermis. Notice that, in the phan mutant, vascular polarity is lost and that the xylem no longer lies adaxial relative to normally abaxial phloem. Images A-C are reproduced with permission from Reinhardt et al. (Reinhardt et al., 2005) and D with permission from Waites and Hudson (Waites and Hudson, 1995).

View larger version (73K): [in this window] [in a new window]   Fig. 4. Genetic interactions that establish adaxial-abaxial polarity in leaves. Pathways are divided into those contributing to adaxial versus abaxial fate and those involving protein components (A) versus the biogenesis and activity of small RNAs (sRNAs) (B). Direct interactions are distinguished from indirect and putative (dashed line) interactions by bold arrows and lines. On the left are false-colored meristems representing generalized expression patterns across plant lineages. The colors of the expression domains correspond to the colors of the factors that they represent.

View larger version (85K): [in this window] [in a new window]   Fig. 5. Hypothetical signals specifying leaf polarity in the meristem. miR166 (blue) accumulates most strongly below the incipient leaf (I1, white dashed line) and in a graded pattern in I1 and older primordia (P1, P2). Such a gradient might be formed by the movement of miR166 itself or may be directed by a secondary mobile signal. A hypothetical lipid signal (red), possibly required for HD-ZIPIII activity, is predicted to act in the meristem (M). If such a lipid contributes to the Sussex signal, it might be restricted to the L1 layer, as shown. The distribution of auxin within the primordia and stem (green), as inferred from the localization of AUX1 and PIN1 (Reinhardt et al., 2003), could contribute to abaxial fate through either ETT and/or ARF4 or perhaps MIR166. Notice that the distributions of auxin within the leaf are speculative. tasi-ARFs (yellow) might also regulate leaf polarity non-cell autonomously, possibly by moving out of the meristem tip, where SGS3/lbl1 is expressed.