The BSE benchmark Sensex rose by nearly 89 points in opening trade today, extending its gains for the fourth straight session on continued buying by funds and retail investors, tracking a firm trend on other Asian bourses.The 30-share BSE index, which has gained nearly 105 points over the past three sessions, rose further by 88.81 points, or 0.50 per cent, to 17,658.34 in the first five minutes of trade today.In a similar fashion, the wide-based National Stock Exchange Nifty Index moved up by 28.15 points, or 0.53 per cent, to 5,317.50.In the Asian region, Hong Kong's Hang Seng Index rose by 1.68 per cent and Japan's Nikkei Index by 0.68 per cent in morning trade today.The US Dow Jones Industrial Average closed 0.84 per cent higher in yesterday's trade.