Biology Reference
In-Depth Information
expression of male germline expressed genes is repressed in all non-germline cells
by a protein called Germline Restrictive Silencing Factor (GRSF), has recently
been proposed [14]. A binding site for the GRSF protein was identified in the
promoter region of the Lily male germline gene LGC1, and mutations in this
sequence led to the ectopic activation of the LGC1 promoter in non-germline
cells in lily and Arabidopsis. Although similar binding sites have been found in
the promoter regions of several germline genes in Arabidopsis, including the ger-
mline-specific transcription factor gene DUO1 [14], a functional role for these
sites or of GRSF activity in regulating gene expression in Arabidopsis pollen has
not been shown.
Germ cell division resulting in the sperm cell pair in each pollen grain, is
essential for double fertilization and recent data supports the capacity of both
sperm cells to fertilize the egg cell in Arabidopsis [15]. Several mutants have
been described in which germ cell division is disrupted [16]-[18]. Mutations in
the conserved cell cycle regulator CDKA1 [16],[17] and in the F-BOX protein
FBL17 [18] prevent germ cell division and result in mature pollen with a single
germ cell. Defects in Chromosome Assembly Factor 1 (CAF1) can also disrupt
germ cell division [19]. Interestingly, the single germ cells in these mutants are
capable of fertilization, with cdka1 and fbl17 mutant germ cells fertilizing the
egg cell to produce an embryo that aborts early in development due to the lack
of endosperm production. These mutations clearly demonstrate that germ cell
division and specification can be uncoupled, but do not identify how these pro-
cesses may be coordinated to produce twin sperm cells competent for double
fertilization.
DUO POLLEN1 (DUO1) is a unique male germ cell-specific R2R3 Myb
protein that is also required for germ cell division in Arabidopsis [20]. Unlike
cdka1 and fbl17 single germ cells, duo1 germ cells do not lead to successful
fertilization, suggesting that in addition to germ cell cycle defects, key features
of gamete differentiation and function are impaired in duo1. Here we further
characterize DUO1 as an essential, positive regulator of sperm cell production
in plants. We use various molecular markers and ectopic expression assays to
show that DUO1 is both necessary and sufficient for the expression of male
germline genes. We show that DUO1 is required for the expression of the Arabi-
dopsis G2/M regulator CyclinB1;1 (AtCycB1;1) in the male germline and that
AtCycB1:1 can partially rescue defective germ cell division in duo1. Our find-
ings reveal a novel integrative role for the germline-specific DUO1 protein, in
cell specification and cell cycle progression necessary for twin sperm cell produc-
tion. Furthermore, we show that restriction of DUO1 expression to the male
germline is not dependent on a putative GRSF binding site but involves positive
elements in the promoter.
