Genetic Control of Anther Cell Division and Differentiation 371
5
Genes Affecting Tapetal Function and Pollen Development
Following the completion of meiosis, the tapetum is critical for normal pollen
development. TheABORTED MICROSPORES(AMS)geneplaysacriticalrole
in the post-meiotic tapetum that supports microspore development (Sorensen
et al. 2002). Theams mutant is male sterile and cannot produce pollen.
Althoughamsis normal in anther histogenesis, meiosis and microspore
formation, the microspores soon degenerate. In addition, theamstapetum
appears to begin premature degeneration. Eventually both microspores and
tapetum cells completely disintegrate.AMSexpression was strong in post-
meiotic floral buds, but not in open flowers, siliques, leaves or roots (Sorensen
et al. 2002). The highest levels of GUS activity from anAMS-GUSfusion were
seen in the tapetum and the nuclei of microspores, consistent with a role for
AMSin promoting tapetum function and pollen mitosis I. The regulation of
pollen mitosis I may be direct, viaAMSfunction in the microspores, or indi-
rect, through regulation of tapetum function, or both. TheAMSgene codes
for a Myc-like bHLH transcription factor (Sorensen et al. 2002) and is homol-
ogous to the rice gene Os02g02820, which is positively regulated byUdt1(Jung
et al. 2005). It is possible that AMS is part of a regulatory network that controls
tapetum differentiation and function (Sorensen et al. 2002).
Part of tapetal function includes biosynthesis of macromolecules that sup-
port pollen wall formation, such as lipids and proteins. One of the major
pathways for Lipid synthesis in plants generally follows one of two major
pathways: incorporation into triacylglycerol (TAG) or into membrane glyc-
erolipids. Glycerolipids are essential components of the cell membrane and
are likely necessary for maintaining membrane integrity as well as mem-
brane biogenesis (Zheng et al. 2003). Glycerol-3-phosphate acyltransferases
(GPATs) are enzymes that mediate the initial step of glycerolipid biosynthe-
sis. Tapetum and microspores are both known to be quite active in lipid
metabolism (Ferreira et al. 1997; Piffanelli et al. 1998; Platt et al. 1998), and
theArabidopsismutantatgpat1is defective in the development of both of
these cell types (Zheng et al. 2003). Although theatgpat1-1mutant appears
normal up to the time of microspore formation, subsequently the tapetal
cells begin to enlarge abnormally (Zheng et al. 2003). This defect underscores
the importance of lipid biosynthesis in tapetum function. In vitro assays
s h o w e d t h at ATG PAT 1 i s a n a c y l t r a n s f e r a s e s t h at c a n s p e c i fi c a l l y a c e t y l at e
glycerol-3-phosphate.
Another gene important for tapetum function is theMALE STERILE1
(MS1) gene, which appears to function later thanAtGPAT1and in a differ-
ent pathway. Thems1mutant appears to develop normally through stage 8,
but the microspores fail to form a complete exine wall (stage 9) and both
the tapetum and microspores become vacuolated and eventually degener-
ate (Fig. 4) (Ito and Shinozaki 2002; Wilson et al. 2001). Also, the cytoplasm