Umami comes from amino acids that occur in tea. The most representative is called theanine (L-theanine). Our teas and other Japanese teas have a lot of theanine in them, resulting in the naturally sweet umami that you find in the cup.
Certain processes will produce more theanine in the tea leaf. Our teas are all Japanese style green teas, which are processed so that the tea has a just-picked green character in it, especially compared to other teas like black teas. This means that in the cup you will taste much of the theanine that these young tea leaves produce.
Gyokuro is especially known for its umami-rich flavor. This is a result of being grown under the shade. When the tea fields are exposed to direct sunlight, theanine (the umami component) is converted into catechin, a component that gives Japanese tea its tannic flavor. So, since gyokuro is grown in the shade, it keeps the theanine it produces without converting much to catechin. The result is a very umami-rich tea when you brew it.
Another reason Japanese tea has umami is because of fertilizer. Fertilizer is used to give the tea trees a lot of energy to grow consistently and energetically, even when grown under the shade as is the case with gyokuro and matcha. The tea plant takes the nutrients from the fertilizer, and uses them to grow powerfully, while converting its energy into amino acids like theanine. So, tea plants grown in well-nourished, lofty soil will produce teas that have more theanine, and more naturally sweet umami taste.