Protein reservoirs of seeds are composites of amyloid and amyloid-like structures facilitating sustained release during germination and seedling growth

The seed protein functions and their localization in seed storage protein bodies (SSPB) are known for several decades. However, the structural and functional complexity of these SSPB is not known. Interestingly, the plant SSPB is morphologically similar to the amyloid-containing protein bodies found in other organisms and individual SSPB proteins were previously shown to form fibrillar structures under non-native conditions in-vitro. Therefore, we hypothesized that the seed storage protein bodies (SSPB) may have similar structures in-vivo for controlling seed functions. Since comprehensive in-vivo characterization of the SSPB and the structure-function relationship remains unexplored, we show firstly that wheat, barley, chickpea, and mungbean SSPB exhibit a speckled-pattern of amyloids interspersed in an amyloid-like matrix in-situ, suggesting their composite nature. This is confirmed by multiple amyloid-specific probes, biophysical characterization, electron-microscopy, peptide-fingerprinting, and differential degradation during germination. Moreover, the role of amyloid composites in seed germination is proved by the effect of signalling molecules and their correlation to germination parameters, using in-situ seed sections, ex-vivo protoplasts and in-vitro SSPB. These results would lay down foundation for understanding the amyloid composite structure during SSPB biogenesis and their structure-function evolution. It would further facilitate the exploration of molecular and atomic-level structural details of SSPB amyloids. Summary Rationale The function of plant seed storage protein bodies (SSPB) in germination is known for decades. SSPB have aggregated and electron-rich morphology. However their structural complexity remains elusive. Based on their morphological similarity to amyloid-containing protein-bodies of other organisms, and amyloid formation by some plant proteins under non-native conditions, we hypothesized that SSPB might contain in-vivo amyloid structures for modulating seed functions. Methods To unambiguously identify seed amyloids in the presence of complex carbohydrate-structures of plant tissues, multi-spectral methods were used including amyloid-staining probes, high-resolution-transmission-electron-microscopy, x-ray diffraction and infra-red-spectroscopy. SSPB amyloid’s role in germination was shown using amyloid probes, MS/MS analysis, and plant hormones/proteases in-situ seed-sections and ex-vivo protoplasts. Key results The SSPB exhibit a composite structure of amyloid, amyloid-like aggregates and soluble proteins. During germination phases, the amyloids degrade slowly compared to the amyloid-like structures. Inhibition of amyloid degradation results in lower germination-index, confirming amyloid’s role in germination and seedling-growth. Conclusion The study for the first time illustrates the presence of composite amyloid structures in-vivo in plant seeds and determines their function in seed germination and seedling-growth. It would open original research questions for decrypting composite amyloid structure formation during SSPB biogenesis and their evolutionary advancement across plant species.