Like other amino acids, it is an important part of cell proteins, especially important for some proteins, since the hydroxyl group of threonine can be phosphorylated (like that of serine or tyrosine) which can be used by cells to perform signaling processes, via phosphorylation/dephosphorylation. It is a precursor of isoleucine, and so it provides energy during dark hours when there is no photosynthesis.