Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2010 Jul;22(7):2113-30.
doi: 10.1105/tpc.110.075853. Epub 2010 Jul 13.

Differentiating Arabidopsis Shoots From Leaves by Combined YABBY Activities

Affiliations
Free PMC article

Differentiating Arabidopsis Shoots From Leaves by Combined YABBY Activities

Rajani Sarojam et al. Plant Cell. .
Free PMC article

Abstract

In seed plants, leaves are born on radial shoots, but unlike shoots, they are determinate dorsiventral organs made of flat lamina. YABBY genes are found only in seed plants and in all cases studied are expressed primarily in lateral organs and in a polar manner. Despite their simple expression, Arabidopsis thaliana plants lacking all YABBY gene activities have a wide range of morphological defects in all lateral organs as well as the shoot apical meristem (SAM). Here, we show that leaves lacking all YABBY activities are initiated as dorsiventral appendages but fail to properly activate lamina programs. In particular, the activation of most CINCINNATA-class TCP genes does not commence, SAM-specific programs are reactivated, and a marginal leaf domain is not established. Altered distribution of auxin signaling and the auxin efflux carrier PIN1, highly reduced venation, initiation of multiple cotyledons, and gradual loss of the SAM accompany these defects. We suggest that YABBY functions were recruited to mold modified shoot systems into flat plant appendages by translating organ polarity into lamina-specific programs that include marginal auxin flow and activation of a maturation schedule directing determinate growth.

