Architecture of light-independent seed germination: a multi-layered regulation in the model plant Cardamine hirsuta

Light is a pivotal environmental factor in plant life cycle, which integrates energy and information on the surrounding environment, affecting many developmental processes. One example is represented by seed germination which, in the model system Arabidopsis thaliana, is induced by light; however, seeds of several plant species can germinate regardless of this stimulus. While the molecular mechanisms underlying light-dependent seed germination are well understood, those governing the light-independent one are still vague, mostly due to the lack of suitable model systems. In this study, we employ Cardamine hirsuta, a close relative of Arabidopsis, as a powerful model to uncover both the molecular and epigenetic mechanisms underlying light-independent germination. In Arabidopsis, the two phytohormones gibberellins (GA) and abscisic acid (ABA) are proved to be antagonistic key regulators in the germination process, with the establishment of a high GA/ABA ratio allowing germination. In our study we show that a high GA/ABA ratio is fundamental also for the germination of Cardamine seeds, in both light and dark conditions, and that the DOF transcriptional repressor ChDAG1, homologous of AtDAG1, a repressor of seed germination able to control both GA and ABA levels, is involved in this process, mitigating GA levels in both conditions and being induced by GAs in darkness, allowing to hypothesize a feedback control. Given the involvement of PRC2 and of its mark H3K27me3 in germination of Arabidopsis seeds, we also investigate how an alternative deployment of this PTM might support Cardamine light-independent germination. Our data reveal a progressive increase of H3K27me3 during the developmental steps preceding germination, and a reduction of this mark uncoupled from light stimulus during early germination. Finally, a time-resolved nuclear profiling reveals a light-independent nuclear reorganization during early germination. Altogheter, our data support Cardamine as a new model system suitable for studying light-independent germination, resolving a long-standing question on the molecular mechanisms controlling it, opening new frontiers for future research on the epigenetic cues driving this process.