Defender Planus poked the ball home in the 62nd minute after keeper Stephane Ruffier had parried a Marouane Chamakh header.
Monaco, who dropped to third place, hardly created a chance as Bordeaux controlled the game in midfield with France playmaker Yoann Gourcuff producing a fine display.
Bordeaux have 25 points from 11 matches, with Olympique Lyon two points adrift after Bafetimbi Gomis sank former team St Etienne with the only goal of the game.
Argentine Gonzalo Bergessio missed a good chance for St Etienne when his lob went just over Hugo Lloris's bar early in the second half.
France striker Gomis, who joined Lyon from Les Verts this season, fired in a Kim Kallstrom corner six minutes from time as the visitors rallied after two consecutive league defeats.
TEN MEN
Brazilian striker Brandao salvaged a 1-1 draw for Olympique Marseille at home to Toulouse, who had opened the scoring through France midfielder Moussa Sissoko despite being down to 10 men.
Toulouse keeper Yohann Pele was sent off after eight minutes following a rough tackle on striker Mamadou Niang but the visitors went ahead when Sissoko drove in a 30-metre free kick 19 minutes later.
Marseille snatched a point 15 minutes from time when Brandao connected with a cross from Ivory Coast striker Bakary Kone.
Surprise packages Montpellier were beaten for the second game in a row, losing 2-1 at AJ Auxerre who moved up to fifth with their fifth straight victory.
Bottom club Grenoble have yet to gain a point this season after losing 2-0 at home to Lille.