Figures

Figure 1.
Figure 1.
Phylogenetic Distribution of Angiosperm Leaf Polarity Genes. Origins of leaves, megaphylls, and microphylls are indicated, as are expression patterns of YABBY genes in seed plants (ab, abaxial; ad, adaxial). Dashed lines indicate that the antiquity of gene families is not known. The phylogenetic distribution of selected genes involved in angiosperm leaf polarity was determined utilizing genome sequence data available at present for land plant lineages (Sawa et al., 1999a; Siegfried et al., 1999; Golz et al., 2004; Yamada et al., 2004; Harrison et al., 2005; Floyd et al., 2006; Prigge and Clark, 2006; Floyd and Bowman, 2007; Rensing et al., 2008).
Figure 2.
Figure 2.
Redundancy of YABBY Gene Activity. (A) to (C) YAB3 (yab3-2), YAB5 (pYAB5:GUS), and YAB2 (pYAB2:GUS) display unique gene expression patterns (9-d-old seedlings, top view). (D) YAB3 is initially expressed throughout the leaf primordium (inset) but is switched off as the leaf differentiates. Note also the absence of expression in the petiole and midrib region. (E) YAB5 expression is more widespread, with highest levels in the petiole and midrib region. (F) YAB2 expression is restricted sharply to the petiole and midrib region. (G) and (H) Cross sections reveal that at early stages, FIL and YAB3 expression is throughout the abaxial regions of leaves (arrowheads), but at later stages, FIL expression becomes localized to the margins (arrows). (I) pYAB5:GUS is initially detected near vascular bundles and is later primarily in the central abaxial region of leaf primordia (arrow). (J) Expression of pYAB2:GUS is limited to the abaxial midrib (arrow). (K) to (N) Whole plant images of wild-type and YABBY multiple mutant plants. (O) to (S) Inflorescence structure of wild-type and YABBY mutant plants. The transition from leaf (l) production to trichomeless bract-like (b) structures is noted in (S). (T) and (U) Radial leaf of a fil yab235 plant (T) and a cross section of a radial leaf (U). p, phloem; x, xylem. (V) Occasionally formed axillary flower (arrow) consisting of a solitary gynoecium. (W) fil yab235 crc pentuple mutants lack carpelloid organs. Bar in (K) to (R) = 1 mm; bar in (U) and (V) = 250 μm; bar in (W) = 500 μm.
Figure 3.
Figure 3.
Morphological and Molecular Markers Show a Loss of Polar Differentiation in yabby Leaves. (A) and (B) Scanning electron microscopy images of wild-type and fil yab235 seedlings showing adaxial trichomes on the first two leaves. (C) to (F) The adaxial and abaxial epidermises of wild-type leaves show distinct cell types, while this is lost in fil yab235leaves. (G) and (H) Transverse leaf sections showing a loss of polar differentiation in fil yab235 leaves and additional cell layers. ab, abaxial; ad, adaxial. (I) to (K) Leaf vasculature: phloem (green arrows) and xylem (red arrows). (L) to (N) GUS expression in yab3-2. (O) The polarity index (right) and color-coded normalized expression of leaf polarity genes (left), derived from genes modified in the adaxial (phb-1d) and the abaxial (pANT>>KAN2) leaves. YABBY mutants show no overall bias in the expression of these genes. Bars = 100 μm. Bar in (A) to (F) = 100 μm.
Figure 4.
Figure 4.
YABBY Mutants Are Defective in Lamina Production and Repression of Meristem Gene Expression. (A) Expression of leaf lamina genes, defined as downregulated in three laminaless genotypes: phb-1d, pANT>>KAN2, and fil yab3 yab5. Note that most of these genes are downregulated in the loss of eight TCP genes (right). (B) The CIN-TCP gene family is downregulated in polarity and YABBY mutants. (C) DDI based on a set of genes with leaf expression modified with age reveals that YABBY triple mutants have a younger transcriptome, on par with that of loss of eight TCPs. Note that severe polarity mutants have a bimodal DDI distribution. (D) pWUS:GUS marker is ectopically expressed in the pANT>>miR-YAB13 leaves.
Figure 5.
Figure 5.
Loss of YABBY Function Is Associated with a Loss of Leaf Margin Cells. (A), (C), and (E) Wild-type margins are characterized by elongate marginal cells (e; red in [A] and [C]) and small isodiametric adaxial cells (s). (B), (D), and (F) yab1235 leaves lack these cell types, with occasional large cells (red in [B]) resembling large abaxial cells (see Figures 3C and 3D). (G) to (J) Marginal cell marker YJ158 is uniformly expressed in wild-type leaf marginal cells ([G] and [I]), and expression is patchy in pANT>>miR-YAB13 leaves ([H] and [J]). Bars = 100 μm.
Figure 6.
Figure 6.
Loss of YABBY Function Is Associated with Defects in Auxin Patterning. (A) to (C) Venation patterns of wild-type (A), fil yab3 (B), and fil yab235 (C) second formed leaves. Leaves of fil yab235 seedlings arise in a variety of unusual shapes (see Supplemental Figure 4 online). (D) Scanning electron microscopy image of a fil yab235 leaf similar in shape to the one shown in (C). (E) to (J) pATHB8:GUS expression in wild-type and pANT>>miR-YAB13 seedlings at 3 ([E] and [F]), 5 ([G] and [H]), and 14 ([I] and [J]) d after germination. (K) to (P) DR5 expression in wild-type and pANT>>miR-YAB13 seedlings. (K) to (L) DR5:GFP signal in leaves 1 and 2 of 4-d-old seedlings. Red signal is chlorophyll autofluorescence. (M) and (N) DR5:GUS expression in leaf 3 of 10-d-old seedlings. (O) and (P) DR5:GUS expression in leaf 2 of 10-d-old seedlings. (Q) and (S) Scanning electron microscopy images of fil/+ yab235 (Q) and fil yab235 (S) seedlings. Note the unusually shaped leaves of the YABBY quadruple mutant (S) with bulges of tissue at the sides (arrows). (R), (T), and (U) to (W) PIN1-GFP (green) expression in the first two to three leaves of yab235 ([R] and [U]; top view) and fil yab235 ([T], [V], and [W]; side view) seedlings. (U) to (W) show longitudinal optical sections showing the different pattern of PIN1-GFP expression in the YABBY quadruple mutant leaf ([V] and [W]) compared with a leaf from a seedling with a functional copy of the FIL gene (U). Arrowheads point to original auxin maxima of leaf primordia, and arrows point to sites of secondary auxin maxima. Bars = 100 μm.
Figure 7.
Figure 7.
Embryogenesis in yabby Mutants. (A) Wild-type embryo. (B) and (C) fil yab235 embryos with three distinct cotyledons (B) or multiple, sometimes fused cotyledons (C). (D) and (E) Reticulate vascular network of wild-type cotyledons as compared with solitary vascular traces of YABBY mutant cotyledons. Note conspicuous expression of PIN1-GFP in the SAM of wild-type plants (arrow). (F) to (M) PIN1:PIN1-GFP in wild-type ([F][I]) and fil yab235 ([J]–[M]) embryos. Red signal is chlorophyll autofluorescence. Arrows in (H) and (L) highlight prominent PIN1-GFP expression in the wild type that is lacking in YABBY quadruple mutants. (N) to (S) pCLV3>>GFP-ER expression in wild-type, fil-8 yab3-2, and pAS1:YAB3 embryos at the torpedo stage ([N][P]) and a later stage ([Q][S]) of embryogenesis. (T) and (U) DR5:GFP in wild-type (T) and fil yab235 (U) embryos. (V) and (W) Wild-type seedling and apex. (X) to (Z) fil yab235 seedlings and apex. M, meristem.
Figure 8.
Figure 8.
The Roles of YABBY Genes during Leaf Development. YABBY genes perform different tasks at different stages of Arabidopsis leaf development. The numbers on the left correspond to the approximate leaf stages illustrated by the leaf primordium numbers on the petunia (Petunia hybrida) SAM, with green shading representing YABBY gene expression. During early stages (1 and 2), YABBY gene expression is activated abaxially in response to earlier acting polarity genes, and YABBY activity during these stages is required for proper signaling between leaf primordia and the SAM (Goldshmidt et al., 2008). YABBY-LEUNIG complexes likely act during these stages, since leunig mutations enhance SAM loss in a YABBY mutant background (Stahle et al., 2009). YABBY activity is required to limit auxin flows to lateral margins (stages 2 and 3), thus influencing leaf margin growth and differentiation and, consequently, reticulate vascularization of the leaf. YABBY activity is also required to initiate leaf-specific genetic programs, such as that defined by CIN-TCP genes, which lead to subsequent events in leaf differentiation (stages 3 and 4).

Comment in

Similar articles

See all similar articles

Cited by 75 articles

See all "Cited by" articles

Publication types

Substances

Associated data

LinkOut - more resources

Feedback