Transcriptional responses of Eucalyptus clones to the gall wasp, Leptocybe invasa

Abstract

Eucalyptus species constitute some of the most widely grown and economically important hardwood trees in global plantation forestry. This is due primarily to their remarkable growth and adaptability. Much of the initial success of exotic Eucalyptus plantations was attributed to the separation from natural enemies. However, there has been a recent increase in the number of introductions of Eucalyptus pests and pathogens in these exotic plantations. One such scenario is the spread of Leptocybe invasa that is currently described as one of the most devastating pests of global Eucalyptus plantations. Leptocybe invasa (Hymenoptera:Eulophidae) is an Australian gall-inducing wasp that oviposits along immature midribs, petioles and stems. The larvae are endophytic herbivores that cause the development of coalescing galls (abnormal plant growths) and lead to a wide range of symptoms such as stunted growth, die-back and death. In their native environment, populations of L. invasa are maintained to almost below observational level; however, once removed from this environment, the pest causes extensive damage in young, susceptible trees. Pesticides are ineffectual against the gall wasp and biological control is considered the key tool in controlling this pest. The molecular interaction between Eucalyptus and L. invasa is poorly understood and limits the design of biotechnological control measures aimed at reducing losses. Plants have evolved a complex, multi-layered system of constitutive and inducible defences that protect against pests and pathogens. Results from numerous studies have shown that there is extensive overlap in the response of plants to a wide variety of stresses. This means that it is possible to develop a hypothetical model of the response of Eucalyptus to L. invasa by incorporating results from studies investigating the response of other plant species to insect pests. This model can then be refined as evidence for the target system is obtained. Transcriptomic analyses are commonly used to investigate the plant response to biotic stress and allow for the identification of genes that may be manipulated to improve plant resistance through genetic engineering.