Four Zea mays homeobox (ZmHox) genes have been analyzed in their spatial expression pattern during development of the maize plant. The ZmHoxJa o r b and ZmHoxZa or b gene pairs encode putative plant transcription factors; the mRNA transcripts belong to the rare class of mRNA molecules. Expression of the ZmHoxI and 2 genes is activated very early in embryonic development and restricted to the embryo proper in the proembryo stage. After establishment of the root/shoot axis, transcripts are prevalent in the embryonic root and shoot apical meristems but later are also found in provascular tissues and young leaf primordia. In the vegetative plant body the transcription of the ZmHox genes marks several types of meristems of the shoot and root system, the descending proliferating regions and provascular strands. Expression persists in the developing reproductive organs, but results obtained in the male flower indicate that transcription of ZmHox genes here may become confined to specific cell types. The data obtained during this study show that these four ZmHox genes are expressed very specifically in cells where developmental decisions contribute to the ontogeny of the maize plant. The overall expression pattern suggests that the ZmHox class of maize homeobox genes will be involved in transcriptional control during maize development from the embryonic to the reproductive phase. 0 1995 Wiley-Liss, Inc.

Clones of two highly related genes, ZmHox2a and ZmHox2b (Zea mays homeobox), were isolated from maize embryo cDNA libraries by screening with the ZmHox1a homeobox sequence. The genes map to chromosomes 3 and 8, respectively, and encode mRNA transcripts of 6kb. The encoded proteins, ZmHox2a and b, share 84% sequence identity and exhibit a modular structure with several novel plant-specific protein domains. Interestingly, each ZmHox2a, gene product contains two complete homeodomains which, for Zmhox2a, were both shown to be functional DNA-binding motifs in vitro. Not only probes encoding the homeobox but also DNA fragments corresponding to other ZmHox2 domains hybridize to multiple bands in genomic Southern blots, indicating that related protein domains may be conserved in other maize genes. The ZmHox2a/b genes, therefore, are members of a novel and large class of maize genes, some of which can be expected to encode new transcription factors.

The ZmHox1a and ZmHox1b (for Zea mays homeobox) genes map on chromosomes 8 and 6, respectively. Both homeobox genes encode proteins that show 91% similarity and are transcribed simultaneously in meristematic and proliferating cells of the maize plant. To gain insight into the biological function of these genes, both open reading frames were expressed in tobacco, under the control of the cauliflower mosaic virus 35S promoter. The resulting transgenic ZmHox1a or ZmHox1b plants showed identical phenotypic alterations that fall into three classes: size reduction, formation of adventitious shoots, and homeotic floral transformations. Approximately 30% of the ZmHox1-expressing plants grew to only one-third of the wild-type size, and most axillary buds gave rise to lateral shoots. Flower abnormalities included formation of petaloid stamens and development of secondary flowers within the primary gynoecium. Therefore, the ectopic expression of the maize ZmHox1 homeobox gene products affects the vegetative as well as the reproductive phase of tobacco plants. All phenotypic alterations were transmitted to the next generation.

The ZmHox1a and ZmHox1b (for Zea mays homeobox) genes map on chromosomes 8 and 6, respectively. Both homeobox genes encode proteins that show 91% similarity and are transcribed simultaneously in meristematic and proliferating cells of the maize plant. To gain insight into the biological function of these genes, both open reading frames were expressed in tobacco, under the control of the cauliflower mosaic virus 35S promoter. The resulting transgenic ZmHox1a or ZmHox1b plants showed identical phenotypic alterations that fall into three classes: size reduction, formation of adventitious shoots, and homeotic floral transformations. Approximately 30% of the ZmHox1-expressing plants grew to only one-third of the wild-type size, and most axillary buds gave rise to lateral shoots. Flower abnormalities included formation of petaloid stamens and development of secondary flowers within the primary gynoecium. Therefore, the ectopic expression of the maize ZmHox1 homeobox gene products affects the vegetative as well as the reproductive phase of tobacco plants. All phenotypic alterations were transmitted to the next generation.

The transcription start site of the maize Shrunken-1 (Sh-1) gene is sufficient for transcriptional initiation in the absence of other promoter elements and is recognized in vitro by the Initiator Binding Protein (IBP). We describe here in situ hybridization experiments performed on various maize tissues to quantify IBP transcription at the cellular level. IBP transcripts are found in the endosperm and in differentiating, enlarging cells of the shoot and the root of the maize seedling. This expression pattern overlaps with that of the Sh-1 gene and is therefore compatible with the hypothesis that the Sh-1 transcription start site is a target for IBP. In the developing spikelets of male and female inflorescences IBP transcript levels are very high in those organs that are later aborted when flowers become unisexual. Overexpression of the maize IBP1 gene product in transgenic tobacco causes a reduction in internodal elongation and effects gibberellin hormonal balance. The cellular expression pattern described here establishes IBP transcripts as an interesting molecular marker for enlarging, and presumably differentiating, cells released from the root or shoot apex.