Rhizobium nod genes are involved in inducing an early nodulin gene

Abstract

Rhizobium bacteria can invade the roots of leguminous plants and elicit the formation of root nodules. This process involves the expression of at least 20 nodule-specific genes encoded by the host plant, the so-called nodulin genes, which include the leghaemoglobin genes^1–3. During development of the root nodules the nodulin genes are differentially expressed¹. In fast-growing Rhizobium species several essential symbiotic genes are located on a large plasmid, and when fragments of the plasmid which contain the genes essential for nodulation (the nod region) are cloned and transferred to Rhizobium strains lacking the symbiotic plasmid (‘cured’ strains), the recipients regain the ability to form nodules. However, these nodules cannot fix nitrogen because essential nitrogen-fixation genes are absent^4–6. We show here that transfer of the nod region (10 kilobases; kb) alone from a Rhizobium leguminosarum symbiotic plasmid into cured Rhizobium strains is sufficient to elicit nodulin gene expression in the host Pisum sativum. We also show that pea root nodules induced by an Agrobacterium strain containing the R. leguminosarum symbiotic plasmid express an early nodulin gene but no other nodulin genes. These results show that at least two signals are involved in the induction of expression of nodulin genes and the presence of the Rhizobium nodulation genes seems to be required to elicit the first signal.