Plant defences can be overwhelmed by a snail’s sweet tooth

Normally a build-up of toxic chemicals is bad news, but not always. Some plants are known as hyperaccumulators, because they build up toxic concentrations of metals in their leaves. Being able to tolerate heavy-metal soils opens up habitats to some plants. Being able to push those metals into leaves helps them survive there as a defence against herbivores.

A new study by Llugany and colleagues examines how these defences work. Ideally, the inorganic defence in the leaves would work with organic defences to deter attackers, the Llugany’s team knew this wasn’t obviously the case with snails. So they ran some experiments with the snail Cantareus aspersus and the hyperaccumulator plant Noccaea praecox.

N. praecox (also known as Thlaspi praecox and early penny-cress) is a herb, about 10 – 20 cm tall (4 – 8 inches) found in southern Austria, Italy and the Balkans. It’s capable of accumulating Cadmium (Cd) its leaves.

The experiments were simple in design. Plants were grouped by rosette size. They were then grown in a nutrient solution. Some plants would have cadmium added to the solution, while others wouldn’t. Then the plants would be given to the snails for three days. After that, they’d look at the leaves so examine the nutritional content as well as defences like glucosinolates, phenolics, tannins, salicylic acid, and jasmonates. They also examined the snails and the snail excrement for cadmium.

The first result they found was that snails tended to go for the small or large plants. The mid-range plants were less popular – and these were the ones where cadmium concentrations in the leaves were highest. The authors also not that the leaves had the highest concentrations of glucosinolates. When the snails bit the leaves, a scent would be released and this could drive off the snails.

However, what really drove the snails to attack was the sugar they found in leaves. They particularly ate leaves high in glucose and fructose. Cadmium and other defences did have an effect, but to prevent a first bite from becoming a second, third or tenth, a leaf also needed low sugars.

Llugany and collegues have a possible explanation for this. Writing in Physiologia Plantarum they say: “Our snails had been starved for 7 days before the experimental feeding; thus, the glucose concentration in the offered plants may have acted as a potent feeding stimulant. Under these circumstances, deterrents such as GLS, tannins or phenolics seem to be less important than the attraction caused by a high sugar concentration.”

Despite the importance of sugar’s Llugany and colleagues have shown that there is a combined organic and inorganic defence in N. praecox leaves. It’s a defence that will work – so long as the leaf doesn’t taste too sweet.