Underlying Biochemical and Molecular Mechanisms for Seed Germination

Abstract

With the burgeoning population of the world, the successful germination of seeds to achieve maximum crop production is very important. Seed germination is a precise balance of phytohormones, light, and temperature that induces endosperm decay. Abscisic acid and gibberellins-mainly with auxins, ethylene, and jasmonic and salicylic acid through interdependent molecular pathways-lead to the rupture of the seed testa, after which the radicle protrudes out and the endosperm provides nutrients according to its growing energy demand. The incident light wavelength and low and supra-optimal temperature modulates phytohormone signaling pathways that induce the synthesis of ROS, which results in the maintenance of seed dormancy and germination. In this review, we have summarized in detail the biochemical and molecular processes occurring in the seed that lead to the germination of the seed. Moreover, an accurate explanation in chronological order of how phytohormones inside the seed act in accordance with the temperature and light signals from outside to degenerate the seed testa for the thriving seed's germination has also been discussed.

Keywords: light; phytohormones; seed germination and dormancy; temperature and endosperm decay.

Figure 1

Graphical abstract: driving forces of seed germination: phytohormones, high temperature, light, and endosperm decay. R (Red), FR (Far-red), Sleepy (SLY), ABA (Abscisic acid), GA (Gibberellins), Reactive oxygen species (ROS), Reduced Dormancy 5 (RD05), RGA-LIKE2 (RGL2), SPINDLY (SPY), Teosinte branches 1/cycloidea/proliferating cell factor (TCP), Ethylene (ETH), Indole-3-acetic acid (IAA), Salicylic acid (SA), PHY-INTERACTING FACTORS (PIF), Delay of Germination (DOG), Brassinosteroids (BRs), ABA HYPERSENSITIVE GERMINATION1 (AHG1), Super oxidase dismutase (SOD), Ascorbate peroxidase (APX), Catalase (CAT), Glutathione reductase (GR), MOTHER OF FT and TFL1 (MFT), ABA-insensitive (ABI), Phytochrome (Phy), Serine palmitoyltransferase (SPT), auxin-responsive factors (ARF), Ethylene responsive transcription factor (ERF), ICE1 (Inducer of CBF Expression 1), Diacylglycerol (DAG), Auxin (AUX), Repressor of ga1-3 (RGA), ABI3-interacting protein (AIP), ETHYLENE RESPONSE1 (ETR1), SOMNUS (SOM, a set of 98 genes), Resolvin E1 (RVE1).

Figure 2

Model showing the effects of extrinsic and intrinsic factors on seed dormancy and germination. During the maturation of the seed (MS), intrinsic ABA is upregulated, and GA is downregulated to inhibit preharvest sprouting (PHS) on the mother plant. After harvest, stratification (STR) and reactive oxygen species (ROS) increase GA biosynthesis and repress ABA biosynthesis, turning the quiescent seed (QS) into a germinating seed (GS). Red bars indicate an inhibition effect, whereas green arrows indicate a promotion effect.

Figure 3

Illustration of the embryo and endosperm in the seeds of tomato (a), Arabidopsis (b), pea (c), and wheat (d). Endosperm (END), embryo (EMB), micropylar endosperm (ME), radicle (RAD), seed coat (SC), cotyledons (COT), embryonic axis (AXS), starchy endosperm (STE), aleurone layer (AL).

Figure 4

Timeline of crucial processes during the germination of eudicot seeds that exhibit separate testa and endosperm rupture (two-step germination). Water uptake, testa and endosperm rupture, and the effect of ABA on these processes are shown for Arabidopsis thaliana seed; control and without hormone CON. Critical biochemical, biophysical, and cellular events during germination are triggered by water uptake and are shown in the diagram. Water uptake phases: Imbibition (I), Post-imbibition (II), and Post germination reserve mobilization (III). The diagram is based on the understanding from [152,153,154,155].