Initiation of Germination

Initiation of Germination

Emergence of the radicle (first root)
• Water and oxygen must reach the embryo
• Exposure to light is require

Germination conditions

Germination can occur over 5-30°C
• Large portion of seeds remain dormant
(=seed bank)

Hormonal Effects

• Germination needs energy → mobilizes starch reserves

• Embryo produces gibberellic acid (GA)

• GA signals aleurone layer to produce α-amylase

• α-amylase breaks down starch into sugars for the growing seedling

Gibberellins

• Also known as gibberellic acid (GA)

• Part of a large group: 100+ plant hormones

• Key role in stem elongation
  → Effect is stronger when auxin is also present

How does gibberellin (GA) promote transcription of α-amylase genes?

GA binds to its receptor GID1
🔹 GA-GID1 complex binds DELLA (a germination-inhibiting protein)
🔹 SCF complex targets DELLA for degradation
🔹 Without DELLA, GA-TRXN can activate transcription
🔹 ➡ α-amylase genes are expressed → starch breakdown for germination

what hormonal changes maintain seed dormancy?

• ↑ ABA synthesis

• ↓ GA levels (GA degradation)

• ↑ DELLA protein synthesis → blocks germination

What hormonal changes trigger germination?

• ↑ GA synthesis

• ↓ ABA levels (ABA degradation)

• ↓ DELLA protein (degraded) → germination proceeds

What is photomorphogenesis in plants?

• Light-mediated development

• Controlled by light sensing proteins like phytochromes

How does exposure to light affect GA levels?

• Phytochromes are red-light sensing molecules

• Activated by red light from the sun

• Activation raises gibberellic acid (GA) levels

• GA promotes seed germination and growth

Why is the phytochrome system important for seed germination?

• Seeds need light exposure at soil surface to germinate

• Phytochromes sense presence or absence of light

• Crucial for species with small seeds (limited food reserves

How do light conditions affect seed germination?

• Below soil surface or in shade → no red light sensed

• Phytochromes not activated → germination is inhibited

What is the Pr​ form of phytochrome?

• Absorbs red light (660 nm)

• Biologically inactive

• Made by dark-grown seedlings

• Converts to Pfr when red light is available

What is the Pfr form of phytochrome?

• Absorbs far-red light (730 nm)

• Active form that triggers germination (via ↑ GA)

• Converts back to Pr when far-red light is available

Which phytochrome form enters the nucleus and what does it do?

• Only Pfr (active form) can enter the nucleus

• Inside the nucleus, Pfr binds to transcription factors

• This triggers transcription of light-regulated genes