Genetic Control of Anther Cell Division and Differentiation 365
Fig. 1Cross-sections of wild-type andems1anthers at late stage 5.AThe wild anther lobe
has the five distinct cell layers.BTheems1anther lobe has a higher than normal number
of PMCs in the center of the lobe, and the tapetum layer is missing. Ep, epidermis; En,
endothecium; ML, middle layer; T, tapetum; PMC, pollen mother cells; MLP, middle layer-
positioned cells
was expanded to the extra PMCs in theems1mutant, whereas no signal was
detected for the tapetum-specific probeATA 7(Zhao et al. 2002).
In a normal stage 6 anther, PMCs are detached from each other and the
tapetum; in contrast, theems1/exsmutant PMCs were abnormally enlarged
and adhered to adjacent cells (Zhao et al. 2002). This may be due to ab-
normal callose accumulation/deposition on the mutant meiocytes (Canales
et al. 2002), supporting the hypothesis that tapetum is necessary for nor-
mal callose deposition and the physical separation of the PMCs. Neverthe-
less, theems1/exsmutants undergo meiotic nuclear events from prophase I
to telophase II (Canales et al. 2002; Zhao et al. 2002). However, theems1
meiocytes fail to undergo cytokinesis and degenerate without forming mi-
crospores (Zhao et al. 2002).
TheEMS1/EXStranscript was detected in archesporial cells at stage 2 and
subsequently in the L2-derived cells at stages 3 and 4 (Canales et al. 2002;
Zhao et al. 2002). At stage 5, theEMS1/EXStranscript was detected strongly in
the tapetum and less so in PMCs. TheEMS1/EXSexpression is greatly reduced
starting at stage 6. Canales et al. (2002) also detectedEMS1/EXSexpression
in the floral meristem and ovule primordia. TheEMS1/EXSgene encodes
a leucine-rich repeat receptor-like kinase (LRR-RLKs) (Canales et al. 2002;
Zhao et al. 2002), a member of the largest family of RLKs in plants (Shiu and
Bleecker 2001). Transient expression in onion epidermal cells of an EMS1-
GFP fusion suggests that EMS1 is localized to the cell surface; furthermore,
an in vitro assay showed that EMS1/EXS has autophosphorylation activity
(Zhao et al. 2002). These results support the hypothesis that it functions as
a receptor-like protein kinase to mediate an important developmental signal.
In addition toEMS1/EXS, two other LRR-RLKs,SERK1andSERK2,play
critical roles in tapetum formation (Albrecht et al. 2005; Colcombet et al.
2005). Although theserk1andserk2single mutants seem normal, theserk1
serk2double mutant has the same male fertility and anther development
phenotypes as those ofems1/exsmutants. Therefore,SERK1andSERK2act