What Drives Symbiotic Calcium Signalling in Legumes? Insights and Challenges of Imaging

We review the contribution of bioimaging in building a coherent understanding of Ca2 + signalling during legume-bacteria symbiosis. Currently, two different calcium signals are believed to control key steps of the symbiosis: a Ca2 + gradient at the tip of the legume root hair is involved in the development of an infection thread, while nuclear Ca2 + oscillations, the hallmark signal of this symbiosis, control the formation of the root nodule, where bacteria fix nitrogen. Additionally, different Ca2 + 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 used 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 Ca2 + spiking is a wide-range signal involving the entire root hair and that the Ca2 + signature may be related to cytoplasmic streaming.