A molecular mechanism for embryonic resetting of winter memory and restoration of winter annual growth habit in wheat.

The staple food crop winter bread wheat (Triticum aestivum) acquires competence to flower in late spring after experiencing prolonged cold in temperate winter seasons, through the physiological process of vernalization. Prolonged cold exposure results in transcriptional repression of the floral repressor VERNALIZATION 2 (TaVRN2) and activates the expression of the potent floral promoter VERNALIZATION 1 (TaVRN1). Cold-induced TaVRN1 activation and TaVRN2 repression are maintained in post-cold vegetative growth and development, leading to an epigenetic 'memory of winter cold', enabling spring flowering. When and how the cold memory is reset in wheat is essentially unknown. Here we report that the cold-induced TaVRN1 activation is inherited by early embryos, but reset in subsequent embryo development, whereas TaVRN2 remains silenced through seed development, but is reactivated rapidly by light during seed germination. We further found that a chromatin reader mediates embryonic resetting of TaVRN1 and that chromatin modifications play an important role in the regulation of TaVRN1 expression and thus the floral transition, in response to developmental state and environmental cues. The findings define a two-step molecular mechanism for re-establishing vernalization requirement in common wheat, ensuring that each generation must experience winter cold to acquire competence to flower in spring.