(Adds an investor, paragraph 4) BEIJING/SAN FRANCISCO, March 31 (Reuters) - U.S. autonomous truck startup Plus said on Wednesday it had raised an extra $220 million from investors in an extension of a $200 million funding round in February, as the global auto industry looks to a driverless future. A source with direct knowledge of the matter also said Plus was in advanced talks with a few special purpose acquisition companies (SPAC) over a potential merger, adding an announcement was expected within weeks. SPACs are becoming an increasingly popular way for private companies to seek public listings as a cheaper and less administratively burdensome alternative to an initial public offering (IPO). The latest funding round was led by FountainVest Partners and ClearVue Partners, Plus said in a statement. Other investors included Phi Zoyi Capital, Quanta Computer Inc and Millennium Technology Value Partners, as well as existing investors including Sequoia Capital China, SAIC Motor and Full Truck Alliance. Plus raised $200 million in February from investors such as Guotai Junan International, CPE, and Wanxiang International Investment. Plus did not disclose its valuation after the fundraising. The Cupertino-based startup, which was founded in 2016, has a joint venture with China’s state-owned automaker FAW to develop and build autonomous trucks. It is testing trucks in the United States, China and Europe. With the funding it will accelerate its production and deliveries of autonomous trucks and improve research capabilities. Automakers and technology firms are investing billions of dollars in autonomous driving, aiming to take an early lead in what many consider the future of road transport, though some industry insiders say it will take time for the public to fully place their trust in these vehicles. Plus’ rival TuSimple filed for a U.S. IPO earlier this month. Other self-driving companies such as Velodyne, Luminar Technologies and Aeva have also recently filed to go public, banking on an historic boom in U.S. capital markets. (Reporting by Yilei Sun in Beijing and Jane Lee in San Francisco. Editing by Tony Munroe, Mark Potter and Jane Merriman)