Seed germination variability: why do genetically identical seeds not germinate at the same time?

This review considers recent developments in the molecular and cellular mechanisms of 'bet-hedging' strategies that generate variability in germination among genetically similar individual seeds. For survival in the natural environment, plants have evolved a 'bet-hedging' strategy where individual variation is high and a range of phenotypes is produced. When faced with unpredictable environmental conditions, fluctuation in seed behaviour is a beneficial trait that allows plant species to survive, particularly if seedlings from early-germinated seeds die. However, this is not a desired trait from an agricultural perspective, where a set of uniformly growing seedlings is required. Whilst variability in seed behaviour is unavoidable, over the centuries humans have attempted to select seeds with minimum variability for agricultural use. In the model plant Arabidopsis, even non-stratified seeds in the same silique germinate variably, and it remains elusive how this variability is manifested from genes to a physiological outcome and what molecular mechanisms of bet-hedging facilitate this diversity. Will the re-introduction of valuable wild alleles into domesticated crops contribute to this variability between individual seeds by promoting bet-hedging? Recent advances have shed light on possible molecular pathways of germination that are affected at the level of single seeds and single cells. Here, we review the hormonal, molecular, and cellular mechanisms that might affect the germination outcome of individual genetically identical seeds.