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Abstract:
Elms (Ulmus spp.) have long been appreciated for their environmental tolerance, landscape
and ornamental value, and the quality of their wood.
Although elm trees are extremely hardy against abiotic
stresses such as wind and pollution, they are susceptible
to attacks of biotic stressors. Over 100 phytopathogens
and invertebrate pests are associated with
elms: fungi, bacteria and insects like beetles and
moths, and to a lesser extent aphids, mites, viruses and
nematodes. While the biology of the pathogen and
insect vector of the Dutch elm disease has been
intensively studied, less attention has been paid so far
to the defence mechanisms of elms to other biotic
stressors. This review highlights knowledge of direct and indirect elm defences against biotic stressors focusing on morphological, chemical and gene regulation
aspects. First, we report how morphological
defence mechanisms via barrier formation and vessel
occlusion prevent colonisation and spread of woodand
bark-inhabiting fungi and bacteria. Second, we
outline how secondary metabolites such as terpenoids
(volatile terpenoids, mansonones and triterpenoids)
and phenolics (lignans, coumarins, flavonoids) in
leaves and bark are involved in constitutive and
induced chemical defence mechanisms of elms. Third,
we address knowledge on how the molecular regulation
of elm defence is orchestrated through the
interaction of a huge variety of stress- and defencerelated
genes. We conclude by pointing to the gaps of
knowledge on the chemical and molecular mechanisms
of elm defence against pest insects and diseases.
An in-depth understanding of defence mechanisms of
elms will support the development of sustainable integrated management of pests and diseases attacking elms.
