Product Backlog Item's estimates?

I got a little doubt over Product Backlog Item's estimates. I read somewhere that the DT is responsible for all the estmations but the PB items are created by the PO and Product Backlog items have the attributes of a description, order, estimate and value.
So who create the estimations for PBI:
(1) PO discussing with DT
(2) DT only
5) DT along with PO after understanding the PBI