Abaxial Cell Fate Research Articles

YABBY polarity genes mediate the repression of KNOX homeobox genes in Arabidopsis.

The YABBY (YAB) genes specify abaxial cell fate in lateral organs in Arabidopsis. Loss-of-function mutants in two early-expressing YAB genes, FILAMENTOUS FLOWER (FIL) and YAB3, do not exhibit vegetative phenotypes as a result of redundancy. Mutations in these genes result in the derepression of the KNOX homeobox genes SHOOTMERISTEMLESS (STM), BREVIPEDICELLUS, and KNAT2 in the leaves and in the partial rescue of stm mutants. Here, we show that fil yab3 double mutants exhibit ectopic meristem formation on the adaxial surfaces of cotyledons and leaf blades. We propose that in addition to abaxial specification, lateral organ development requires YAB function to downregulate KNOTTED homeobox genes so that meristem initiation and growth are restricted to the apex.

Zinc Release from the CH2C6 Zinc Finger Domain of FILAMENTOUS FLOWER Protein from Arabidopsis thalianaInduces Self-assembly

The FILAMENTOUS FLOWER gene from Arabidopsis thaliana is a member of a gene family whose role is to specify abaxial cell fate in lateral organs. Analysis of the amino-terminal region of the FILAMENTOUS FLOWER protein suggests that seven cysteine residues at positions 14, 26, 30, 33, 54, 56, and 57, and two histidine residues at positions 18 and 24 contribute to a putative zinc finger motif, Cys-X(3)-His-X(5)-His-X-Cys-X(3)-Cys-X(2)-Cys-X(20)-Cys-X-Cys-Cys. Zinc determination experiments revealed that the FILAMENTOUS FLOWER protein binds two zinc ions per molecule. Chemical modification was required to release one zinc ion, whereas the other was released spontaneously or more rapidly in the presence of metallochromic indicator. The loss of a zinc ion and the subsequent structural change of the zinc finger domain were correlated with the multimerization of the FILAMENTOUS FLOWER protein. A cysteine residue at position 56 in the FILAMENTOUS FLOWER protein potentially interferes with zinc ligation within the zinc finger and causes this zinc release. In support of this, substitution of the Cys(56) by alanine suppressed both the zinc release and the multimerization of the FILAMENTOUS FLOWER protein. Deletion analysis showed that the region between positions 45 and 107 functions in the intermolecular contacts between FILAMENTOUS FLOWER proteins. This region corresponds to the carboxyl-terminal half of the zinc finger domain and the following hydrophobic region containing two putative alpha-helices. Our results suggest that the FILAMENTOUS FLOWER protein forms a range of different conformers. This attribute may lead to a greater degree of functional flexibility that is central to its role as an abaxial cell fate regulator.

Plant development: YABBYs claw to the fore

The YABBY gene family was identified recently by homology to CRABS CLAW, a gene involved in carpel and nectary development in Arabidopsis. Several of the transcription factors encoded by the YABBY genes appear to have conserved roles in specifying abaxial cell fate in leaves, floral organs and ovules.

Members of the YABBY gene family specify abaxial cell fate in Arabidopsis.

Lateral organs produced by shoot apical and flower meristems exhibit a fundamental abaxial-adaxial asymmetry. We describe three members of the YABBY gene family, FILAMENTOUS FLOWER, YABBY2 and YABBY3, isolated on the basis of homology to CRABS CLAW. Each of these genes is expressed in a polar manner in all lateral organ primordia produced from the apical and flower meristems. The expression of these genes is precisely correlated with abaxial cell fate in mutants in which abaxial cell fates are found ectopically, reduced or eliminated. Ectopic expression of either FILAMENTOUS FLOWER or YABBY3 is sufficient to specify the development of ectopic abaxial tissues in lateral organs. Conversely, loss of polar expression of these two genes results in a loss of polar differentiation of tissues in lateral organs. Taken together, these observations indicate that members of this gene family are responsible for the specification of abaxial cell fate in lateral organs of Arabidopsis. Furthermore, ectopic expression studies suggest that ubiquitous abaxial cell fate and maintenance of a functional apical meristem are incompatible.