Histone H4 gene expression in shoot apices associated with floral initiation in lettuce

The in situ hybridization of using histone H4 gene was carried out to check whether the earliest event in the floral initiation of lettuce plants can be detected by the distribution patterns of cells in the shoot apices that express this particular gene. The histone H4 gene that was isolated from lettuce yielded an amino acid sequence that had more than 98% identity to those of known plant histone H4 genes. The vegetative lettuce shoot apices probed with this histone H4 gene exhibited a well-defined zonate structure; DNA replication, as marked by histone H4 gene expression, was most frequent at the periphery of the shoot apex (the peripheral zone), but seldom at the center of the shoot apex (the central zone). Thereafter, this zonate pattern changed dramatically; the apices became domed where the DNA replicating cells were distributed uniformly and more frequently associated with floral initiation. The results of this study reveal that in lettuce, (1) the cell cycle is dramatically accelerated in the central zone of a bud induced to flower. This phase is preceded in the apices before becoming dome-shaped in a few days; and (2) in situ patterns of histone H4 gene expression are excellent markers for floral initiation.