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The structures of the three Myb-related genes Hv1, Hv5 and Hv33 from barley were determined. They contain a single intron located in the second repeat unit of the Myb-related domain. By analogy to the animal MYB oncoproteins this conserved region of the gene product was shown by filter-binding experiments to exhibit nucleic acid-binding activity. Tobacco plants transgenic for chimeric Myb promoter/Gus genes express the enzyme in a developmentally controlled and tissue-specific manner. During germination and early stages of plant growth, GUS activity is seen in the root cap and adjacent meristematic tissue. At later stages of plant development, GUS activity is predominantly observed in the shoot apical meristem, the roots and the nodal regions of the stem. Within the stem at stages of secondary growth, Myb promoters are active in defined cell types. In the internode low GUS activity is displayed by the innermost cell layer of the cortex, the starch sheath, that surrounds the vascular cylinder of secondary xylem and phloem tissue, as well as in pith rays originating from vascular cambium initials. In the nodal region Myb promoter-controlled Gus expression is mainly confined to the abaxial starch sheath of the leaf trace, to the branch traces and to internal strands of primary phloem. It is suggested that in addition to their activity in meristematically active plant tissues Myb genes are expressed in conductive tissues that are closely associated with vascular bundles.

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.

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