Advertising and PR giant WPP has reached an agreement to acquire a "significant" minority stake in German independent agent Hirschen Group GmbH for an undisclosed amount.The FTSE 100 group said its J Walter Thompson agency will partner with Hirschen following the acquisition, adding that existing team will continue to manage the German group and retain its independent position.Last year, Hirschen posted revenue of €37.3m (£26.9m) and had gross assets of €17m.WPP shares were down 0.55% to 1,456.00p at 11:06 on Tuesday.