Based on complementary vascular and leaf phenotypes of class III HD-ZIP and KANADI mutants, we propose that a common genetic program dependent upon miRNAs governs adaxial-abaxial patterning of leaves and radial patterning of stems in the angiosperm shoot. This finding implies that a common patterning mechanism is shared between apical and vascular meristems.
The phenotypes of KANADI loss- and gain-of-function alleles suggest that fine regulation of these genes is at the core of polarity establishment. As such, they are likely to be targets of the PHB-mediated meristem-born signaling that patterns lateral organ primordia. PHB-like genes and the abaxial-promoting KANADI and YABBY genes appear to be expressed throughout primordia anlagen before becoming confined to their corresponding domains as primordia arise. This suggests that the establishment of polarity in plant lateral organs occurs via mutual repression interactions between ab/ad factors after primordium emergence, consistent with the results of classical dissection experiments.
Asymmetric development of plant lateral organs is initiated by a partitioning of organ primordia into distinct domains along their adaxial/abaxial axis. Two primary determinants of abaxial cell fate are members of the KANADI and YABBY gene families. Progressive loss of KANADI activity in loss-of-function mutants results in progressive transformation of abaxial cell types into adaxial ones and a correlated loss of lamina formation. Novel, localized planes of blade expansion occur in some kanadi loss-of-function genotypes and these ectopic lamina outgrowths are YABBY dependent. We propose that the initial asymmetric leaf development is regulated primarily by mutual antagonism between KANADI and PHB-like genes,which is translated into polar YABBY expression. Subsequently, polar YABBY expression contributes both to abaxial cell fate and to abaxial/adaxial juxtaposition-mediated lamina expansion.
Lateral organs of plants display asymmetry with abaxial identity being specified by members of the Arabidopsis YABBY gene family. Mutations in CRABS CLAW, the founding family member, display ectopic formation of adaxial carpel tissues only when the functions of other genes, such as GYMNOS or KANADI, are also compromised. Mutations in these genes alone do not result in loss of polar differentiation, and therefore, they act redundantly with CRABS CLAW to establish polarity. As GYMNOS encodes a uniformly expressed homolog of the chromatin-remodeling protein, Mi2, we argue that the unique genetic interactions do not reflect a molecular redundancy. Rather, CRABS CLAW regulates transcription spatially, whereas GYMNOS regulates downstream targets temporally to ensure proper differentiation of the carpels.
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