Early nodulin genes are induced in alfalfa root outgrowths elicited by auxin transport inhibitors (original) (raw)

1989, Proceedings of the National Academy of Sciences of the United States of America

Rhizobium nod genes are essential for root hair deformation and cortical cell division, early stages in the development ofnitrogen-fixing root nodules. Nod-mutants are unable to initiate nodules on legume roots. We observed that N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid, compounds known to function as auxin transport inhibitors, induced nodule-like structures on alfalfa roots. The nodule-like structures (pseudonodules) were white, devoid of bacteria, and resembled nodules elicited by Rhizobium meliloti exopolysaccharide (exo) mutants at both the histological and molecular level. Two nodulin genes, ENOD2 and Nms-30, were expressed. RNA isolated from the nodule-like structures hybridized to pGmENOD2, a soybean early nodulin cDNA clone. RNA isolated from roots did not hybridize. We determined by in vitro translations of total RNA that the alfalfa nodulin transcript Nms-30 was also expressed in the nodule-like structures. The late expressed nodulin genes, such as the leghemoglobin genes, were not transcribed. Because N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid induce the development of nodules on alfalfa roots, we suggest that the auxin transport inhibitors nmimic the activity of compound(s) made upon the induction of the Rhizobium nod genes. The nodulation (nod) genes of Rhizobium play an essential role in the induction of nodules on the roots of leguminous plants. The importance of the nod genes has been demonstrated in at least two ways. First, it has been shown that Rhizobium that have mutated common nod genes, nodABC, lose the ability to curl root hairs (1) as well as to initiate cellular divisions within the root cortex. Cortical cell divisions mark the beginning of root nodule formation (2). Second, by transferring the nod region of Rhizobium meliloti or Rhizobium leguminosarum to Agrobacterium tumefaciens, Agrobacterium transconjugants acquire the ability to form nodules on alfalfa and Vicia, respectively (3, 4). Although the nod genes are required for the earliest stages