Lecture 5 – Seed Maturation & Germination

Seed Maturation

Has various steps starting with maturation of the embryo. Remember that the different parts of the seeds are derived from different parts of the mother/father.

• Endosperm from the mother (two polar nuclei) and father (sperm)

• Embryo also from both (egg & sperm)

• Seed coat from the mother

The endosperm is comprised of starches (glucose) that is important in sustaining the developing seed.

Monocot seeds tend to have larger endosperm reserves while eudicots tend to focus on larger cotyledons

Steps of Seed Maturation

1. Embryogenesis finishes, alongside endosperm growth to initiate maturation

2. Cell division stops and storage compounds accumulate (protection proteins, reserves, and others to prep for germination)

3. Embryo becomes tolerant to dessication (drying) while seed coat dehydrates to enter a dormant state

How these steps occur will determine germination and also seed quality (controlled by development, environmental conditions, and genetic factors)

• If any of the processes go wrong, germination may occur too early or not at all. Early germination is bad for us because it makes the plant/seeds inedible

Seed Dormancy

Halts the seed from germinating in unfavourable conditions, waiting until conditions are right to ensure the most sucessful germiantion & development. These would be ensuring conditions such as light, temperatures, moisture, and so on are just right for further development.

However, it dormancy can still continue even in favourable conditions. There are various reasons why it might continue:

• As a means of dispersal, so that the seed can go further away from the mother plant

• May stay in dormancy because seasonal changes are about to happen (if it germinates only for it to be winter soon, it’d be a waste)

There are two types of induced dormancy:

• Physical, coming from the seed coat itself, preventing moisture from entering or escaping which is important to intitiate germination.

• Hormonal, self-imposed dormancy by the embryo itself.

Seed Germination

Growth resumes after dormancy period is broken. To initiate it, water must be returned to the seed through the process of imbibition. Then, the seed will be released from dormancy, which is typically trigerred by specific conditions of some sort of cue.

• This could be things such as having abrasions on the seed coat, going through a digestive tract, light, going through a cold period, and so on and so forth.

Once dormancy is released, temperatures and oxygen levels must be right to support growth and respiration.

Vivipary & Mutants

Being able to skip dormancy and germinating while still on the mother plant. Essentially, an early germination.

Vivipary14 mutants in maize disrupt the biosynthesis of abscisic acid, making it germinate early by skipping dormancy,

Abscisic Acid

From the information above, we can infer that abscisic acid is able to keep a seed in dormancy and prevent vivipary (promotes dormancy).

Gibberelins (GA)

Another phytohormone involved within germination. An experiment was conducted to see how seeds would germinate in dark conditions, initial light then dark conditions, then dark conditions in a GA medium.

It was shown that germiantion didn’t occur in just dark condition. However, the initial light then dark and dark and GA conditions did experience germination. From this we can infer that GA has a role in intitiating and promoting germination and that light could be a stimulus that activates GA biosynthesis.

So, any mutation that disrupts GA biosynthesis will likely disrupt the germination process

Basic Signalling Pathway

• Stimulus: biotic, abiotic, developmental, etc

• Reception: typically by proteins inside/on cells

• Signal Transduction: from cells-to-cells involving many hormones, proteins, enzymes, signalling molecules, and secondary messengers (cAMP, calcium phosphorylation, etc)

• Response: how a plant responds by changing gene expression

Cereal Aleurone Layer

A layer of living endosperm cells the encloses the outer surface of the starchy endosperm in cereal grains. It is the source of the enzyme α-amylase, which is required to break down the starches within the endosperm.

The endosperm is comprised of amylose (linear) and amylopectin (branched) starches which need to be broken down into glucose so that the embryo can use it. The enzyme, α-amylase, will break it down into dextrins, then into maltose, then into glucose.

The signalling pathway for this looks like:

• A signal from GAs from the embryo is recepted by receptors on the aleurone layer

• A signalling transduction pathway tramits the from the receptors to the nucleus of the aleuron layer cells (through secondary messengers) to initiate a response in gene expression (activation of gene for α-amylase)

After α-amylase is made, it is transported to the endosperm to breakdown glucose. This glucose is then transported to the embryo.