Journal Article Research Support, Non-U.S. Gov't nitrogen fixation symbiosis ACC oxidase ACC synthase CCAAT box-binding factor Hemoglobins Medicago truncatula NF-YA Plant Proteins Review Rhizobium Transcription Factors hormones infection infection thread leghemoglobins model legumes nitric oxide nodule development nodules oxygen transport phytoglobins rhizobium symbiosis.