370 C.L.H.Hord·H.Ma
sistent with the tapetum hypertrophy in themyb33 myb65double mutant at
this stage. TheMYB33andMYB65genes are also post-transcriptionally regu-
lated by microRNAs (miRNAs) (Millar and Gubler 2005). miRNAs can guide
cleavage of the cognate mRNAs, and cleavage products of bothMYB33and
MYB65have been isolated (Palatnik et al. 2003). In addition, it was shown that
overexpression of the miRNA miR159a causes cleavage ofMYB33and male
sterility (Achard et al. 2004).
A rice mutant namedundeveloped tapetum1(udt1)wasrecentlyisolated
from a T-DNA insertional screen (Jung et al. 2005). In theudt1mutant, anther
development appears normal through the pre-meiotic stages (stage 5) (Jung
et al. 2005). However, at the time of meiosis the mutant tapetal cells become
highlyvacuolatedandcontinuetoincreaseinsizethroughstage7;atthesame
time, the middle layer cells do not flatten or degenerate. From meiosis onward
all of the cell layers of the anther wall cell layers appear irregularly shaped
and abnormally stained. Theudt1PMCs undergo normal nuclear division,
forming dyads at the end of meiosis I. However, at the stage where tetrads
are normally formed the meiocytes are degraded. Thus,Udt1plays an im-
portant role in tapetum differentiation, beginning around meiosis, and may
be involved in the development of the other anther cell wall layers (Jung et al.
2005).
Semi-quantitative RT-PCR showed thatUdt1is strongly expressed within
the anther, from meiosis to pollen release. Using a GUS reporter gene, re-
searchers first observedUdt1expression after the initiation of tapetum de-
velopment.Udt1encodes a protein with a predicted basic helix-loop-helix
(bHLH) domain (Jung et al. 2005), which define a large family of putative
transcription factors, including theArabisdopsisABORTED MICROSPORES
(AMS, see below) and mammalian Myc proteins. Transient expression of
a35S:Udt1-GFPfusion construct revealed that GFP signal was localized to
the nucleus, indicating that UDT1 is likely a nuclear protein (Jung et al.
2005). Therefore,Udt1appears to be a key regulator of tapetum differentia-
tion and/or function at the time of meiosis and soon afterwards.
BecauseUdt1encodes a putative transcription factor, the transcript level
of three rice tapetum-specific markers was tested:Osc4andOsc6,whichen-
code protease inhibitors, andCys protease1(Jung et al. 2005; Lee et al. 2004;
Tsuchiya et al. 1994). Each of these genes is expressed in the wild type from
meiosis (stage 6) through the vacuolated pollen stage (about stage 10). In the
udt1mutant anthers, however, transcripts for these genes were undetectable
(Jung et al. 2005). This suggests that they may be downstream ofUdt1(Jung
et al. 2005). In addition, a potential rice homolog (Os02g02820) ofAMS,also
important for tapetum development (see below), is strongly expressed in the
wild-type anther but reduced in theudt1mutant (Jung et al. 2005). Further
investigation using microarray experiments revealed a large number of genes
that show reduced expression in theudt1mutant, including genes coding for
bHLH, MYB, WRKY, and APETELA2 transcription factors (Jung et al. 2005).