Nature’s seed secret: Elusive plant amyloids as facilitators of seed germination

How would you explain your research outcomes to the non-scientific community?

Proteins are one of the major biomolecules for providing structure and nutrition to any life form. However, under certain aberrant conditions or to perform specialized functions, some proteins might aggregate and form structures called amyloids. These amyloid structures are much more resilient to environmental perturbations than normal proteins as they are formed by layering and twisting of individual proteins. Amyloids are involved on one hand in multiple pathological conditions including diabetes, cataract, and neurodegenerative diseases including Alzheimer’s. On the other hand, they also perform functional roles, for example melanin synthesis in humans. These amyloids were known to be present in almost every kingdom of life except plants. Thus, it was surprising that despite the conservation of amyloids in almost all types of species, these remained elusive in the plants. 

On searching the scientific literature, we found similarities (size, shape and appearance) between plant seed protein storage compartments (seed storage protein bodies) and amyloid-containing protein bodies of other organisms (bacterial inclusion bodies for example), and thus proposed that seed storage protein bodies (SSPB) might contain amyloid structures.

In this study, we have first employed multiple amyloid-specific dyes (ThT, Proteostat, and Congo red) that are used to detect amyloids in any cell or tissue. All the amyloid-specific probes showed a positive signal of amyloid structures in the cereal (wheat, barley) and pulse (mungbean, chickpea) seeds. Among these dyes, Congo red (CR) is known as the “gold-standard” probe for detecting and confirming amyloids in human and animal tissues. However, CR also binds to the plant cell walls and can yield similar signals to amyloids. To remove the ambiguity of staining among proteinaceous amyloids and cellulose-like carbohydrates, we digested the plant cell walls and isolated protoplasts (living plant cells without cell walls). The protoplasts also exhibited amyloid signatures with all the dyes, proving the presence of proteinaceous amyloids. The isolated SSPB similarly showed amyloid-signatures and confirmed amyloid presence in plants. We next dissected the amyloid areas, specifically from the seed tissues, using a high-precision laser and performed mass spectrometry to identify the amyloid-forming proteins. It was found that the seed globulin proteins are the major proteins that form amyloids.

The next question was why nature has provided amyloids in plants. To establish this, we allowed seed germination and seedling growth and checked the pattern of total protein vs amyloid degradation throughout germination. We found that the total proteins remain constant throughout germination; the less-stable amyloid-like structures degrade fast, whereas the more stable amyloid structures degrade slower, maintaining a sustained degradation. Further, we confirmed that the stepwise degradation of the amyloids plays a critical role in maintaining the optimum germination index and is regulated by plant germination/dormancy inducing hormones (gibberellic acid, abscisic acid) and proteases.

Thus, we have proved in the current study for the first time that plant seeds contain amyloid structures, and these play a critical role in seed germination and seedling growth.