ALFALFA ENOD12 GENES ARE DIFFERENTIALLY REGULATED DURING NODULE DEVELOPMENT BY NOD FACTORS AND RHIZOBIUM INVASION

MsEnod12A and MsEnod12B are two early nodulin genes from alfalfa (Medi cago sativa). Differential expression of these genes was demonstrated using a reverse transcription-polymerase chain reaction approach. MsEn od12A RNA was detected only in nodules and not in other plant tissues. In contrast, MsEnod12B transcripts were found in nodules and also at low levels in roots, flowers, stems, and leaves. MsEnod12B expression was enhanced in the root early after inoculation with the microsymbion t Rhizobium meliloti and after treatment with purified Nod factors, wh ereas MsEnod12A induction was detected only when developing nodules we re visible. In situ hybridization showed that in nodules, MsEnod12 exp ression occurred in the infection zone. In empty Fix(-) nodules the Ms Enod12A transcript level was much reduced, and in spontaneous nodules it was not detectable. These data indicate that MsEnod12B expression i n roots is related to the action of Nod factors, whereas MsEnod12A exp ression is associated with the invasion process in nodules. Therefore, alfalfa possesses different mechanisms regulating MsEnod12A and MsEno d12B expression.