Term 2 Lecture 8: Plants With Partners Flashcards
Types of interaction
Mutualism - benefits both
Neutralism - benefits neither
Competition - benefits neither
Parasitism - benefits parasite at expense of host
Commensalism - one party benefits but the other is neither benefited nor harmed
amensalism - one party is always harmed, but the other is neither harmed nor benefited
Mutualism - both parties benefit
1) symbiosis in fungi, fungi colonise plant roots helping it to take up minerals like nitrates and phosphates from the soil by secreting enzymes. They alter the pH around the roots making it easier to take up minerals and increasing root surface area vastly in exchange the plant provides carbohydrates to the fungi.
Mass of hyphae can be similar to the root mass.
There are 2 main types - exto and endo mycorrhizal fungi
Ecto-mycorrhizal fungi
Associate with only 3% of higher plants - mostly trees in the form of the wood wide web - a signalling mechanism for trees to signal to one another associated with basidiomycetes (club fungi) and ascomycetes (sac-like fungi) the fungi form a loose sheet-like structure over roots and root hairs - they do not enter the root cells.
They allow the plant to reach beyond the nutrient depletion zone where it would normally not be able to reach alone
Endo-mycorrhizal fungi
Aka arbuscular mycorrhizae
Produce arbuscles (blobs) of material within the plant root cells.
Often found in herbaceous flowering plants and associated with many crop plants.
Aid plants uptake of nutrients and create an environment in which they can grow successfully.
Looser hyphal arrangement making up around 10% of root mass.
They penetrate between and into cells - inside the cell walls but not the inner membrane.
Membrane invaginates around them increasing surface area in the cell for nutrient exchange - in this area carbohydrate exchange takes place - the plant provides food for the fungus in exchange for it extending the plants ability to take up mineral nutrients
They also produce a soil protein called glomalin - a protein important in soil structure - soil is a carbon sink due to the presence of this glomalin - a carb/protein complex
Symbiosis with rhizobium bacteria (another mutualism)
Allows nitrogen fixation in roots.
N is often the limiting nutrient as there is a lot of N in the air but very little in the soil.
Some plants particularly legumes associate with rhizobium bacteria to fix nitrogen effectively.
This is the reason why in crop rotation farmers often grow clover as it possesses the root nodules containing this n fixing bacteria
Process:
2N + 3H2 → 2NH3
The bacteria that live in the modules fix N and supply it to the plant in exchange the plant provides carbohydrate nutrients.
The reason most plants can’t take up N directly is the triple bond in an N3 molecule is very strong requiring a lot of energy to break and this must be done in the absence of O2 by nitrogenase enzyme.
Root nodules are pink due to the pigment leghaemoglobin which like the Hb in our blood has a high O2 affinity.
Leghaemoglobin takes up O2 so that there is not a lot ‘floating around’ to damage the nitrogenase enzyme.
The bacterium itself needs some O2 to respire/grow so leghaemoglobin only takes up the free O2 allowing the bacteria to grow as well.
N fixation is important for many crops and is part of crop rotation.
Where you find legumes growing naturally it usually means soil conditions are poor.
E.g. the legume Oxytropis cachemeriana grows high up in the Himalayas on dry, cold, bare rock with hardly any soil
Mutualism with ants
E.g. vicia sativa has extrafloral nectaries to attract ants that then defend the plant against herbivorous insects (in exchange for nectar)
E.g. Myrmecodia beccarii which grows epiphytically in north Queensland. It has ‘ant chambers’ similar to galls for the ants to live in - providing food and shelter in return for the ants aiding by pollination, seed dispersal and defence
Mutualistic coevolution of figs and wasps
Figs are infact not fruits, a fig is a swollen stem filled with tiny pink fruits that resemble seeds. Figs and wasps have been in an obligate mutualistic relationship for 75 million years.
No figs without fig wasps and no fig wasps without figs.
Approximately 750 species of ficus (figs) and most of these species are pollinated by just one specific wasp specie.
A fig is technically a syconium - a tiny group of inverted flowers opening inside a pod, when fertilised by wasps developing into tiny fruits.
E.g. Ficus sycomorus (Ethiopia) is pollinated by the ceratosolen arabicus wasp.
Wasps and figs (2)
Ficus sycomorus is monoecious meaning it has male and female flowers on the same tree some other fig trees are diecious.
