What Drives Symbiotic Calcium Signalling In Legumes? Insights And Challenges Of Imaging

We review the contribution of bioimaging in building a coherent understanding of Ca$^{2+}$ signalling during legume-bacteria symbiosis. Currently, two different calcium signals are believed to control key steps of the symbiosis: a Ca$^{2+}$ gradient at the tip of the legume root hair is involved in the development of an infection thread, while nuclear Ca$^{2+}$ oscillations, the hallmark signal of this symbiosis, controls the formation of the root nodule, where bacteria fix nitrogen. Additionally, different Ca$^{2+}$ spiking signatures have been associated with specific infection stages. Bioimaging is intrinsically a cross-disciplinary area that requires integration of image recording, processing and analysis. We use experimental examples to critically evaluate previously established conclusions and draw attention to challenges caused by the varying nature of the signal-to-noise ratio in live imaging. We hypothesise that nuclear Ca$^{2+}$ spiking is a wide-range signal involving the entire root hair, and that Ca$^{2+}$ signature may be related to cytoplasmic streaming.