Abstract
Seeds provide survival and dispersal capabilities by protecting the dormant mature plant embryo. Germination and resumption of development under favourable conditions requires the reinitiation of cell growth and division through poorly understood processes. Here we show that four phases of cell division activation during germination in Arabidopsis are related to external morphological changes. Cell division initiates in the root apical meristem (RAM) before root protrusion, followed by sequential activation of cell division in the cotyledons, shoot apical meristem (SAM), and secondary meristems. Major changes in transcript levels of >2,000 genes precede root emergence, including expression peaks of six D-type (CYCD) and two A-type cyclins. Two further CYCDs are activated later with the SAM. Early activated CYCDs play key roles in regulating the extent of cell division, because loss-of-function alleles of early CYCDs display reduced division activation and consequential delayed root emergence. Conversely, elevation of early CYCDs increases cell cycle activation in the RAM and promotes embryonic root (radicle) protrusion, whereas a later-acting CYCD does not. These phenotypes, together with their overlapping expression domains, support a cumulative action of a subset of CYCDs in cell cycle reactivation, rather than a complete functional redundancy. This analysis reveals a phenotype associated with loss-of-function of a plant cyclin and demonstrates that D-type cyclins regulate cell cycle reentry during meristem activation to promote successful germination and early seedling growth.