1In 2-quart saucepan, heat 6 cups lightly salted water to boiling. Add potatoes; return to boiling. Reduce heat; simmer 5 minutes. Add green beans; cook uncovered 8 to 12 minutes longer or until potatoes and beans are crisp-tender.
2Meanwhile, in small bowl, mix yogurt, dressing, horseradish, salt and pepper; set aside.
3Drain potatoes and green beans; rinse with cold water to cool. In large serving bowl, mix potatoes, green beans, chicken and celery. Pour yogurt mixture over salad; toss gently to coat. Line plates with greens; spoon salad onto greens.