MERISTEMATIC PRECURSORS OF VASCULAR PARENCHYMA DIFFERENTIATE FROM G₂ PHASE AFTER REPLICATING DNA DISCONTINUOUSLY

A small population of cells representing 1% or less of those in the root-tip meristem was identified as the precursor of vascular parenchyma and certain root-cap cells in carbohydrate starved cultured pea roots. Autoradiography and cytophotometric measurements of nuclei labeled with [³ H]-thymidine showed that in the absence of carbohydrate the precursor cells replicate their DNA discontinuously accumulating temporarily in late S phase prior to differentiating from the G₂ phase. Besides discontinuity of DNA synthesis, the nuclei of precursor cells undergo a change in morphology. The nuclei are shaped round when replicating DNA but later on, while differentiating, they become oblong. This transformation occurs within 72 hr after the starved roots are fed sucrose. Autoradiograms of serial cross-sections of pulse-labeled roots indicate that the cells in late S phase differentiate forming a ring around the stelar cylinder and a ring around the periphery of the root. These observations suggest that during the last half of the final S phase the precursor cells modify their chromosomal DNA and that this modification is associated with the initial steps of differentiation.