Trust income taxable to whom?

Grandparent established trust for grandson. Trust has its own tax ID. Grandparent in the past had reported trust income on her own return, then died in 2008. Grandparent's daughter (grandson's mother) is the successor trustee. Trust income for 2009 includes OID and capital gains. Who must report trust income for 2009? Can grandson report? Grandson's mother? The trust?