Transition from Seeds to Seedlings: Hormonal and Epigenetic Aspects

Abstract

Transition from seed to seedling is one of the critical developmental steps, dramatically affecting plant growth and viability. Before plants enter the vegetative phase of their ontogenesis, massive rearrangements of signaling pathways and switching of gene expression programs are required. This results in suppression of the genes controlling seed maturation and activation of those involved in regulation of vegetative growth. At the level of hormonal regulation, these events are controlled by the balance of abscisic acid and gibberellins, although ethylene, auxins, brassinosteroids, cytokinins, and jasmonates are also involved. The key players include the members of the LAFL network—the transcription factors LEAFY COTYLEDON1 and 2 (LEC 1 and 2), ABSCISIC ACID INSENSITIVE3 (ABI3), and FUSCA3 (FUS3), as well as DELAY OF GERMINATION1 (DOG1). They are the negative regulators of seed germination and need to be suppressed before seedling development can be initiated. This repressive signal is mediated by chromatin remodeling complexes—POLYCOMB REPRESSIVE COMPLEX 1 and 2 (PRC1 and PRC2), as well as PICKLE (PKL) and PICKLE-RELATED2 (PKR2) proteins. Finally, epigenetic methylation of cytosine residues in DNA, histone post-translational modifications, and post-transcriptional downregulation of seed maturation genes with miRNA are discussed. Here, we summarize recent updates in the study of hormonal and epigenetic switches involved in regulation of the transition from seed germination to the post-germination stage.

Transcripts Expressed during Germination Sensu Stricto Are Associated with Vigor in Soybean Seeds

“…The expression of these transcripts changed throughout the germinative process ( Figure 5 ) due to the signaling necessary for the growth of the embryonic axis. According Smolikova et al [ 21 ], this phase is marked by rearrangements of signaling pathways and a switching of gene expression programs. This explains the intercorrelations between the genes here evaluated, by demonstrating the interactivity between processes necessary for the occurrence of germination ( Figure 6 ), i.e., different signals are required for its success.…”

Seed-to-Seedling Transition in Pisum sativum L.: A Transcriptomic Approach

“…As known, the master negative regulators of seed germination include the transcription factors LEC1, ABI3, FUS3, LEC2 (so-called LAFL), and DOG1 [16,41,92]. Switching the developmental program from maturation to germination is accompanied by gene suppression of the LAFL network and activation of the genes involved in vegetative growth [15].…”

“…Seed germination also includes such critical processes as silencing of seed development genes and activation of vegetative growth genes [16,38,39]. Key modulators of the transition from seeds to seedlings are epigenetic modifications such as DNA methylation, demethylation, histone modifications, and sRNA pathways [40][41][42]. Apparently, at this moment, the program controlled by the LAFL network is blocked [43,44].…”

Exploring Breakthroughs in Three Traits Belonging to Seed Life

“…(vi) All the above vital events that occur in seeds are coordinated by transcription factors (TFs) such as ABI3, ABI4, and ABI5 [18][19][20][21][22][23], and phytohormones, being the regulatory mechanisms underlying abscisic acid (ABA) and gibberellins (GAs) crosstalk, intensively documented during seed dormancy and germination. However, identification of the ABA and GAs synthesis/degradation pathways, their feedbacks, and their impact on the regulation of dormancy and germination is far from clarified [23][24][25][26]. On the other hand, the involvement of ethylene in the regulation of seed dormancy and germination was recently updated and discussed [27][28][29][30]; and (vii) the life of this propagule is strongly influenced by external signals (e.g., light, nitrate, humidity, temperature) [31][32][33][34][35].…”

“…ABI3 and ABI5 control the seed sensitivity to ABA [23]. ABI5-binding protein (AFP) induces ABI5 degradation [24,59]. In order to understand the mechanisms underlying seed dormancy or PHS tolerance in common wheat (Triticum aestivum), the possible role of TaAFP in seed dormancy was developed, concluding that TaAFP is a negative regulator in seed dormancy [60].…”