368 C.L.H.Hord·H.Ma
mac1mutant, the primary parietal cells do not seem to undergo any peri-
clinal division, and the sporogenous cells proliferate abnormally (Sheridan
et al. 1999) (Fig. 2). The innermost sporogenous cells enter meiosis, but they
are abnormal in shape, organization, and callose-deposition, and fail to pro-
ceed to beyond prophase I. Likemsp1in rice, themac1mutation in maize
also results in the over-proliferation of archesporial cells in the ovule (Nono-
mura et al. 2003; Sheridan et al. 1999). This implies that these genes act to
negatively regulate sporogenous cell division and to positively regulate pari-
etal cell differentiation, similar toEMS1/EXS,SERK1, SERK2,andTPD1in
Arabidopsis.
The results fromArabidopsis, rice, and maize suggest that there is a con-
served cell-cell signaling pathway that promotes the formation of the tapetum
layer and limits the number of PMCs. The mutant phenotypes also sup-
port the idea that the default developmental program favors the formation
of PMCs. Furthermore, molecular studies described here suggest that these
genes represent signaling pathway(s) that respond to signal(s) from the cen-
tral region of the PMCs and direct differentiation of the tapetum, which then
support meiosis and pollen development (Ma 2005).
4
Regulation of Normal Tapetum Development and Function
The coordinated differentiation and function of tapetal cells and the male
gametophyte is essential for normal pollen development. Several genes have
been identified that are important for tapetum differentiation and/or func-
tion, including the closely relatedMYB33and MYB65genes (Millar and
Gubler 2005). Disruption of either gene has no apparent effect on plant de-
velopment, whereas themyb33 myb65double mutant plants are conditionally
male sterile and fail to produce pollen, indicating that they function re-
dundantly (Millar and Gubler 2005). Cross sections of the mutant anthers
revealed that the male sterile phenotype was due to hypertrophy (abnormal
enlargement) of the tapetal cells from stages 5 to 6 (Fig. 3A and B). In addition
to having an increased size, the tapetal cells do not separate from one another,
as they do in the wild type. Furthermore, the tapetum appears unable to per-
form its normal functions. For example, callose deposits remain in the locules
where the tapetum has expanded, indicating that the tapetum is not releasing
callase (Millar and Gubler 2005).
Interestingly, male sterility in themyb33 myb65double mutant is affected
by environmental conditions. Occasionally normal locules were seen adjacent
to hypertrophied locules (Fig. 3C), suggesting that theMYB33/MYB65func-
tion might not be absolutely essential, but is important under certain condi-
tions. Fertility of the double mutant is significantly increased under relatively
high light (∼ 300 μmolμm–2s–1vs. 95 μmolμm–2s–1)andatlowtempera-