Secondary metabolite imagery techniques

Most of the time, LAESI and DESI require little or no preparation of the sample, and take place at atmospheric pressure. On the other hand, MALDI and SIMS require a very important sample preparation step as well as observation under vacuum, which prevents localizing very small metabolites. For the range of these techniques, there are already a series of works presenting localizations of metabolites, specialized metabolites and lipids with a resolution of 10-100 pm. Increasingly rapid progress will likely lead to venturing below the scale of micrometers and thus to a better understanding of intercellular traffic of metabolites as well as their secretion in the external environment. 

When photons arrive at a molecule, they can be absorbed or scattered. In the case of MPF, the photons which arrive together are resonantly absorbed and part of the energy is transformed into fluorescence. The phenomenon is referred to as incoherent because the incident photon phase and that of the fluorescence are not correlated. To the contrary, in MHG, the incident photons are not absorbed but scattered in a single energy photon corresponding to the sum of the energies on the incident photons. The phenomenon is coherent because the phases of the excitation and emission photons are correlated. Thus, when several molecules emit in phase. 

In many studies, the simultaneous use of multiple techniques (CARS, THG, TOF, Raman, etc.) helps to localize secondary metabolites and/or lipophile molecules such as the triterpenoids and phytoalexins. There are also numerous other imagery techniques which could prove useful for localizing the specialized metabolites and/or lipids in situ. Thus, the Magnetic Resonance Imaging (MRI) technique has already shown its capacity to furnish images of lipid distribution in intact organisms with a 10 pm order of resolution. 

This technique uses the detection of spins of nuclear magnetic moments to produce a virtual section in the organ. Fourier-Transform InfraRed (FTIR) imaging, X-ray imaging or even the techniques of imaging by tomography help to obtain resolutions near the micrometer. These techniques are particularly useful for identifying stmctural chemical elements and also, in some cases, specialized metabolites. 

The improvement of resolution and the progress in the identification of molecules should increase their use in the problems of localization of specialized metabolites at the organ level as well as at the intracellular level. The interest in non-invasive, and therefore non-destmctive, methods is that we can hope to follow in real time the d5mamic of displacement or mobilization of specialized metabolites, and, therefore, have a new element to understand chemical communication. 

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