Induced plant defence and species specificity

Plants have faced the threat of being consumed by hungry herbivorous insects for over 350 million years. During this time they have evolved many sophisticated strategies to protect them from their enemies, including physical barriers, toxins and compounds that stop or inhibit feeding (antifeedants).

However, defence is costly in plants; therefore, it is believed that induced defence, which is only activated when the plant is attacked, evolved as a means to reduce these costs. The INDUCED DEFENCE (Herbivore induced plant defences: The evolution and genetic basis of their specificity) project investigated whether this form of defence is species specific.

Specificity in plant defences is believed to be facilitated by chemical cues known as herbivore-associated elicitors (HAEs); but how specificity evolved is unclear. INDUCED DEFENCE therefore studied the evolution and specificity in HAE-induced plants' defences and identified the genetic basis of the specificity.

Researchers investigated induced defences in closely related species of tobacco plants (Nicotiana) and elicitors from different insect oral secretions. Results revealed that HAE defences are highly specific to different Nicotiana species. Single Nicotiana species were found to elicit distinct responses to different HAEs, and one HAE can induce different defence responses among closely related Nicotiana species.

The effect of HAE defences on herbivores was found to differ between Manduca sexta, which is a specialist feeder on plants from the Solanaceae family (which includes tobacco), and the generalist feeder Spodyptera littoralis.

Leaf transcriptomes of six Nicotiana species, which were induced by different HAEs, were sequenced and a key co-expression network was identified. The co-expression network matched the specificity of HAE-induced defences in Nicotiana, both within and between species. Functional analysis confirmed that the identified co-expression network was involved in HAE-induced early defence signalling.

A phylogenomic approach was used to clarify understanding of the identified co-expression network. It indicated that whole-genome duplication events in Solanaceae contributed to the evolution of a HAE-induced early signalling network in Nicotiana.

The identified herbivore-induced co-expression network and the discovery that genome-wide duplication significantly influences the evolution of herbivore-induced defences will benefit crop breeders. In addition, the new methods developed will enable future researchers to rapidly identify genes involved in complex traits.

Candidate genes identified by INDUCED DEFENCE could lead to a breakthrough in understanding how plants perceive herbivores, thereby benefiting crop protection around the world.