Functional redundancy of the homeobox genes ZmHox1a and ZmHox1b
--Bärbel Überlacker, Claudia Mehlem and Wolfgang Werr
ZmHox1a and ZmHox1b (Zea mays homeobox) are a pair of highly related homeobox genes encoding proteins with 87% identity on the amino acid level (91% similarity). The genes are located on chromosomes 8 (8L-17) and 6 (6L-118), respectively, within larger chromosomal regions known to be duplicated in the maize genome. No phenotypic mutant maps close to either of these positions. All Northern and in situ hybridization experiments performed so far show very similar transcription patterns of both genes, and the ZmHox1a and ZmHox1b mRNAs are generally restricted to meristematic and proliferating cells of the maize plant.
To gain insight into possible biological functions of each ZmHox1 gene we ectopically expressed the ZmHox1a and 1b coding frames behind the leader sequence of TMV under the control of the CaMV 35S promoter. The regenerated ZmHox1a (42) and ZmHox1b (137) transgenic tobacco plants show identical and heritable alterations in the development of the vegetative plant body and the flower. The phenotypic changes fall into three different classes: 1) A reduction in size. 2) An outgrowth of few to all axillary buds. The adventitious shoots often overgrow the primary shoot which indicates reduced apical dominance. 3) Alterations of the flower development in the third and fourth whorl. The stamens are homeotically transformed to petaloid organs, the stigma is changed in shape, the position of the ovaries is displaced towards the stigma and the number of ovules is significantly reduced. In severely affected plants a secondary flower develops inside the primary gynoecium. While each phenotypic alteration may vary from weak to strong, the grade of severity is highly correlated between the three classes (P< 0.001 in (2-tests). The ectopic expression of the maize ZmHox1 homeobox gene products therefore affects different phases of the tobacco life cycle, which coincides with the transcription in different types of meristems in the maize plant.
The identical phenotypes of transgenic ZmHox1a/1b tobacco plants are presumably due to the 91% amino acid similarity shared between both gene products. It remains unclear why the expression patterns and the gene function of both ZmHox1 genes have been highly conserved during evolution after gene duplication, while the individual genes in other pairs of transcriptional regulators (e.g. C/Pl or R/B) acquired very distinct transcription patterns after duplication. The similar functions of ZmHox1a and ZmHox1b implied by transgenic tobacco experiments indicate a functional redundancy in the maize genome, which may explain the lack of corresponding mutants.
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