THE ELICITOR-INDUCIBLE ALFALFA ISOFLAVONE REDUCTASE PROMOTER CONFERS DIFFERENT PATTERNS OF DEVELOPMENTAL EXPRESSION IN HOMOLOGOUS AND HETEROLOGOUS TRANSGENIC PLANTS

In legumes, the synthesis of infection- and elicitor-inducible antimic robial phytoalexins occurs via the isoflavonoid branch of the phenylpr opanoid pathway, To study transcriptional regulation of isoflavonoid p athway-specific genes, we have isolated the gene encoding isoflavone r eductase (IFR), which is the enzyme that catalyzes the penultimate ste p in the synthesis of the phytoalexin medicarpin in alfalfa, Chimeric gene fusions were constructed between 765- and 436-bp promoter fragmen ts of the IFR gene and the beta-glucuronidase reporter gene and transf erred to alfalfa and tobacco by Agrobacterium-mediated transformation. Both promoter fragments conferred elicitor-mediated expression in cel l suspension cultures derived from transgenic plants of both species a nd fungal infection-mediated expression in leaves of transgenic alfalf a. Developmental expression directed by both promoter fragments in tra nsgenic alfalfa was observed only in the root meristem, cortex, and no dules, which is consistent with the accumulation of endogenous IFR tra nscripts. However, in transgenic tobacco, expression from the 765-bp p romoter was observed in vegetative tissues (root meristem and cortex, inner vascular tissue of stems and petioles, leaf tips, and stem perip heries adjacent to petioles) and in reproductive tissues (stigma, plac enta, base of the ovary, receptacle, seed, tapetal layer, and pollen g rains), whereas the 436-bp promoter was expressed only in fruits, seed , and pollen, These data indicate that infection/elicitor inducibility of the IFR promoter in both species and developmental expression in a lfalfa are determined by sequences downstream of position -436, wherea s sequences between -436 and -765 confer a complex pattern of strong e ctopic developmental expression in the heterologous species that lacks the isoflavonoid pathway.