Differential Rna Synthesis in the Shoot Apex of Pharbitis Nil During Floral Evocation

Abstract

Early events in the evocation of the flower in Pharbitis nil Chois seedlings were investigated by following the incorporation of tritiated uridine into the shoot apex. The uridine was applied to the expanded cotyledons of seedlings at 8 hr into the inductive dark period. The shoot tips were fixed at 20 hr (a 12-hr labeling period). After the 12-hr labeling period there was considerable label throughout the shoot tips of both control plants (dark period interrupted with 5 min of red light at 8 hr) and plants induced to flower. Both RNase and acid hydrolysis removed the nonexchangeable label and the Azure B staining, thus leading to the conclusion that the uridine was incorporated into RNA. Induction in the cotyledons was followed quickly by an increased synthesis of RNA in the rib meristem region of the receptor bud at the time when the floral stimulus is assumed to be arriving. The increase in RNA synthesis is revealed by an increase in the rib meristem/central zone ratio of counts due to the incorporation of tritiated uridine. A comparison of counts in each of the two regions revealed that the change in ratio was due to an increase in the rib meristem and not due to a decrease in the central zone in induced shoot apices. The initial activation of the rib meristem probably occurred by 16 hr from the beginning ofthe dark period. Tendencies in the literature to disregard the role ofthe rib meristem in giving rise to part of the flower are discussed.

Gene Expression in the Early Floral Meristem.

“…The pattern of CDCP expression appears to be modified during the I -, F transition. High levels of CDC2 transcript in the rib meristem and developing pedicel may reflect the increased mitotic and metabolic activity in this region during floral meristem initiation (Vaughn, 1955;Wetmore et al, 1959;Bonner et al, 1991). Martinez et al (1992) also noted that the expression pattern of CDC2 in the early floral meristem is the reciproca1 of the pattern displayed by several of the floral homeotic genes, and they suggested that antagonistic interactions between CDC2 and the homeotic genes may determine these expression patterns.…”

Activation of Basal Cells of the Apical Meristem during Sepal Formation in Tomato

“…Another example is provided by the study of Melzer et al (1990), in which a number of genes whose transcripts are up-regulated during floral transition in mustard were found to show high levels of mRNA at the base of the meristem and pedicel. Furthermore, Bonner et al (1991) reported an increased synthesis of RNA at the base of the meristem of Pharbitis nil following the induction of flowering. Even earlier studies (e.g., Gifford and Tepper, 1962;Marc and Palmer, 1982) have suggested that a relative metabolic activation of the lower, interna1 cells of the meristem is a significant occurrence within the meristem during floral transition.…”