Plant Defence 3 - Acquired Resistance

In posts one and two of this mini-series I explored how plants can defend against bacteria by releasing dangerous chemicals and by killing off cells. This post looks at how surviving one bacterial attack can make plants more able to survive subsequent ones with both local and systemic acquired resistance.

Locally acquired resistance is the simplest to manage, and provides a clear advantage. If cells have been attacked once it makes sense to defend them in case of a second attack. Plants achieve this by strengthening the cell walls in cells that have survived the bacterial attack. Experiments adding elicitors (bits of bacteria that stimulate the plant pathogen receptors) to plant cells showed that proteins in the cell wall became oxidatively cross-linked as they sense the bacteria. Interestingly the molecule responsible for this is hydrogen peroxide, one of the molecules also involved in the cell-death response discussed in post two. If it doesn't kill the plant, it makes it stronger.

The cell membrane is the blue box at the bottom, whereas the cell wall is the light blue rods in the middle. It is the cell wall which is strengthened. Image from wikimedia commons.

This response is all very well for plant cells which happened to be near the site of infection, but what about the rest of the plant? Is it possible for cells on the other side of the plant to be warned and ready for a pathogen attack? Despite the inability of plant cells to move, the answer surprisingly is yes. Cells at the site of infection can release a chemical called salicylic acid which moves through the plants vascular system (the system which also delivers sugars and other important nutrients to all parts of the plant).

The chemical structure of salicylic acid, which is chemically similar to the active component of aspirin.

Following an infection, the levels of salicylic acid were found to rise dramatically in cells around the zone of infection, before spreading through the rest of the plant. This isn't a species specific response either but one found in many different species; grafting parts of one plant onto another did not stop either plant from acquiring resistance. In response to the salicylic acid signal cells start accumulating small amounts of hydrogen peroxide, which can lead to the same cell wall strengthening seen around the area of infection.

As well as salicylic acid it has also been suggested that infected areas of the plant can release the volatile molecule methyl salicylate, commercially known as oil of wintergreen. Rather than travelling through the plant this signal is airborne, allowing transmission not just to other parts of the plant, but to neighbouring (and therefore likely to be related) plants as well. As the only difference between these two signalling molecules is the addition of a small CH3 group, the methyl salicylate can easily be converted back into salicylic acid once it reaches the cells where it can cause the same downstream response.

If anyone was wondering quite why I've suddenly been into plants part of the reason is that the BBC is showing a program called "Botany - a blooming history" and I've been catching the episodes. Despite the slight naffness of the title, it's actually a really good program showcasing experiments, personalities, and the scientific method as it unfolds the history of plant science. You can catch the episodes here on iPlayer.

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Brisson, L., Tenhaken, R., & Lamb, C. (1994). Function of Oxidative Cross-Linking of Cell Wall Structural Proteins in Plant Disease Resistance The Plant Cell, 6 (12) DOI: 10.2307/3869902

Durrant, W., & Dong, X. (2004). SYSTEMIC ACQUIRED RESISTANCE Annual Review of Phytopathology, 42 (1), 185-209 DOI: 10.1146/annurev.phyto.42.040803.140421

Shulaev, V., Silverman, P., & Raskin, I. (1997). Airborne signalling by methyl salicylate in plant pathogen resistance Nature, 385 (6618), 718-721 DOI: 10.1038/385718a0

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