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My training as a scientist is in [tag]plant genetics[/tag], but my [tag]Science[/tag] Tuesday posts to date have all been medical in nature. This is because science that has a direct impact on people is “sexier”. This week, however, I’ve started feeling some guilt about spurning my botanical roots (pun intended) in favor of the glamour of research into human disease. So let’s see if I can sexy up some plant science.

It was a good week for plant molecular genetics. A report in Science from Dan [tag]Klessig[/tag]’s laboratory at Cornell University elucidated elegantly how plants defend themselves. One of the tough things about being a plant is that you are generally pretty tied down to your birthplace and thus at the mercy of whatever gets thrown at you. Plants aren’t stupid, however, and they have developed numerous ways to protect themselves from extremes in environment, herbivores and pests. One way of dealing with the latter is known as [tag]systemic acquired resistance[/tag] or SAR. This response is is similar to the animal immune system; when a pathogen – bacteria, fungus or virus – infects a plant, the plant recognizes that something is wrong and responds. Tissue immediately surrounding the site of infection quickly dies, preventing the invader from spreading easily. In addition, a signal is sent from the site of infection alerting the rest of the plant as to the nature of the invader. It is this signal which Klessig’s group is interested in determining what the plant uses as this signal.

This isn’t the first attempt to determine the SAR signal, in fact it has been an active are of research for half a century. Near the turn of the century, researchers pointed towards the plant hormone salicylic acid, but this has recently been debunked. However, the [tag]Cornell[/tag] researchers worked within this context and serendipitously stumbled on an enzyme involved in the metabolism of salicylic acid as a crucial component of the SAR picture. The enzyme, referred to as [tag]SABP2[/tag], converts [tag]methyl salicylate[/tag] (the active ingredient in heat rubs like Icy Hot – see, plants are sexy) into [tag]salicylic acid[/tag]. The researchers found that SABP2 is required for SAR signal perception – in other words after a plant is infected, this enzyme is necessary for further defense. More importantly, when they engineered plants with an enzyme that uses up all the methyl salicylate, the plants neither responded to the initial infection nor exhibited SAR. What this means is that methyl salicylate is acting as the signal molecule to tell the plant about the infection and to call up defenses.

The thing that I like about this research is the “old school” approach that Klessig’s group takes to work out the nature of the signal. Keen gardeners may be familiar with the technique of grafting, in which the top (scion) of one plant is fused to the roots (rootstock) of another. Plants have a primitive circulatory system, so by [tag]grafting[/tag] it can be determined how molecules from one plant effect another. In this research, tobacco plants that had a defective SAPB2 were grafted with plants that were normal. The interaction between these scion and the rootstock provides information about where SAPB2 is involved in SAR. For example, without getting too technical, plants that had a normal rootstock and a defective SAPB2 scion showed a normal response at the point of infection but no SAR in the rest of the plant. This result tells us that SAPB2 is not required to perceive the threat but is necessary in spreading the word.

The take home message from this study is that methyl salicylate is acting as the long range signal in the plant immune response. The SABP2 enzymes acts as the receptor which recognizes that signal in uninfected tissues. This discovery has implications for agriculture – by modifying levels of methyl salicylate or SABP2 we may be able to improve defenses in crop plants. This in turn could reduce the requirement for pesticide applications, making agriculture both cleaner and more efficient. On a geekier level, this study fairly definitively answers a 50 year old scientific puzzle.