Genetic Control of Anther Cell Division and Differentiation 363
The establishment of anther cell layers, histogenesis, occurs within the first
five stages. Although the relative roles that position and cell lineage play in an-
ther cell differentiation have yet to be elucidated (Goldberg et al. 1993; Scott
et al. 2004), by early anther stage five, the cell layers are fully established with
the endothecium, middle layer and tapetum forming rings around the pollen
mother cells (PMCs). At this stage, the PMCs and tapetal cells are connected to
each other and to their own cell type by plasmodesmata (Owen and Makaroff
1995). Post-histogenesis growth also requires the coordinated development
and function of sporophytic and gametophytic cells, particularly the tape-
tum, which supports the growth of the developing pollen at later stages. In
addition, tapetum degeneration and its timing are very important for proper
microspore development (Wu and Cheun 2000). Following tapetum degener-
ation, the anther becomes bilocular as the septum separating the locules on
each side of the anther breaks. Subsequently, anther dehiscence occurs to al-
low the release of pollen. The morphological description ofArabidopsisanther
development provides a basis for the characterization of mutants defective in
anther development (Owen and Makaroff 1995; Sanders et al. 1999). This chap-
ter presents recent advances in our understanding of the control of anther cell
division and differentiation from genetic and molecular analyses.
2
Control of Early Anther Development by SPL/NZZ
An early acting anther gene isSPOROCYTELESS/NOZZLE(SPL/NZZ), which
is required for the formation of the PMCs and also has a role in ovule devel-
opment (Balasubramanian and Schneitz 2000; Schiefthaler et al. 1999; Yang
et al. 1999). In the stage 5splanthers, there are no normal somatic or sporoge-
nous cells; however, the nature of the earliest defect is not clear. Yang et al.
(1999) indicated that the archesporial cells ofsplanthers underwent a normal
periclinal cell division to form the primary parietal cells (PPCs) and primary
sporogenous cells (PSCs) at stage 3. On the other hand, Schiefthaler et al.
(1999) reported that thenzzmutant anther failed to show differentiation of
the archesporial cells. It is possible that the difference in phenotypic descrip-
tions reflect allelic variations and/or environmental effects. However, the lack
of molecular markers for early anther cell types makes it difficult to iden-
tify them. Nevertheless, it is clear thatSPL/NZZis required for normal early
anther development.
SPL/NZZencodes a putative transcription factor (Schiefthaler et al. 1999;
Yang et al. 1999). It was recently shown that the floral MADS-box protein
AGAMOUS (AG) can directly bind to the 3′end ofSPL/NZZand induce its
transcription (Ito et al. 2004). Using a fusion ofSPLto the rat glucocorticoid
receptor gene (35S::SPL-GR), it was shown that SPL/NZZ is sufficient to in-
duce microsporogenesis in a strongagmutant background, indicating that