Development in plants

Question 1

What adaptations does plant development show to the sessile habitat?

Answer 1

Indeterminate growth

Continuous (not just embryonic)

Branching

Iterative

Plastic

Question 2

When is animal development completed?

Answer 2

During embryogenesis. Their body plan is established and highly determinate. Most adult organs are present and the germline is established early. Can easily predict the number and position of organs in the body.

Question 3

How is plant development divided into stages?

Answer 3

Embryonic and Meristematic

Question 4

Describe embryonic development?

Answer 4

Organs are formed with predetermined number and arrangement. The seedling body plant is established. Adult organs and reproductive structures are missing. No germline. Has apical meristems, which give rise to all other plant structures.

Question 5

Describe meristematic development?

Answer 5

Meristems produce new cells for all post embryonic development of root and shoot. Produces all vegetative and floral organs from seedling germination onwards. Responsible for the characteristic features of plant development (continuous, branching, iterative, highly in/determinate). The shoot apical meristem gives rise to an indeterminate number of “phytomers” in an iterative series.

Question 6

What is a phytomer?

Answer 6

Consists of a stem and a node. Node - lateral organ (leaf) and an axillary meristem (bud).

Question 7

What is the advantage of branching and iterative growth?

Answer 7

Allows for the colonisation of 3D space. Insurance against predation and physical damage. Axillary meristems have the same developmental potential as the apical meristem.

Question 8

Describe a meristem?

Answer 8

Maintains a population of stem cells at their centre. These are undifferentiated, and pluropotent. They can divide slowly to provide cells for organogenesis.

Question 9

Describe plant stem cells?

Answer 9

When one stem cell divides, one remains a stem cell whilst the other gets committed to organogenesis. Can be identified by their slow division rates. The stem cells define the central zone of meristems.

Question 10

Is there a single stem cell that is the ultimate source of all other cells in plants?

Answer 10

In angiosperms, there are at least 3 stem cells as there are at least 3 distinct layers of the meristem. Each represents a different cell lineage.

L1 and L2 divide almost exclusively in the anticlinal plane, whilst L3 divides in various planes.

Question 11

Does each cell layer derive from a single stem cell?

Answer 11

No, each layer is derived from between 2 and 5 stem cells

Question 12

How is the small stem cell population maintained in the plant?

Answer 12

Lineage dependent model:
Central zone stem cells have a unique competence to remain as undifferentiated stem cells and this competence is passed onto only one daughter cell at each division.

Position dependent model:
Central zone stem cells occupy a unique position within the meristem and receive signals from surrounding cells to maintain their undifferentiated state.

Maintenance mechanism operates at population level, whilst fates of individual daughter cells can be variable. One daughter cell must ON AVERAGE remain a stem cell.

Question 13

How is plant development extremely plastic?

Answer 13

One genotype translates to more than one phenotype, dependent on environmental factors.

Allows plants to alter their development and body plan to suit the prevailing conditions, and to respond to local competition and opportunity.

Question 14

How is branching in plants advantageous?

Answer 14

Allows for colonisation of 3D shape, and can act as insurance against predation and physical damage.

Branches arise from the axillary meristems in the axil of each phytomer.

Question 15

Different types of axillary meristem?

Answer 15

Vegetative (indeterminate)

Inflorescence (determinate or indeterminate)

Floral (highly determinate)

Their versatility can be influenced by diverse genetic and environmental factors.

Question 16

How is environmentally induced plasticity controlled?

Answer 16

Genetic programs don’t control plant form in detail but they instead set the rules for a competitive self organising process through which branching forms emerge.

Competitive resource allocation is a key facotr. The total amount of resource is limiting and so individual meristems compete. The allocation is influenced by genetic and environmental factor.

Question 17

What is apical dominance?

Answer 17

Strongly growing apex, which outcompetes the branch growth. Environmental and genetic influences on branching can often be considered to act through the regulation of apical dominance.

Question 18

Role of auxin in pant development?

