When you mean requiring signatures on a PO, do you mean that someone has to "sign" a PO to ensure its validity?
I assume by required, you mean internal procedures require all POs to be signed.
From a supplier perspective,
The buyer orderganization can document a MOU (memo of understanding) or some other psuedo legal consideration that states that the electronic submission of the purchase order serves as a signed purchase order.
Internally our procedures require purchase orders to be signed but we have worked with the exception that extranet or integrated orders serve the same consideration.