Tuesday, 5 March 2013 Dani Cooper
Biologists have uncovered a yellowbox eucalyptus tree that is able to change the smell of its leaves from one side of the tree to the other to protect itself against predation.
The finding, published in the online journal BMC Plant Biology, answers a 20-year-old mystery surrounding a eucalyptus tree in a sheep paddock at Yeoval, New South Wales.
The tree at the centre of the study was almost totally defoliated by insects in 1990, but one branch was left completely untouched.
Lead author Amanda Padovan, a doctoral student at the Australian National University’s Research School of Biology, says their study shows the yellowbox Eucalyptus melliodora is able to control which leaves are attacked by predators by alterations in its genes.
Padovan says the tree, which is estimated to be 75 years old, has developed this ability known as “genetic mosaicism” as a survival mechanism.
“If an insect outbreak occurs then a part of the plant won’t be eaten and therefore it will still be able to grow and reproduce,” she says.
‘Cocktail of oils’
The research team collected leaves from both sides of the tree and through gene sequencing found there were 10 genes that contained differences between the leaves from each side.
Padovan says one of the main defences the eucalyptus uses against predation is its distinctive smell, which is the result of a “cocktail of terpene oils”, including monoterpenes and sesquiterpenes, and formylated phloroglucinol compounds or FPCs that make animals nauseous.
The gene sequencing revealed leaves that were predation-resistant had five fewer monoterpenes and nine fewer sesquiterpenes than the leaves that were “tastier”.
However the concentration of FPCs and the remaining monoterpenes was far higher.
As a result, says Padovan, the leaves on the part of the tree that was not eaten had a strong eucalyptus smell whereas the leaves that were attractive to the insects had a stronger florally smell.
Padovan says it appears the impact on vertebrates such as koalas is similar as feeding experiments in the laboratory show koalas reject the same leaves as the insects.
She says although they have searched the area nearby they have only found one yellowbox tree like this, however she suspects the trait “is more common than we know”.
“Trees can’t get up and walk away from unfavourable conditions and so we believe this genetic mosaicism is a way for trees to survive changing conditions throughout their life,” she says.
“We believe all trees have the ability in that they can acquire mutations in their stem cells, however we believe the mutation must be favourable – in this case the mutation lead to resistance against feeding ¬- to allow an entire branch to develop.”
Padovan is now using gene sequencing on an ironbark eucalyptus Eucalyptus sideroxylon to see if it has similar mosaic properties.