Ovule Development: Identification of Stage- and Tissue-specific cDNAs
J. A. Nadeau, X. S. Zhang, J. Li and S. D. O'Neill
The Plant Cell 8: 213-239 (1996)
ABSTRACT
A differential screening approach was employed to identify seven
ovule-specific cDNAs representing genes that are expressed in a stage-specific
manner during ovule development. The Phalaenopsis orchid takes 80 days to
complete the sequence of ovule developmental events, making it a good system to
isolate stage-specific ovule genes. We constructed cDNA libraries from orchid
ovule tissue during archesporial cell differentiation, megasporocyte formation,
and the transition to meiosis, as well as during the final mitotic divisions of
megagametophyte development. RNA gel blot hybridization analysis revealed that
four clones were stage-specific and expressed solely in ovule tissue, whereas
one clone was specific to pollen tubes. Two other clones were not
ovule-specific. Sequence analysis and in situ hybridization revealed the
identities and domain of expression of several of the cDNAs. O39 encodes a
putative homeobox transcription factor that is expressed early in the
differentiation of the ovule primordium; O40 encodes a cytochrome P450
monooxygenase (CYP78A2) that is pollen tube specific. O108 encodes a protein
of unknown function that is expressed exclusively in the outer layer of the
outer integument and in the female gametophyte of mature ovules. O126 encodes
a glycine-rich protein that is expressed in mature ovules, and O141 encodes a
cysteine proteinase that is expressed in the outer integument of ovules during
seed formation. Sequences homologous to these ovule clones can now be isolated
from other organisms that should facilitate their functional
characterization.