Molecular memories in the regulation of seasonal flowering: from competence to cessation.

Plants commit to flowering based on endogenous and exogenous information that they can remember across mitotic cell divisions. Here, we review how signal perception and epigenetic memory converge at key integrator genes, and we show how variation in their regulatory circuits supports the diversity of plant lifestyles.

VAL- and AtBMI1-mediated H2Aub initiate the switch from embryonic to postgerminative growth in Arabidopsis.

Plant B3-domain transcription factors have an important role in regulating seed development, in particular seed maturation and germination. Among the B3 factors, the AFL (ABSCISIC ACID INSENSITIVE3 [ABI3], FUSCA3 [FUS3], and LEAFY COTYLEDON2 [LEC2]) proteins activate the seed maturation program in a complex network, while the VAL (VP1/ABI3-LIKE) 1/2/3 proteins suppress AFL action in order to initiate germination and vegetative development through an as yet unknown mechanism. In addition, the AFL genes and LEAFY COTYLEDON1 (LEC1), referred as seed maturation genes, are epigenetically repressed after germination by the Polycomb group (PcG) machinery via its histone-modifying activities: the histone H3 lysine 27 trimethyltransferase activity of the PcG repressive complex 2 (PRC2) and the E3 H2A monoubiquitin ligase activity of the PRC1.

Regulation of the new Arabidopsis imprinted gene AtBMI1C requires the interplay of different epigenetic mechanisms.

Recently, it has been shown that plants contain homologs to the animal Polycomb repressive complex 1 (PRC1) components BMI1 and RING1A/B. In Arabidopsis, there are three BMI1-like genes, two of which, AtBMI1A and B, are required during post-embryonic plant growth to repress embryonic traits and allow cell differentiation. However, little is known about the third BMI1-like gene, AtBMI1C.

Keeping cell identity in Arabidopsis requires PRC1 RING-finger homologs that catalyze H2A monoubiquitination.

Polycomb group (PcG) proteins form conserved regulatory complexes that modify chromatin to repress the genes that are not required in a specific differentiation status [1]. In animals, the two best-characterized PcG complexes are PRC2 and PRC1, which respectively possess histone 3 lysine 27 (H3K27) trimethyltransferase [2-4] and histone 2A lysine 119 (H2AK119) E3 ubiquitin ligase activities [5-7]. In Arabidopsis, PRC2 activity is also required for the gene silencing mechanism [8]; however, the existence of PRC1 has been questioned, because plant genomes do not encode clear PRC1 components and H2A monoubiquitination has not been detected [6, 9].