Divergent functions of three Kunitz trypsin inhibitor (KTI) proteins in herbivore defense in poplar
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Background
Climate warming promotes the expansion of insect pests. Among the inducible defense responses activated by attacked plants, Kunitz trypsin protease inhibitors (KTIs) play an outstanding role. KTIs affect food digestion and thereby control the fitness of herbivorous insects. Poplars contain an expanded family of KTIs, whose distinct intrinsic functions are under investigation. Here, we set out to identify KTIs with anti-herbivore activity and assessed the potential growth trade-off incurred by high KTI expression levels.
Results
We identified 28 KTIs in the haploid genome of Populus x canescens , 21 of them were responsive to herbivory. The greatest induction was observed for KTI_400, KTI_600 and KTI_0882 ( P. trichocarpa orthologues Potri.019G124400, Potri.019G124600, Potri.019G088200), whereas a moderate response was found for KTI_53200 (Potri017G153200 orthologue), a protein mainly localized in the xylem sap. Mechanical wounding and methyl-jasmonate resulted in fast and strong induction of KTI_400 and KTI_600 and moderate or lacking responses in KTI_0882 and KTI_53200 . Increased KTI expression levels were associated with upregulation of ALLENE OXIDE SYNTHASE (AOS), whereas exposure to compounds eliciting ethylene or salicylic acid signaling did not affect KTI s. We generated stable CRISPR/Cas12a-mediated knock-out and p35S -mediated overexpression lines of KTI_400, KTI_600 and KTI_53200 in Populus x canescens. Among the wildtype and transgenic lines, only kti_400+kti_600 double knock-out lines produced greater biomass. Larvae of Helicoverpa armigera, a pest expanding in Europe due to a warmer climate, were allowed to feed on wildtype and transgenic poplar lines. Transgenic poplars overexpressing KTI_400 or KTI_600 resulted in reduced and their double knock-out lines in increased weight gain of the larvae. In contrast, overexpressing or knock-out lines of KTI_53200 had no effect on larval weight gain compared with controls.
Conclusion
KTI_400 and KTI_600 are potent, natural in-planta anti-herbivorous agents. Their expression is associated with larval growth reductions. Modulation of KTI_53200 levels had no direct effects on the fitness of leaf-feeding H. armigera or on plant growth. This study sheds light on the potential application of KTI in plant defenses and biocontrol against herbivores in trees and presents new options to investigate growth-defense theories.
