Protease's function to break down proteins. When a protease inhibitor is present it prevents the breakdown of proteins. This is extremely important during the seed stage of a plant. Without a protease inhibitor the seed's proteins would be broken down by the protease enzyme. This would cause the seed to exhaust its energy before germination and growth. Protease inhibitor crystals are found in higher concentrations in areas of viral infection, meaning it may also play a role in immune protection.
Protein crystals in plants are also thought to be involved in material storage. This is seen in the case of plastids which transform into chloroplasts for photosynthesis. It is thought that these crystals contain reserve material that is called upon during this transformation. They disappear after the plastid becomes a chloroplast.
When a plant cell undergoes stress from drought or plasmolysis, it forms protein crystals. These may act as emergency material reserves to jump-start cell operation once the drought is over and water can return to the cell. Its function could also include protease inhibition to prevent the plant cell from consuming its energy resources during a drought, leading to increased longevity.