Pine defense responses to eggs of an herbivorous sawfly are elicited by
an annexin-like protein
Known elicitors of plant defenses against eggs of herbivorous insects
are low-molecular-weight organic compounds associated with the eggs.
However, previous studies provided evidence that also proteinaceous
compounds present in secretion associated with eggs of the herbivorous
sawfly Diprion pini can elicit defensive responses in Pinus sylvestris.
Pine responses induced by the proteinaceous secretion are known to
result in enhanced emission of (E)-β-farnesene, which attracts egg
parasitoids killing the eggs. Here, we aimed to identify the
defense-eliciting protein and elucidate its function. After isolating
the defense-eliciting protein from D. pini egg secretion by
ultrafiltration and gel electrophoresis, we identified it by MALDI-ToF
mass spectrometry as an annexin-like protein, which we named
“diprionin”. Further GC-MS analyses showed that pine needles treated
with heterologously expressed diprionin released enhanced quantities of
(E)-β-farnesene. Our bioassays confirmed attractiveness of
diprionin-treated pine to egg parasitoids. Expression of several pine
candidate genes involved in terpene biosynthesis and regulation of ROS
homeostasis was similarly affected by diprionin and natural sawfly egg
deposition. However, the two treatments had different effects on
expression of pathogenesis related genes (PR1, PR5). Diprionin is the
first egg-associated proteinaceous elicitor of indirect plant defense
against insect eggs described so far.