Lecture 6 – Seedling Establishment

Seedling Establishment

When the seedling is able to photosynthesise, assimilate water and nutrients from the soil, has differentiated tissue, and responds to the environment

Gravitropism

Growth in response to gravity, which tends to be the first thing the seedling perceives, as shown with how the root is the first part growing out.

While growing, there is no guarantee that the seedling is in the right orientation and must sense gravity to orientate itself. This is done within the root tip/meristematic zone (protected by the root cap) with the help of amyloplasts

• Amyloplasts are heavy starches that fall towards gravity (to wherever the bottom is)

• Where the amyloplast accumulates will signal to the cell that that is where gravity is falling, which will then trigger the relocalisation of PIN (PIN3, PIN7) transporters to that side

• Here, auxin will accumulate on the bottom side. But, auxin inhibits root elongation, so that side will not elongate, while the top side will elongate

• This will then cause the root to bend down towards gravity

Phototropism

Growth in response to light, first discovered through a series of experiments that tested how plants would bend if the tip was covered, uncovered, covered with transparent, and so on. These experiments showed that plants would bend towards the light if it was able to perceive it.

This bending is caused by the accumulated of auxin (IAA) to the opposite side of where the light is being shown

• Auxin (produced by the meristem) is transported (by PIN) to accumulate on the opposite side which will elongate more

• This then causes it to bend towards the light

Transgenics & Auxin

An experiment was conducted to observe the effect of auxin transport on plant bending.

This was done by having a transgenic plant that had a reporter gene that would fluoresce in the presence of auxin and show us where the auxin was being transported and accumulating. Over time, we’d see that fluorescent side would elongate and therefore bend towards the light

• Remember that the transgenic plant would’ve been made with agrobacterium and a modified T-DNA

• Within the left and right borders, there would’ve been an auxin-responsive promoter, the reporter gene (GUS, GFP, fluorescent gene), and then a selectable marker (constitutive promoter, antiobiotic gene)

Auxin & Tropisms

Both gravi and phototropism is driven by auxin redistribution.

Phototropism takes advantage of how auxin promotes shoot elongation

Gravitropism takes advantage of how auxin inhibits root elongation (at high concentrations)

• However, auxin is needed for root initiation

So, there is a dose-response curve of auxin for these different parts

• The shoot requires more auxin than the root to experience elongation

• There is an optimal amount of auxin to trigger elongation

Etiolation

The process of plants growing within the dark. When grown in the dark, it has a pale colour, a long shoot, reduced cotyledons, and a hook-shaped tip

• Plants that grow in the dark have a hook-shaped tip because it perceives itself as being underground still and or needs to push through obstacles to find light

• The hook-shaped tip gives it protection and aids it in pushing

Triple Ethylene Response

An ethylene-induced response to stressful events such as meeting with an obstacle. It’ll cause the plant to:

• Gain an exagerrated hook shaped tip for more protection and push

• A shorter thicker hypocotyl

• A shorter root

Phytoreceptors

Used by plants to perceive different wavelengths and ‘quality’ of light to grow better.

Phototropins & Cryptochromes

Perceive blue light that is associated with bending (phototropism). A mutation within these receptors would result with a plant phenotype that couldn’t bend towards blue light because it can’t perceive it

Phytochromes

Made up of a protein and chromophore that undergo reversible photoconversion between it’s active and in-active state.

• As Pr (inactive), it can perceive red light to entere it’s active state (Pfr) when there is a high red:far red light ratio.

• As Pfr (active), it will perceive far-red light to enter it’s inactive state (Pr)

• In it’s active state, it will transduce a message for some sort of response

Red Light & Far Red Light

The ratio between the two determines whether the plant is in shade or in light

A high ratio means more red light to far red light, aka it is in the light

A low ratio means the opposite, aka it is in shade

This ratio is also important in seed germination. When there is lots of red light (high ratio), it’ll be perceived and activate a pathway for giberellins (GA) to initiate germination