Ceratosolen arabicus females have a wedge shaped head whereas males have a square head and are born blind (as are most male fig wasps) because they spend almost their whole life inside the fig and don’t go anywhere. Males also have a reduced genome and no wings - a very simple organism compared to the females.
Females are winged, tough jawed to chew through into an unfertilised fig to enter and fertilise it.
Females travel long distances in a short time - upto 160km in a space of 48 hours to carry out pollination.
The rest of their life apart from those 48 hours is spent inside the fig as well.
Females carry pollen from the fig where they were born to the new fig fertilising it.
To make sure the correct species of wasp is attracted green figs emit a mix of volatile organic chemicals to attract them.
The wasp enters a small hole in the bottom of the fix chews in and walks around spreading pollen.
In some cases female wasps lay fertilised eggs into the flowers. The males are first to hatch, they hunt for the females and fertilise them before they leave their flower ‘galls’ when the females hatch the males chew open the fig to release the females and as they have no wings they fall to their deaths (no wings) having fulfilled their purpose. This creates a path for females to leave to pollinate new figs.
Problems:
Sometimes ants will catch and eat the wasps as they are emerging.
Sometimes wasps do not take pollen with them
Sometimes the wrong wasp species enters a fig
Figs are a keystone species
Fig trees produce figs year-round due to the short window for female wasps to pollinate. This provides a reliable food source for many organisms.
Natural greenhouse and bug hotel
Himalayan rhubarb (Rheum nobile) grows at 4800m altitude
It is very high so cold at night and lots of damaging UV radiation.
This creates extreme condition for both plants and pollinators.
The rhubarb produces flowers inside bracts (modified leaves) that are translucent allowing in heat and warmth whilst screening out most UV creating a ‘greenhouse’ up to 10°c warmer inside than out. This is ideal for the flowers to develop protected and once seeds are set the bracts fall away to release them.
The heat speeds up seed development and provides an appealing environment for pollinators such as fungus gnats to forage in. The gnats lay their eggs in the flowers and larvae consume some of the developing seeds but enough are left to be dispersed for the next generation of rhubarb - so it is a trade off for pollination.
Commensalism - one organism benefits whilst the other is not effected
Epiphytes
orchids, ferns and Myrmecophytes grow high up in trees. The seeds germinate in little pockets of soil that accumulate in the rainforest canopy ‘hitching a lift’ allows new seedlings to access light without causing damage to the tree. Giving them access to water, light and nutrients for photosynthesis.
Climbers
Such as ivy do not damage the tree they are climbing except in some cases by making trees top heavy. The tips of the plant are thin and flexible to grasp, anchoring with adventitious roots that secrete a polysaccharide mix that glues them to the surface they are growing on
Competition
Sharing resources e.g. light
Trees often show ‘canopy shyness’ forming a pattern where they grow very close but branches do not touch in the canopy to prevent shading out others
Amensalism - one organism inhibiting another - physical inhibition
Physical inhibition - strangler fig, like epiphytes it germinates in the canopy but then it produces incredibly long roots reaching down to the forest floor where they become bigger by taking up water from the forest floor.
The roots grow so big that they crush their host tree ‘strangling it’ . It’s a highly competitive specie, fast growing and prevalent
Amensalism: allelopathy
E.g. black walnut
Chemical inhibition of one plant (or other organism) by release of substances into the environment that inhibit growth or germination of other species. Black walnut (Juglans niger) produces juglone a chemical respiration inhibitor present throughout the tree but concentrated on buds,nuts,hulls and roots.
Juglone is not very soluble so it does not move far in soil. It’s toxicity is concentrated close to the tree due to root density and accumulation of leaves and hulls - plants attempting to grow under the tree will die as far as the extent of the root radius
E.g. halogeton glomeratus a plant native to the Himalayas and invasive in the US can kill sheep. The plant accumulates toxic levels of Na and K oxalates to deter herbivores. It is a halophyte, able to accumulate salt in cell walls or vacuoles allowing them to tolerate high levels of salinity. When they die the salts are released back into the soil preventing non-halophyte seedlings from germinating.
Therefore halophytes create an environment that gives their own seeds an advantage to germinate
Parasitism
There are 4000 species in 17 families.
Approximately 1% of all flowering plants are parasitic to some degree.
Probably parasitism evolved convergently as it is present in many families.
Hemiparasites - use another plant for minerals and water
Holoparasites - are fully parasitic, they do not photosynthesise and rely wholly on their hosts
