The amino acid behind the savory sweetness. It is made in the roots and sent up to the leaves. It dissolves even in cool water, which is why brewing cool brings the savory side forward. Sunlight drives its breakdown while catechins build up, which is what shade growing exists to prevent.
In more detail
It makes up more than half of the amino acids in tea. Besides the savory taste, it is often credited with softening the stimulant effect of caffeine. Gyokuro is rich in it; bancha has almost none. Cold brewing holds back the more astringent catechins so the amino acid character comes forward — though caffeine still extracts.