41 Mutualism involving Plants

Key Idea: Plants form mutualistic relationships with animals, fungi, bacteria, and other plants. Because plants are unable to move they must have effective strategies for reproduction and acquiring nutrients. Most plants form mutualistic relationships with fungi that help to provide nutrients to the roots in exchange for carbohydrate from the plant. Plants use many different types of animals to facilitate pollination.

Nitrogen fixation in root nodules

Nitrogen fixation is a crucial part of the nitrogen cycle. Nitrogen is an abundant element, but biologically available nitrogen compounds are relatively scarce, so plants that are able to form a mutualistic relationship with bacteria to fix nitrogen have a nutritional advantage. Plants in the legume family (e.g. peas, beans, and clover) and nitrogen fixing bacteria (e.g. Rhizobium) form a mutualistic relationship with considerable nutritional benefits to both parties.

Nitrogen fixing bacteria reduce nitrogen from the atmosphere to ammonium ions, combining them with organic acids to produce amino acids. The amino acids provide a nitrogen supply to the plant and the bacteria gain a supply of carbohydrate and a suitable environment in which to grow. Nitrogen fixation in legumes occurs within root Root nodule nodules, which are extensions of the root tissue formed in response to bacterial entry. The nodules provide the low oxygen environment necessary for nitrogen fixation. The presence of nodules allows plants to grow successfully even when soil nitrate is low.

WBS

Mycorrhizal associations

Mycorrhizae are formed by the mutualistic association between a fungus and the roots of a vascular plant. The fungus colonises the plant roots, either intracellularly or extracellularly, forming the mutually beneficial mycorrhizal association.

A B

Around 85% of vascular plant species have mycorrhizal associations and they are vitally important to plant health and forest ecology. The fungal mycelium provides a vast surface area, improving the plant's capacity to absorb minerals and water. In addition, the fungus can access phosphate ions, which are largely unavailable to the plant roots, and transfers them to the plant. In return, the plant provides the fungus with a supply of carbohydrate (produced by photosynthesis). Many conspicuous fungi in the forest, e.g. fly agaric (A), are ecotomycorrhizal. The mycorrhizal roots (B) are short and stubby with a film of fungal threads enveloping them.

1. Root nodules are a mutualistic relationship between a bacterium and a plant. Describe the benefits of the relationship to:

(a) The plant:

(b) The bacterium:

2. When might legumes have a clear competitive advantage over plants that cannot fix nitrogen? Explain your answer:

` Mistletoes are hemiparasitic (partly parasitic) flowering plants. They are photosynthetic but use specialised roots to gain water and nutrients from their host plant. There are eight species of mistletoe native to New Zealand. Unlike mistletoes in other countries, New Zealand mistletoes do not harm their hosts. ` The two species of New Zealand beech mistletoes (Peraxilla) are pollinated by tui and bellbirds, which are nectar feeders and have adaptations to penetrate the mistletoe flower and access the nectar. Even when the flower is ready for pollination (ripe), the petals are closed. When tui and bellbirds give the ripe flower a squeeze with their beak, the petals spring open to reveal the inside of the flower. The birds can then feed, collecting pollen on their heads, which they transfer to different plants as they move between flowers. Native solitary bees are also known to open the flowers by chewing the tips until they spring open. ` Tui and bellbirds feed on a variety of plant species but are primarily responsible for beech mistletoe pollination and are also the main dispersers of the pea-sized fruits. Seed dispersal is critical to mistletoes because the seed must land on a suitable host tree.

Bellbird on beech mistletoe Tui on beech mistletoe

3. Describe the mutualism between vascular plants and mycorrhizal fungi, including benefits to plant and fungus:

4. (a) Horticulturists frequently add a mycorrhizal inoculum when transplanting plants. Suggest why they would do this:

(b) Why would they not add phosphorus fertiliser when attempting to establish the mycorrhizae?

5. (a) Explain how the mutualism between NZ beech mistletoes and its bird pollinators benefits both parties:

(b) How is the dependency of the relationship different for the mistletoe and the bird pollinator?

(c) Describe possible benefits and risks to the mistletoes of such a specialised pollination system: