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Review
. 2004 Jun;135(2):685-94.
doi: 10.1104/pp.104.040394.

Promoter bashing, microRNAs, and Knox genes. New insights, regulators, and targets-of-regulation in the establishment of lateral organ polarity in Arabidopsis

Eric M Engstrom  1 , Anat Izhaki, John L Bowman
Affiliations
Review

Promoter bashing, microRNAs, and Knox genes. New insights, regulators, and targets-of-regulation in the establishment of lateral organ polarity in Arabidopsis

Eric M Engstrom et al. Plant Physiol. 2004 Jun.
No abstract available

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Figures

Figure 1.
Figure 1.
Four axes of development in seed plants. The apical-basal axis (1) of the plant represents a polarity established in embryogenesis with the shoot apical meristem being at the apical end and the root meristem residing at basal tip shown here in a longitudinal section of an Arabidopsis shoot (A). The other 3 axes depicted are defined with reference to this apical-basal axis. The central (ce)-peripheral (pe) axis (2) in the stem (B) is analogous to the adaxial (ad)-abaxial (ab) axis (4) of lateral organs (D) with the central/adaxial end being adjacent to the center of the shoot and the peripheral/abaxial end being away from the center of the shoot meristem (m). Lateral organs such as leaves also exhibit a proximal-distal axis (3) that can also be defined with respect to the stem, with the distal end away from the stem and the proximal end attached to the stem, as shown in the Acer leaf in C. Positions of tissues within vascular bundles develop with respect to the central-peripheral axis with the phloem (ph) positioned peripherally, the xylem (xy) positioned centrally in the stem (B), and the xylem positioned adaxially and phloem abaxially in lateral organs (E). In addition to polar differentiation of vascular tissues, leaf asymmetry is evident with palisade mesophyll (pm) differentiating adaxially and spongy mesophyll (sp) positioned abaxially, as shown in E, a longitudinal section of an Arabidopsis leaf.
Figure 2.
Figure 2.
Schematic representation of the FIL promoter as elucidated by Watanabe and Okada (2003). The start-codon in the first FIL exon is located at the far right. Position 5′ of the Start-codon is indicated above the promoter. Regions containing the two enhancer elements and the FIL activation element are delineated by solid bars. The 12-bp Adaxial exclusion element is defined by the solid box. Relative sizes of all regions are depicted to scale.
Figure 3.
Figure 3.
Conceptual framework of adaxial-abaxial polarity establishment in lateral organs of Arabidopsis. Organs arise from the flanks of shoot meristems. The organ anlage is marked by the expression of AS1 and concomitant repression of class I Knox genes. The apical meristem is the source of a putative signal(s). Reception of the signal activates the PHB protein (PHB*). Activated PHB* promotes PHB transcription and represses transcription of the YABBY and KANADI genes. PHB in turn is negatively regulated at the transcriptional and posttranscriptional levels by the KANADI genes and microRNAs 165,166, respectively. The dashed line linking KAN to MIR165,166 reflects the hypothesis that negative regulation of PHB expression by KANADIs could occur in part or in whole through KANADI activation of MIR165,166 transcription. PHB and KANADI are predicted to sort into adaxial and abaxial domains of expression, respectively, and this may establish the adaxial-abaxial axis of the lateral organ. Expression of YABBY genes is predicted based on the structure of the FIL promoter, to be localized to the abaxial domain of the organ anlagen by a combination of transcriptional activation throughout the anlage coupled with repression in the adaxial region. YABBY genes can promote differentiation of abaxial cell fates. AS2 is predicted to be expressed in the adaxial organ domain where AS2 forms a complex with AS1 (AS2/AS1) to promote repression of KANADI and differentiation of adaxial cell fates. Interactions between AS1/AS2 and other components of polarity establishment remain unclear, as does the extent to which AS1/AS2 promotes axis establishment, polar differentiation, or both processes. Both YABBY genes and AS1/AS2 repress expression of class I Knox genes. Juxtaposition of adaxial and abaxial organ domains is a requirement for the establishment of lateral growth.
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