The 4th instar larva of the diamondback moth

Two hormones which are key to moulting and metamorphosis have another role in protecting insects from a bacterial pathogen, according to recent research.

The hormones ecdysone and juvenile hormone have long been known to play a key role in the timing and transition of moulting and metamorphosis in insects.

However, research involving Dr Neil Crickmore has now demonstrated these hormones have another important function in the diamondback moth.

The hormones were shown to play a role in protecting the moth from the bacterial pathogen Bacillus thuringiensis, which is commonly used as biological pesticide.

This bacterium produces toxins that bind to proteins on the surface of the insect gut before attacking the gut cells through pore-formation, which can have deadly consequences for the insects it infects.

However, in response to this challenge the diamondback moth can downregulate the expression of the toxin-binding proteins, therefore reducing the pathogenic effect. The two hormones ecdysone and juvenile hormone are key to this process.

Dr Crickmore said: “Importantly though our work has shown that the insect can simultaneously upregulate the expression of paralogous proteins which can maintain the function of the downregulated ones but not act as surrogate receptors for the toxins. In dissecting the signalling pathway involved in this mechanism, we established the role of the two hormones, acting upstream of a MAPK cascade. In insects that are exposed to toxins over an extended period of time, a mutation results in the response mechanism being permanently turned on.”

While Bacillus thuringiensis and its toxins have been used commercially as biopesticides for several decades, the researchers argue that the complexity of the response suggests that the moths have been fighting it for a much longer time.

Dr Crickmore added: “The observation that the MAP4K4 enzyme appears to act as a bottleneck in the signalling pathway opens up the possibility of using inhibitors or RNAi to potentiate the effect of B. thuringiensis-based products.”

MAPK-dependent hormonal signaling plasticity contributes to overcoming Bacillus thuringiensis toxin action in an insect host is published in Nature Communications.