Amazon.com Inc plans to open up to 400 physical bookstores, an executive of a major U.S. mall operator said on Tuesday. Amazon dipped its toe into the waters of brick-and-mortar stores with the opening of a bookstore in its home city of Seattle in November.An expansion of bookstores, which the company has not confirmed, would be a surprise reversal from the online retailer credited with driving physical booksellers out of business. "You've got Amazon opening brick-and-mortar bookstores and their goal is to open, as I understand, 300 to 400 bookstores," Sandeep Mathrani, chief executive of General Growth Properties Inc, said on Tuesday, responding to an analyst's question after it reported earnings.On the call, Mathrani compared Amazon's plans to similar moves by eyeware company Warby Parker or men's clothing retailer Bonobos, both of which opened physical stores after finding success online. Before branching out to offer everything from fresh groceries to original TV programming, Amazon got its start as a bookseller 20 years ago. It has since revolutionized the publishing industry by introducing its popular e-reader, the Kindle.Amazon's bookstore in Seattle carries books selected based on customer ratings and popularity on Amazon.com.