Separation of root nodule cells and identification of tissue-specific genes

The interior tissues of the alfalfa (Medicago sativa) root nodule differ in form and function from the peripheral layers. The interior tissues are specialized for the fixation of nitrogen in cells infected by rhizobia. In contrast, the peripheral nodule tissues perform roles that assist the interior tissues: they provide metabolic support and protect the interior tissues from damaging levels of oxygen. We used a novel microdissection technique to separate these tissue types, allowing immunological and molecular comparison between the nodule interior and periphery. Using differential mRNA display reverse transcription and polymerase chain reaction, we compared the mRNA profiles of the separated tissues and identified a transcript specific to the nodule interior, and several peripheral-specific candidate genes.