Just as “prions” were first discovered in humans — and now are realized to exist in not only all animals but also yeast, fungi and plants — and carry out important functions — so we learn [from the attached article] that “amyloid”, discovered early on in human disease — also exists not only in all animals but also in plants — and even bacteria(!!) and carries out important critical-life functions. In fact, prions represent a subset of amyloids. [By the way, discoveries such as these create a very strong case for evolution of life on this planet from a common ancestor.] 😊 The present study [see attached] shows that amyloid participates in transfer of “environmental signal(s)” to elicit a “response by the genome” in carrying out an important function in plants.
Amyloids (seen under pathological conditions such as Alzheimer disease, Parkinson disease, Creutzfeldt–Jakob disease, motor neuron diseases, the large group of polyglutamine disorders including Huntington’s disease, as well as diseases of peripheral tissue such as hATTR amyloidosis and familial amyloid polyneuropathy) — represent protein aggregates having an unusual structure formed by intermolecular beta-sheets and stabilized by numerous hydrogen bonds. Such a structure (called “cross-β”) gives amyloids the morphology of predominantly unbranched fibrils with unique physicochemical properties.
The biological significance of amyloids is thus based on two aspects, i.e. pathological and functional. Besides amyloid deposition being associated with development of more than 40 incurable human and animal diseases — amyloids also have important intracellular functions as well. A growing number of studies demonstrate that amyloids play vital roles in archaeabacteria and eubacteria, and in eukaryotes (i.e. having chromosomal pairs) including humans. Amyloids in prokaryotes (i.e. having single unpaired chromosomes) fulfill mostly structural (biofilm and sheath formation) and storage (toxicant accumulation) functions. In fungi, infectious amyloids (called prions) are beneficial in that they regulate heterokaryon incompatibility (a fungal nonself-recognition system), multicellularity, and drug resistance. In animals, amyloid formation is important for various functions (e.g. long-term memory potentiation, melanin polymerization, hormone storage, and programmed necrosis). On the other hand, plants remain poorly studied in the field of amyloid biology.
The term “amyloid” was initially introduced in 1838 to describe plant cell carbohydrates, but then attributed to Rudolf Virchow in 1854 for forming pathological protein deposits in human tissues. Early studies (from the 1920s through the 1950s) led to hypotheses on the existence of amyloids in plant seeds; however, these structures turned out to be xyloglucans (the major cell wall matrix polysaccharides). Nevertheless, recently, some plant proteins were shown to form fibrils (having properties of amyloids under denaturing conditions), suggesting that plants might form bona fide amyloids in vivo. Authors [see attached article] thus hypothesized that amyloid formation might occur at seed maturation to stabilize storage proteins, thus preventing their degradation and misfolding during seed dormancy. In order to test this hypothesis, authors analyzed whether amyloid proteins are present in seeds of the garden pea, Pisum sativum L.
Authors showed that 7S globulin vicilin (the most abundant amyloid-like aggregates of storage proteins) forms bona fide amyloids in vivo and in cell culture. Full-length vicilin contains two evolutionary conserved β-barrel domains that self-assemble in vitro into amyloid fibrils with physicochemical properties similar to amyloids. In vivo, vicilin forms amyloids in cotyledon cells that bind amyloid-specific dyes and are resistant to detergents and proteases. Vicilin amyloid accumulation increases during seed maturation, and wanes at germination. Vicilin amyloids resist digestion by gastrointestinal enzymes (which persist in canned peas), and exhibit toxicity for yeast and mammalian cells. This study thus demonstrates that amyloid formation plays an important role in accumulation of storage proteins in plant seeds. 😊
PLoS Biol July 2020; 18: e3000564