Genetic Control of Anther Cell Division and Differentiation 369
Fig. 3 Cross-sections ofmyb33 myb65double mutant anthers.AAround anther stage 6,
pre-meiotic PMCs have a thick callose layer, but have become irregularly shaped under
an abnormally expanding tapetum layer.BA later stage anther showing tapetal cells that
are highly vacuolated and have expanded radially inward around degrading PMCs.CAn
anther with adaxial lobes undergoing normal development and abaxial lobes with hyper-
trophied tapetal cells. T, tapetum; PMC, pollen mother cells. This figure was modified
from Figs. 3 and 4 from Millar and Gubler (2005) with permission from the American
Society of Plant Biologists
tures ( 16 ◦Cvs. 22 ◦C), and were almost as fertile as the wild type ( 75 %vs.
90 – 96 % filled siliques). Both high light and low temperatures increase the
level of soluble carbohydrates in plants (Hurry et al. 1995). Carbohydrate
reserves are mobilized in the tapetum just before meiosis; this timing coin-
cides with the mutant defect (Clément et al. 1994; Millar and Gubler 2005).
ThusMYB33andMYB65may play a role in starch mobilization similar to the
function of a knownMYBgene from barley (HvGAMYB) (Millar and Gubler
2005). Indeed,MYB33andMYB65were shown be capable of activating the
α-amylase promoter in the place ofHvGAMYB(Gocal et al. 2001). Further-
more, theGAMYBmutants in rice and barley have similar anther defects,
exhibiting tapetum hypertrophy prior to meiosis (Kaneko et al. 2004; Mur-
ray et al. 2003). Therefore, it is possible that the role ofMYBgenes in anther
development may be conserved between monocots and eudicots (Millar and
Gubler 2005).
MYB33is expressed at low levels in anthers at stages 5 and 6, and at much
higher levels at stage 7 in all four cell layers of the anther wall, as well as
in the tetrads.MYB33expression in the tapetum just before meiosis is con-