THE PATTERN OF HISTONE H4 EXPRESSION IN THE TOMATO SHOOT APEX CHANGESDURING DEVELOPMENT

Two histone H4 cDNA clones were isolated from a tomato (Lycopersicon e sculentum Mill.) shoot-tip cDNA library using a heterologous probe fro m barley (Hordeum vulgare L.). Both cDNAs, which are 81 % identical in the coding region, are polyadenylated and belong to a small gene fami ly in the tomato genome. Histone H4 message is abundant in young tissu es and rare in older tissues. In the shoot apical meristem, the distri bution of H4-expressing cells changes during development. In a juvenil e vegetative apex, H4 message is detectable in the central region and the peripheral parts of the meristem. In a mature vegetative apical me ristem, H4-expressing cells are localized in the peripheral zone exten ding into the provascular strands and the rib meristem whereas the cen tral zone is almost devoid of H4 mRNA. After floral transition, H4 mRN A is found throughout the floral meristem, indicating a second change in the pattern of H4 expression. The observed changes in H4 expression are indicative of changes in the distribution of mitotic activity in the shoot apical meristem during plant development. In addition, H4-ex pressing cells were found to occur frequently in clusters, which may i ndicate a partial synchronization of cell divisions in the shoot apex.