Feeding the World

Key Stage5-
Fixation with intimacy

Pupil worksheet

Feeding the world

Gaining a better understanding of the mechanisms underlying the symbiotic relationship between some plants, such as peas, and nitrogen-fixing rhizobia bacteria which live in their root nodules will help us to increase crop yields - something that is vital for a growing population.

Important crop plant such as wheat and corn do not have this useful relationship with rhizobia. Instead, they rely on nitrates in the soil as a source of nitrogen. Fertilisers are used to increase the nitrate concentration of the soil and increase crop yield.

Scientists at the University of Oxford are researching into how we can increase the uptake of rhizobia by pea plants. It is hoped that in the future we may be able to developother crop plants, whichalso have this beneficial relationship.

Your task

Use the supplied sources to support you in answering the following questions.

Sources you can use include the film 'Give peas a chance', information on the infection process, the image showing metabolic interactions of an infected plant cell and the extracts from the article in Microbiology Today co-authored by University of Oxford Professor Philip Poole.

Questions

1. Explain why the relationship between rhizobia and legume is an example of symbiosis.

1. Explain the importance to the plant of the signalling between plant and rhizobia prior to infection.

1. Sucrose and the amino acid glutamine are exchanged across the transport system of an infected plant cells. Describe the biochemical pathways inside the cell that exist to bring about this exchange.

1. The bacteroid switches off its enzyme glutamine synthetase (GS). Why is this vital for the symbiotic relationship?

1. How might this research lead to a decrease in the use of artificial fertilisers? Why is this important?

1. Suggest ways that researchers could go about producing crop plants that have a symbiotic relationship with rhizobia.

The infection process

• Plant roots release chemicals called flavonoids into the soil.

• Rhizobia bacteria recognise the flavonoids, which induces the expression of rhizobial nodulation (nod) genes.Nod gene products include highly specific plant signals called nod factors.

• The nod factors bind to plant root cells via receptors. Activation results in the induction of signalling pathways required for nodule development and infection thread growth.

• Some rhizobia can deliver other nod gene products into the plant cells to assist plant infection, probably by suppressing plant defence systems.

• Rhizobia grow down infection threads found in root hair cells. The process of infection can take 10–14 days and during this period the rhizobia do not fix nitrogen.

• They are then engulfed into cells by endocytosis. They differentiate into nitrogen-fixing bacteroids inside the plant cells.

• Bacteria export ammonia to the plant. In return, the plant supplies the bacteroid with metabolic products, mainly malate, which is fed into the TCA (Krebs) cycle.