Data from: Nuclear-localized β-(1→3)-glucan polymers and expression of the candidate metabolic genes provide evidence of a novel photosynthate storage system in Paulinella micropora

Paulinella micropora is a testate amoeba that acquired a photosynthetic organelle (the chromatophore) from a cyanobacterial donor. This event is independent from the origin of the widespread primary plastid in Archaeplastida. Whereas photosynthate storage mechanisms are well-characterized in plants, little is known about how P. micropora stores and utilizes photosynthetically fixed carbon. Here, we examined β-glucans in P. micropora, focusing on their identification, subcellular localization, and associated gene expression patterns to assess their putative roles in the cell. Comparative analyses suggest that photosynthetic Paulinella species lack the genes required for α-glucan (starch or glycogen) storage but contain β-glucans, as confirmed by enzymatic hydrolysis in P. micropora that released glucose monomers. Using immunolabeling transmission electron microscopy with a monoclonal anti-(1→3)-β-D-glucan antibody, we report that β-1,3-glucan is localized to the chromatophore and cytosol, and unexpectedly also to the nucleus, where labeling was preferentially enriched in heterochromatin. The latter result has not previously been reported for β-glucans. In addition, co-expression analysis of transcriptome data identified candidate genes likely involved in β-glucan metabolism in Paulinella, several of which show signatures of horizontal gene transfer, suggesting a mosaic evolutionary origin. Our results provide novel insights into the evolution of photosynthate storage strategies and the potential roles of β-glucans in an independent example of primary plastid endosymbiosis.