Discovering the cover : molecular imaging of Populus trichocarpa leaf surface by FT-IR spectroscopy and mass spectrometry techniques
All terrestrial plants are covered by cuticle and its most outer layer is called epicuticular waxes (EWs). This layer forms an actual leaf surface, therefore is a first line of defence against any environmental stress. Despite its crucial role in plant survival, for decades leaf surface was studied without required selectivity. In the present research, the leaf surface was investigated using selective sampling methods and molecular imaging tools: (1) MALDI-TOF-MS (matrix assisted laser/desorption ionization), (2) TOF-SIMS (time-of flight secondary ion) mass spectrometry imaging, (3) FT-IR (Fourier transform infrared) as well as (4) Raman spectroscopy imaging. These tools provide molecular specificity and spatially resolved information. The results were complemented with GC-MS, SEM (scanning electron microscopy), behavioral experiments and statistical analysis. The first sequenced tree, Populus trichocarpa, along with the leaf beetle Chrysomela populi were chosen as a model system. This system represents naturally occurring interaction between a specialist herbivore and its host plant. Following aspects were investigated: (1) Characterization of structure and chemical composition of leaf surface (2) Investigation of role of EW layer in the host recognition process (3) Analysis of wound-healing processes on the leaf surface in response to insect infestation (4) Distribution of leaf surface constituents with high resolution imaging techniques. Results allow to conclude, that: (1) Higly non polar aliphatic compounds detected on the leaf surface of P. trichocarpa play protective role rather than informative (2) EW layer lack compounds that would be necessary in the host recognition process (3) EWs are involved in early stage wound-healing process by their deposition on the injury area (4) Leaf surface compounds co-aggregate and form two distribution patterns, possibly transport of EW constituents depends on their chain length.