372 C.L.H.Hord·H.Ma
Fig. 4 Cross-sections of wild-type No-0 andms1anther lobes.AWild-type at anther
stage 9 has microspores with an exine layer.BThems1mutant microspores at stage 9
have no exine layer.CInms1, subsequent to stage 9, the tapetum and microspores be-
come vacuolated and eventually degradeDleaving the locules empty. Ep, epidermis; En,
endothecium; T, tapetum; Msp, microspores (Courtesy of Takuya Ito)
of the microspores appears granular and vacuolated prior to degeneration
(Wilson et al. 2001).MS1expression was only found in the inflorescence
(Ito and Shinozaki 2002; Wilson et al. 2001). In addition, in situ hybridiza-
tion showed that in anthers,MS1transcript could only be detected in the
tapetal cells at the tetrad stage (stage 7) (Ito and Shinozaki 2002). Locules of
the same anther at a slightly different developmental stage did not show any
MS1expression, indicating a very short period of expression. Taken together,
the mutant phenotype and expression pattern imply thatMS1expression at
stage 7 is required for the tapetum to produce materials essential for sub-
sequent exine development on microspores at stage 9 (Ito and Shinozaki
2002). Therefore,MS1function is necessary for normal tapetum function
after stage 8.
MS1codes for a protein with three predicted domains: a nuclear local-
ization signal, a PHD-finger motif and a leucine zipper-like sequence with
sequence similarity to anArabidopsismitochondrial ORF. PHD-finger motifs
are thought to be involved in protein-protein interactions and proteins con-
taining these motifs may participate in transcriptional regulation (Aasland
et al. 1995; Ito and Shinozaki 2002). Using a GFP fusion construct and particle
bombardment, it was shown that the N-terminal region of MS1 is sufficient to