75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

305 Role of HD2 family in Seed Development/Embryogenesis
Adam Colville 2, 1 , Tim Xing 2 , Brian Miki 1
1
Agriculture and Agri-Food Canada, Ottawa, ON, CAN , 2 Carleton University, Ottawa, ON, CAN
Histone modification plays a critical role in maintaining and altering the epigenetic state of the cell. This process is
carried out by the acetylation/deacetylation of core histones of a nucleosome enabling the temporal and spatial expression
of distinct genes required for normal plant growth and development. The plant kingdom contains the HD2 family of
histone deacetylases, bearing no sequence similarity to previously characterized histone-modifying enzymes. Arabidopsis
thaliana genome contains 4 HD2 family members (HD2A, HD2B, HD2C, HD2D). In situ hybridization of HD2A, B
and C show high levels of expression in ovules, embyros, shoot apical meristem and primary leaves. HD2A suppression
(missense) and overexpression lines showed pleiotropic developmental abnormalities including aborted seed development
indicating a possible role of HD2A in seed and embryo development.
Analysis of HD2 suppression (RNAi) lines for all HD2 genes showed no visible phenotype. Single loss-of-function
mutant lines for HD2A and HD2C, the two most structurally similar HD2s each with three domains were identified
but showed no visible phenotype indicating possible functional redundancy. Double mutant of hd2a and hd2c loss-offunction
mutants are identical to wildtype until embryogenesis and seed development. Siliques contain ~23.5% seeds
with an abnormal appearance and 27% of progeny failed to germinate. Analysis of these seeds found embryos arrested
at all major stages of embryogenesis (globular, heart, torpedo and bent-cotyledon).
306 The Nuclear Actin-Related Protein ARP6 is a Pleiotropic Developmental Regulator Required
for the Maintenance of FLOWERING LOCUS C (FLC) Expression and Repression of Flowering
in Arabidopsis
Roger Deal, Muthugapatti Kandasamy, Elizabeth McKinney, Richard Meagher
Department of Genetics, University of Georgia, Athens, GA
Actin-related proteins (ARPs) are found in the nuclei of all eukaryotic cells, but their functions are generally understood
only on the basis of their enigmatic presence in various yeast and animal chromatin-modifying complexes. Arabidopsis
thaliana ARP6 is a clear homolog of S. cerevisiae ARP6, which was identified as a component of the SWR1 chromatin
remodeling complex. The yeast SWR1 complex deposits the conserved histone variant H2A.Z into euchromatic regions,
where it acts to antagonize the encroachment of silent heterochromatin or to poise quiescent promoters for full activation.
In order to address the function of ARP6 in Arabidopsis we have examined the subcellular localization, expression patterns,
and loss-of-function phenotypes for this protein. We found that Arabidopsis ARP6 was localized to the nucleus and a large
part of the cellular pool of this protein was engaged in a high molecular weight complex. ARP6 expression was observed
in all vegetative tissues and in a subset of reproductive tissues. Null mutations in ARP6 resulted in a multitude of defects
including altered development of the leaf, inflorescence, and flower, as well as reduced female fertility and early flowering
in both long- and short-day photoperiods. The early flowering of arp6 mutants was associated with reduced expression
of the central floral repressor gene FLOWERING LOCUS C (FLC), as well as MADS AFFECTING FLOWERING
4 (MAF4) and MAF5. In addition, arp6 mutations suppressed the FLC-mediated late flowering of a FRIGIDA (FRI)
expressing line, indicating that ARP6 is required for the activation of FLC expression to levels that inhibit flowering.
Together these results indicate that ARP6 acts in the nucleus to regulate multiple aspects of plant development, and that
it does so at least in part by regulating the expression of developmentally important genes. We discuss evidence for the
existence of a SWR1-like complex in plants and present a model for how the H2A.Z variants might serve to control
epigenetic switches during plant development.