Plant Physiology Spring 2006
Tuesday 4 April
Homework assignments due
1. Project Proposals (10 minutes)
2. How do plants grow (5 minutes) – Dave’s problem with the split tree.
Review Quiz
1. How do plants grow? Where do they produce new tissues?
2. What are the three main tissue systems found in plants?
3. What cell types make up the tissue systems?
4. What are the main biotic and abiotic factors that will influence the growth and
development of a plant throughout its life cycle? You should be able to think of
seven different ones.
3. Hormones – What are they?
They are chemical messengers that are produced in small quantities and are transported
over relatively long distance to bring about a physiological effect. They require a protein
receptor in the plasma membrane that will allow it to transmit a biochemical response
known as signal transduction.
What are the five main hormones found in plants?
New class of brassinosteroids – they have a similar amino acid sequence to the
mammalian steroid 5α-reductases involved in the NADPH-dependent conversion of
testosterone to dihydrotestosterone.
Seems to have very similar effects to auxin – we’ll examine this next week.
4. Ethylene (Chapter 22)
What is its structure?
It is a gas.
How was it discovered?
In the early 1900’s, A Russian physiologists found that pea seedlings had reduced stem
elongation, increased lateral stem growth and grew horizontally in the dark in the lab.
When exposed to fresh air, it regained its normal morphology. He later identified
ethylene in the laboratory.
Bananas would ripen in the presence of oranges. It was later discovered that a fungus,
Penicillum, produced copious amounts of ethyelene.
Where is it produced?
It is produced from the amino acid, methionine. In particular, labeling experiments show
that it is derived from carbons 3 and 4.
When ACC is supplied exogenously, ethylene production increases.
ACC synthase is found in the cytosol and is unstable. Its very low abundance makes it
hard to purify.
ACC oxidase catalyzes the last step in ethylene biosynthesis. The gene that encodes ACC
oxidase has been cloned.
It is lighter than air (molecular mass of 28).
It can diffuse readily through the intercellular air spaces.
Scientists use Potassium permanganate to trap ethylene.
Mostly produced by senescing and ripening fruits.
Only need 1 ul per L for it to be physiologically active. A ripe apple can have
concentrations as high as 2500 uL L-1.
Environmental stresses and auxins can promote ethylene biosynthesis
Figure 22.2 shows that as fruits ripen, the ACC content and ACC oxidase activity
increases as fruits ripen.
Stress-induced ethylene production
Stress conditions such as drought, flooding, chilling. Exposure to ozone or mechanical
wounding can increase ethylene synthesis.
Physiological processes
1. Fruit ripening:
Tissues soften due to enzymatic break down of cell walls, starch hydrolysis, sugar
accumulation and disappearance of organic acids and phenolic compounds, including
tannins.
Anthocyanins and carotenoids typiocally accumulate in the epidermis of ripened fruits.
Figure 22.4 shows that respiration increases as a function of number of days after harvest
and one sees an ethylene pulse at about 8 days after harvest. The term climacteric refers
to fuits that show this characteristic respiratory rise before ripening.
2. Ethylene promotes lateral cell expansion.
3. Ethylene causes hook opening when seedlings germinate.
4. Ethylene breaks seed dormancy and bud dormancy.
5. Ethylene promotes the elongation growth of submerged aquatic species.
6. Ethylene induces the formation of roots and root hairs.
7. Ethylene induces flowering in the pineapple family.
8. Ethylene enhances the rate of leaf senescence.
Healthy leaves maintain a high enough auxin concentration that removes ethylene.
sensitivity in the abscission zone and prevents leaf shedding.
As leaves receive signals, a reduction in auxin concentration from the leaf increases the
ethylene production and ethylene sensitivity in the abscission zone. This triggers the
shedding phase.