Dealing with shortages and nutrient partition

Question 1

Approaches to dealing with shortages

Answer 1

-Altering carbon allocation
- Nitrogen fixation
- Parasitism
- Carnivory

Question 2

Impact of nitrogen levels

Answer 2

Nitrogen levels impact species in different ways - more is not always better
e.g. at high nitrogen levels certain grasses such as Mat Grass show decreased growth as they are adapted to poor soil whereas Carpet bentgrass thrives under high nitrogen levels and is often seen in well fertilised lawns

Question 3

Plants have a range of options when nutrient availability is low:

Answer 3

• lower tissue nutrient concentrations – less herbivory results
• lower maximum rates of net photosynthesis
• increase carbon allocation to roots (for growth or to support mutualistic associations)
• increase leaf longevity – as in coniferous trees to reduce nutrient wastage
• increase nutrient translocation before senescence – reabsorbing nutrients before shedding leaves – this is why leaves of deciduous plants change colour before dropping as nitrogen is reabsorbed
  Most of these strategies lead to overall lower maximum growth rates, even in high nutrient conditions – a trade-off (as seen in mat grass in the graph above)

Question 4

Altering carbon allocation: root growth

Answer 4

When nutrients are abundant shoot mass is higher and root mass is less as they don’t need to grow as far to scavenge from the soil. When nutrients are low root mass is higher and shoot is smaller – this is seen in Milk Thistle.

A similar effect is seen in limited water conditions

Question 5

Altering carbon allocation: longevity

Answer 5

Higher nitrogen levels leads to shorter leaf lifespan. Lower nitrogen availability results in leaves being retained longer as seen in the ‘needles’ of Pinus sylvestris

Question 6

Nitrogen fixation: Rhizobium leguminosum in legume root nodules

Answer 6

2N + 3H2 >nitrogenase> 2NH3

Nitrogen fixing requires a lot of energy

Bacterium with nitrogenase are sequestered in root nodules
Nitrogenase is damaged by oxygen and this is why these nodules only occur in the roots.
These nodules are pink due to high concentrations of leghaemoglobin whihc absorbs excess oxygen to protect the nitrogen-fixing bacteria

e.g. Zigzag clover: these plants can survive in poor soils and rocky conditions due to their nitrogen fixing abilities. This means that species like these are often pioneering/ colonisers

For more see: https://heatherkellyblog.wordpress.com/2015/05/02/from-malta-to-bowburn/

Question 7

Nitrogen fixation: Nod and nod genes

Answer 7

Nod (nodulin) genes come from the plant – produce the nodule

nod (nodulation) genes come from rhizobia

*nodA, nodB, nodC - common to all rhizobial strains, others are host specific. nodD is constitutively expressed and its product (NodD) regulates transcription of other nod genes

*Roots secrete chemicals which attract rhizobia and activate the NodD transcription factor, causing it to induce other nod genes

*nod genes encode proteins involved in biosynthesis of
Nod factors – signalling molecules (like plant growth hormones)

*Particular legume host will respond to specific Nod factors and start expressing Nod genes, leading to nodule production – the symbiotic pathway starts

Question 8

Roots secrete chemicals

Answer 8

– rewatch this section:
https://durham.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=f4f21cc9-f1ac-48cb-a5fa-acaf00bcc7d5&start=374.876928

Question 9

Process of root nodule formation

Answer 9

Infection of pea root by Rhizobium leguminosarum bv. viviae
(see diagram)
- Preinfection thread structures in cortical cells
- Infection thread
- Cells dividing to produce nodule meristem

Question 10

Nitrogen fixation: leghaemoglobin

Answer 10

Like our haemoglobin has a haem in the centre surrounded by protein
- The haem is very highly concentrated
- Affinity for O2 is 10x higher than ours to effectively protect nitrogenase
- Transports enough O2 to bacteria for their survival but limits O2 presence to prevent damage to nitrogenase
* Globin from host is combined with haem from bacterium
see:
http:/W.yw.calvin.edu/academic/chemistry/faculty/amoys/amOys- chem324-Ieghemog obin.html

Question 11

Nitrogen fixation: terminal bacteroid differentiation

Answer 11

e.g. Medicago trunculata (Barrel clover)
Barrel Clover is a small, annual legume native to the Mediterranean, grown as forage crop in Australia used in genomic research. This plant foms symbiosis with Sinorhizobium meliloti.

*NCR (nodule-specific cysteine-rich) peptides targeting symbiosomes allow plant to dominate interaction

*Rhizobial HrrP (host range restriction peptidases) attack NCR peptides preventing bacteria from becoming dominant in the relationship

Question 12

Other plants with root nodules (Alnus)

Answer 12

Nodules contain Frankia

Question 13

Free living cyanobacteria use symbiosis too

Answer 13

Azolla fern can clog waterways due to a lack of herbivory in its non-native environment
Deep water rice species in Bangladesh also has epiphytic cyanobacteria symbioses to take up nitrogen

Question 14

Parasitic plants: root and stem holoparasites

Answer 14

holoparasite = no leaves at all, produces no chlorophyll so does not appear green and doesn’t photosynthesise
e.g. Dodder and Broomrape (Orobanche)
Takes up all its nutrients from its host plant either by parasitising the root or stem of the host plant. Complete reliance on the host for water, nutrients and carbohydrates.

Question 15

Root parasite example: Broomrape (Orobanche)

Answer 15

Produces seeds that can live up to 5 years in the soil.
The seeds react to strigolactone released by nearby germinating plants (a chemical used to attract symbiotic bacteria and mycorrhizal relationships) the seed connects to the young plant by a haustorium and acts as a holoparasite fully dependent on its host.

Parasitic plants often favour nitrogen fixing species since they are more nutrient rich.

These parasites have a huge impact on crop yield.

Question 16

Parasitic plants: Striga

Answer 16

Striga was the first plant from which strigolactone was isolated
Striga is a parasite of crop plants in sub-saharan africa

As seen in the graph as Striga mass increases crop yield is reduced.

Question 17

Parasitic plants: stem holoparasite Dodder

Answer 17

Stem parasite, Dodder is a holoparasite that behaves like a bindweed.
Lacks roots and is basically just a long stem which sends out pegs into the vascular tissue of the host to take up minerals, water and carbs. During germination the dodder seed has a temporary root which dies back quickly therefore Dodder must find a host quickly.

Dodder can spread parasites and disease between plant species as it grows and attaches to other stems.

Question 18

Parasitic plants: hemiparasites e.g. yellow rattle

Answer 18

semi- parasitic = hemiparasite

Yellow Rattle – collects water and minerals from grass species but also photosynthesises – it controls grass growth allowing for other species to grow and is therefore effective for conservation/ rewilding purposes

Question 19

Carnivorous plants e.g. Venus Fly trap

Answer 19

Utilising insects as a nitrogen source

Venus Fly traps are triggered to close when the trigger hairs are tripped by insects

Question 20

Carnivorous plants: Pitcher plants

Answer 20

See: Mechanism for rapid passive-dynamic prey capture in a pitcher plant Bauer et al. (2015)

Downward pointing hairs trap insects in the interior, the walls of the pitcher are also slippery

Question 21

Carnivorous plants: pollinator-prey conflict

Answer 21

how do carniverous plants differentiate between prey and pollinators?
Flowers on long stems far from the sticky traps e.g. on venus fly traps
And/or: different colour flower compared to traps – white attracts flower visitors
And/or: a diff odour: flower visitors are more attracted to floral odour

See:
Carnivorous plants - from David Attenborough’s ‘The Private Life of Plants’
^ Venus fly traps and Pitcher plants on Mt Roraima