Confocal imaging of metabolism in vivo: pitfalls and possibilities

Confocal laser scanning microscopy (CLSM) has had wide application in morphological studies and ion imaging in plants, but little impact so far on biochemical investigations. This position is likely to change as the range of fluorescent probes increases. To illustrate the type of kinetic information that can be obtained using CLSM in an intact, living system, an analysis has been made of the two-step detoxification of monochlorobimane (MCB) following conjugation to glutathione (GSH) by a glutathione S-transferase in the cytoplasm and vacuolar sequestration of the fluorescent glutathione S-bimane (GSB) by a glutathione S-conjugate (GSX) pump. Fluorescence from the cytoplasm and vacuole of individual trichoblasts and atrichoblasts was measured from time-series of (x, y) optical sections in the elongation zone of Arabidopsis root tips. Intensity changes were calibrated and converted to amounts using compartment volumes, measured by stereological techniques. The data were well described using pseudo-first-order kinetics for the conjugation reaction and either Michaelis-Menten kinetics (Model I), or, as the GSX-pump was operating close to V(max), a pseudo-zero-order reaction (Model II), for the GSX-pump. Analysis of 15 individual cells from two roots gave [GSH](cyt) in the range 1.8-4 mM. GST activity was relatively constant on a cell basis in one root, but increased markedly in the other, giving a net increase in conjugation activity as cells progressed through the elongation zone. In contrast, GSX-pump activity increased in parallel with the increase in cell size in both roots, effectively maintaining a constant transport activity per unit root length or estimated vacuole surface area.