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