Seed germination and flowering, two critical developmental transitions in plant life cycles, are coordinately regulated by genetic and environmental factors to match plant establishment and reproduction to seasonal cues. The DELAY OF GERMINATION1 (DOG1) gene is involved in regulating seed dormancy in response to temperature and has also been associated genetically with pleiotropic flowering phenotypes across diverse Arabidopsis thaliana accessions and locations. Here we show that DOG1 can regulate seed dormancy and flowering times in lettuce (Lactuca sativa, Ls) and Arabidopsis through an influence on levels of microRNAs (miRNAs) miR156 and miR172. In lettuce, suppression of LsDOG1 expression enabled seed germination at high temperature and promoted early flowering in association with reduced miR156 and increased miR172 levels. In Arabidopsis, higher miR156 levels resulting from overexpression of the MIR156 gene enhanced seed dormancy and delayed flowering. These phenotypic effects, as well as conversion of MIR156 transcripts to miR156, were compromised in DOG1 loss-of-function mutant plants, especially in seeds. Overexpression of MIR172 reduced seed dormancy and promoted early flowering in Arabidopsis, and the effect on flowering required functional DOG1. Transcript levels of several genes associated with miRNA processing were consistently lower in dry seeds of Arabidopsis and lettuce when DOG1 was mutated or its expression was reduced; in contrast, transcript levels of these genes were elevated in a DOG1 gain-of-function mutant. Our results reveal a previously unknown linkage between two critical developmental phase transitions in the plant life cycle through a DOG1-miR156-miR172 interaction.T he life cycles of flowering plants are characterized by distinct phase transitions such as from seed to seedling (germination) or from vegetative to reproductive development (flowering) (1). The timing of germination and flowering both require precise environmental sensing and integrated responses to multiple inputs so that developmental transitions can be accurately matched to seasonal conditions (1-3). Seeds use temperature as a signal of the seasonal and current environmental conditions to determine opportune times to germinate with respect to the potential for seedling survival (2, 4, 5). Similarly, in many plants the transition from vegetative to floral development occurs in response to environmental cues, particularly temperature and day length (6, 7). Ecological and evolutionary studies have found that seed germination and flowering traits within species are coadapted across habitat ranges (8-11). Seed dormancy and germination are regulated primarily by the antagonistic actions of the plant hormones gibberellin (GA; promotive) and abscisic acid (ABA; inhibitory), whose synthesis and action vary in response to environmental signals (12). Recent studies indicate that canonical genes regulating flowering, such as FLOWERING LOCUS T (FT) and FLOWERING LOCUS C (FLC), are also involved in the transition from seed dormanc...