The ability of lipids in complex mixtures to form liquid-ordered, sterol-rich phases, together with the association of specific proteins with sterol-enriched biochemical fractions extracted from biological membranes, gave raise to the "lipid rafts" hypothesis. This suggests the presence within biological membranes of small-sized, ordered domains, of particular lipid and protein composition, which could serve as signal transduction platforms, particularly during plant-microorganisms interactions. However, the precise organization of living cell plasma membrane is still poorly described and the spatial distribution of segregating liquid-disordered and liquid-ordered phases has, for example, not been analyzed in plant cells with a resolution compatible with the estimate size of such assemblies. We developed a multispectral confocal microscopy approach generating ratiometric images of the whole plasma membrane surface of tobacco suspension cells, suggesting its mosaic organization into areas exhibiting various levels of order. Probabilistic modelisation of acquired images led to the description of the spatial repartition of the most ordered domains into areas randomly organized all over the membrane. Moreover, we showed that modifications of the plasma membrane fluidity, together with an increase of the proportion of ordered domains, transiently occurred in the early steps of signaling triggered by cryptogein, an elicitor of defense reaction.