Combining ocFLIM and FIDSAM reveals fast and dynamic physiological responses at subcellular resolution in living plant cells

FRET-FLIM applications in plant systems

A hallmark of cellular processes is the spatio-temporally regulated interplay of biochemical components. Assessing spatial information of molecular interactions within living cells is difficult using traditional biochemical methods. Developments in green fluorescent protein technology in combination with advances in fluorescence microscopy have revolutionised this field of research by providing the genetic tools to investigate the spatio-temporal dynamics of biomolecules in live cells. In particular, fluorescence lifetime imaging microscopy (FLIM) has become an inevitable technique for spatially resolving cellular processes and physical interactions of cellular components in real time based on the detection of Förster resonance energy transfer (FRET). In this review, we provide a theoretical background of FLIM as well as FRET-FLIM analysis. Furthermore, we show two cases in which advanced microscopy applications revealed many new insights of cellular processes in living plant cells as well as in whole plants.

Fluorescence microscopy

Optical microscopy has developed as an indispensable tool for Arabidopsis cell biology. This is due to the high sensitivity, good spatial resolution, minimal invasiveness, and availability of autofluorescent proteins, which can be specifically fused to a distinct protein of interest. In this chapter, we introduce the theoretical concepts of fluorescence emission necessary to accomplish quantitative and functional cell biology using optical microscopy. The main focus lies on spectroscopic techniques, which, in addition to intensity-based studies, provide functional insight into cellular processes.

Application of FLIM-FIDSAM for the in vivo analysis of hormone competence of different cell types

Background fluorescence derived from subcellular compartments is a major drawback in high-resolution live imaging, especially of plant cells. A novel technique for contrast enhancement of fluorescence images of living cells expressing fluorescent fusion proteins termed fluorescence intensity decay shape analysis microscopy (FIDSAM) has been recently published and is applied here to plant cells expressing wild-type levels of a low-abundant membrane protein (BRI1-EGFP), demonstrating the applicability of FIDSAM to samples exhibiting about 80% autofluorescence. Furthermore, the combination of FIDSAM and fluorescence lifetime imaging microscopy enables the simultaneous determination and quantification of different ligand-specific responses in living cells with high spatial and temporal resolution even in samples with high autofluorescence background. Correlation of different responses can be used to determine the hormone ligand competence of different cell types as demonstrated here in BRI1-EGFP-expressing root and hypocotyl cells.