No, sugar is not stored in the root of an onion plant.

Here's why:

* Onions store sugar in their bulbs: The bulb, the part we eat, is actually a modified leaf structure where the plant stores sugars and other nutrients. This is why onions are sweet and can be used to add flavor to dishes.

* Roots are for absorption: The roots of the onion plant are responsible for absorbing water and nutrients from the soil, not for storing sugar.

So while onions have roots, their primary storage organ is the bulb, which is the modified leaf structure.