Why plants' soapy defences against disease don't always wash.

Natural soaps are an important weapon in the armoury that plants deploy to protect against disease attack, but a report today, in the international journal Nature, describes how disease-causing microbes can turn these plant defences to their own advantage. Scientists at the Sainsbury Laboratory (SL)[1] Norwich, UK, have discovered that fungi that attack tomatoes break down the natural soaps that help protect the plant against infection. Even worse for the plant, these breakdown products then interfere with the internal communication system that the plant relies on to actively fight off invaders.

"Our discovery shows that plants face a double whammy from attacks by fungi", said Dr Anne Osbourn (leader of the research team at the SL). "The first line of defence is transformed into a weapon that makes the plant unable to protect itself against further attack."

Many plants produce natural soaps (called saponins). These chemicals are toxic to bacteria and fungi and so form part of the plant's protection against disease [2]. Researchers have known for some years that microbes that can successfully infect saponin-containing plants often produce enzymes that break down the saponins into less toxic chemicals. This enables the invader to breach the plant's first line of defence.

Working with tomato [3] and tobacco the SL scientists have shown that saponin breakdown products interfere with essential communication processes in the plant. Signalling pathways that would normally set off the alarm system leading to the activation of defence responses are disabled. So in overcoming one line of defence the microbe also disrupts the plant's ability to trigger its other defence systems.

"A better understanding of how plants and their diseases interact will eventually help scientists and breeders who are trying to breed plants with improved natural disease resistance. The next challenge for us is to find out how the saponin breakdown products interfere with internal plant signalling systems, and to establish how common this phenomenon is", concludes Dr Osbourn.

John Innes Centre. August 2002.