Answer 18

Necessary and sufficient to inhibit growth of axillary meristems.

IAA is synthesised in tissues near the shoot apex. IAA applied to the apex of a decapitated stem can restore apical dominance.

Gets transported via polar auxin transport in xylem parenchyma near the xylem vessels. Gets moved via an active process.

Mechanism allows apical dominance and axillary outgrowth to be moderated.

Question 19

How do seasons affect apical dominance?

Answer 19

Axillary meristems can be activated in summer and form lateral organs (branches).

Secondary dormancy is imposed over autumn and winter (hormone - ABA). May be reactivated over spring to form new branches.

Question 20

How does shade affect apical dominance?

Answer 20

Shade inhibits branching, which promotes elongation of existing stems as there are more resources available for them.

Resources get committed to exploration rather than colonisation. This is genetically encoded and gets adapted depending on the shade!

Shade is measured by a photoreceptor called phytochrome. Can undergo photo-interconversion between two forms as it has a wavelength sensitive switch.

In full daylight, high Red:Far Red ratio so promotes short, bushy, colonising form.

In canopy shade, low Red:Far Red ratio so promotes elongated, unbranched, exploratory form.

Question 21

How does flowering change the axillary meristem?

Answer 21

Changes axillary meristem activity using environmental and genetic cues.

Apical dominance is strongly suppressed upon flowering. Change in meristem identity and activity.

Flower is equivalent to a vegetative axillary shoot, floral organs are equivalent to leaves of an axillary shoot.

Question 22

Advantages of root branching?

Answer 22

1. Colonisation of 3D space to acquire water and minerals
2. Increased anchorage
3. Insurance against predation and physical damage
4. Lateral roots emerge from within the mother root
5. Roots are initiated from pericycle cells
6. Each lateral root has a meristem at its apex

Question 23

How is root branching regulated?

Answer 23

Endogenous factors - auxin promotes formation of lateral root meristems

Environmental factors - water and mineral availability, shoot vigour, shading of the root can also affect lateral root growth

Question 24

Why is coordination required between shoot and root growth?

Answer 24

They develop from individual sets of meristems but are mutually dependent. Requires coordination.

“Allometric growth”

Plants in a constant environment, maintain constant root:shoot ratios over wide size ranges.

Allocates more resources to shoot growth when in shade.

Question 25

How do soil conditions influence root branching?

Answer 25

Roots actively forage for soil resources and competitively deny them to others.

Roots colonise space to minimise self competition and maximise resource acquisition.

Control of elongation vs branching which requires communication between the root systems.

Question 26

Control of lateral root growth by nitrate?

Answer 26

Important source of nitrogen, often limiting.

Little effect on primary root growth

When N is high everywhere…
Inhibits lateral root growth at a certain concentration, and commits resources to growing the shoots.
This happens as nitrate reductase converts nitrate -> nitrite -> ammonia -> glutamine. Nutritional signalling system. ABA also necessary to control lateral root growth.

When N is high in a local patch…
Patchy high nitrate stimulates lateral root growth. Allows for colonisation of nitrate rich patches.
Local nitrate signalling system.

If N is generally low…
Exploratory growth of existing lateral roots is permitted.

Question 27

Control of lateral root growth by phosphate?

Answer 27

Low phosphate…
Promotes initiation of lateral roots, but inhibits elongation.
Results in a highly branched compact root system - specialised form of development “proteoid roots”

No phosphate…
Must be liberated from the soil. Its mobilisation requires secretion of phosphatases and organic acids into the rhizosphere. Proteoid root systems are highly branched with a large surface area and are specialised for liberation and assimilation of phosphate. Stimulates intiation of new roots and inhibits elongation.

Question 28

Control of lateral root growth by water?

Answer 28

Low water potential causes ABA to go up, which inhibits rate of root and shoot growth. Causes root elongation to be favourable against shoot elongation.

ABA inhibits root initiation, as no point growing more in areas of dry soil. Promotes elongation of existing roots.