15.2. Control and coordination in plants

Question 1

Importance of control and coordination in plants

Answer 1

• allows plants to communicate with one another
• allows them to coordinate activities appropriately and respond to changes in its internal or external environment
• plant communication systems involve receptors and effectors
• have a system of electrical communication similar to that of an animal’s nervous system, but action potentials are slow and weak

Question 2

Electrical Communication in Plants

Answer 2

• Plant cells have electrochemical gradients across their cell surface membranes in the same way as in animal cells.
• They also have resting potentials.
• In at least some species, some responses to stimuli are coordinated by action potentials.
• ex. Venus Fly Trap

Question 3

Venus Fly Trap

Answer 3

• grows in soils low in nitrate
• in order to obtain sufficient nitrogen for protein synthesis, the Venus fly trap digests insects and absorbs nutrients from them
• leaf is separated into 2 lobes, which are red has nectar - secreting glands around the edge to attract insects
• each lobe has three stiff sensory hairs that respond
  to being defected.
• the outer edges of the lobes have stiff hairs that interlock to trap the insect inside.
• the surface of the lobes has many glands that secrete enzymes for the digestion of trapped insects

Question 4

Venus Fly Trap Mechanism

Answer 4

• The touch of a fly or other insect on the sensory hairs on the inside of the folded leaves of the Venus fly trap stimulates action potentials that travel very fast across the leaf causing it to fold over and trap the insect
• The defection of a sensory hair activates calcium ion
  channels in cells at the base of the hair to open
• Calcium ions flow in to generate a receptor potential
• If two of these hairs are stimulated within a period of 20 to 35 seconds, or one hair is touched twice within the same time interval, action potentials are generated and travel across the trap.
• The action potential travels across the leaf to hinge cells and causes the lobes to change to a convex shape, which makes them fold together and close the trap very quickly
• The stiff spikes on the outer surface of the trap slot together, sealing the trap
• Hydrolytic enzymes are then secreted, and these digest the prey to soluble substances such as amino acids, which are absorbed into the leaf

Question 5

Chemical Communication in Plants

Answer 5

• plant growth regulators are responsible for most communication within plants.
• plant growth regulators are not produced in specialised cells within glands, but in a variety of tissues.
• move in the plant either directly from cell to cell (by difusion or active transport) or are carried in the phloem sap or xylem sap.
• interact with receptors on the surface of cells or in the cytoplasm or nucleus.
• These receptors usually initiate a series of chemical or ionic signals that amplify and transmit the signal within the cell

we study 3 types of plant growth regulator:

• auxins
• gibberellin
• abscisic acid

Question 6

Auxin Function

Answer 6

influence many aspects of growth including elongation growth which determines the overall length of roots and shoots

Question 7

Auxins and elongation growth

Answer 7

• Auxin stimulates cells to pump hydrogen ions (protons)
  into the cell wall.
• This lowers the pH and acidifes the cell walls which leads to breaking of the bonds between cellulose microfbrils and the matrix that surrounds them due to activated expansins. The expansins loosen the linkages between cellulose microfbrils.
• The cells absorb water by osmosis and the pressure potential causes the wall to stretch so that these cells become longer, or elongate.

Question 8

Gibberellin Function

Answer 8

involved in seed germination and controlling stem elongation.

Question 9

Gibberellins and Stem Elongation

Answer 9

• The height of some plants is partly controlled by their
  genes.
• Tallness in pea plants is afected by a gene with two alleles; if the dominant allele, Le, is present, the plants can grow tall, but plants homozygous for the recessive allele, le, always remain short.
• with the Le allele, inactive gibberellin → enzyme → active gibberellin which leads to growth of plant
• with the homozygous le allele, it codes for a faulty enzyme which does not catalyze inactive gibberellin to active gibberellin There is no growth - dwarf plant

Question 10

Seed Structure

Answer 10

• When the seed is shed from the parent plant, it is in a state of dormancy; that is, it contains very little water and is metabolically inactive.
• This is useful because it allows the seed to survive
  in adverse conditions, such as through a cold winter, only germinating when the temperature rises in spring.
• The seed contains an embryo, which will grow to form
  the new plant when the seed germinates.
• The embryo is surrounded by endosperm, which is an energy store containing the polysaccharide starch.
• On the outer edge of the endosperm is a protein-rich aleurone layer.
• The whole seed is covered by a tough, waterproof, protective layer

Question 11

Gibberellins and Seed Germination

Answer 11

• The absorption of water at the beginning of germination stimulates the embryo to produce gibberellins.
• These gibberellins diffuse to the aleurone layer and stimulate the cells to synthesise amylase.
• The amylase mobilises energy reserves by hydrolysing starch molecules in the endosperm, converting them to soluble maltose molecules.
• The amylase mobilises energy reserves by hydrolysing starch molecules in the endosperm, converting them to soluble maltose molecules.
• These maltose molecules are converted to glucose and transported to the embryo, providing a source of carbohydrate that can be respired to provide energy as the embryo begins to grow.
• Gibberellins cause these effects by regulating genes that are involved in the synthesis of amylase.

Question 12

Gibberellin Function

Answer 12

1) Gene expression in plants
2) Cause cell elongation, division
3) Involved in germination of seeds

Question 13

How Gibberellin stimulates stem

elongation.

Answer 13

• without Gibberellin, the transcription factor/PIF is attached to DELLA protein, preventing PIF from binding onto the gene promoter
• Gibberellin binds to receptor (complex) and causes DELLA (protein) destruction
• transcription factor/PIF can now bind to promoter and
  (growth) genes are switched on / expressed /transcribed
• This causes cell division and cell elongation
• This increases internode length, loosens cell walls /acid growth so cells can expand when water enters