Transcription of the ZmHox homeobox genes is activated early in embryogenesis and is abundant in meristematic tissues
--Bettina Klinge and Wolfgang Werr

Four different ZmHox genes have been analyzed in their spatial expression pattern during development of the plant. The ZmHox1a /1b and ZmHox2a /2b gene pairs encode putative plant transcription factors and their transcripts belong to a class of rare mRNAs. The spatial expression pattern of the four homeobox genes was found to be very similar in all tested organs. Expression of the ZmHox1 and 2 genes is activated very early in development and restricted to the embryo proper four days after pollination. After establishment of the root/shoot axis, ZmHox transcripts are prevalent in the embryonic root and shoot apical meristems and are found in provascular tissues and young leaf primordia. No signals were observed in the scutellum, the coleoptile or the coleorhiza of the embryo. In the seedling, the transcription of the ZmHox genes marks all kinds of meristems of the shoot and root system, the descending proliferating regions and the provascular strands. As the vegetative shoot apical meristem undergoes transition to a floral meristem, ZmHox expression persists in the developing tassel. At the same time high transcript levels are detectable in the initiating axillary buds. In differentiated inflorescences ZmHox transcripts become more restricted to the nucellar region and the silk of the female flower, and to the innermost layer of the sporangium wall, the tapetum cells of the developing anthers. The data show that all four ZmHox genes are expressed in meristems and proliferating tissues where developmental decisions contribute to the ontogeny of the plant. The overall expression pattern suggests that the ZmHox class of homeobox genes might be involved in transcriptional control during development from the embryonic to the reproductive phase, which is supported with the pleiotropic effects observed in transgenic tobacco plants.
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