Production of root hair deformation factors by Rhizobium meliloti nodulation genes in Escherichia coli: HsnD (NodH) is involved in the plant host-specific modification of the NodABC factor.

The role of the hsnD (nodH) gene in the determination of the host-specific nodulation ability of Rhizobium meliloti was studied by expressing the common nodulation genes (nodABC) with or without the hsnD gene in Escherichia coli and testing for biological activity on various leguminous plants. In this way, four categories of plants were established. Upon infection with E. coli carrying the nodABC construct, root hair deformation (Had) was detected on clovers while the hsnD gene was additionally needed for the elicitation of the same response on alfalfa and sweet clover. A weak root hair deformation was seen on siratro by inoculation with E. coli harbouring the nodABC genes and was highly increased when hsnD was also introduced. Cowpea and Desmodium did not respond to any of the E. coli strains constructed. Exudates or cytosolic fractions of the respective E. coli derivatives elicited the same root hair deformation as the intact bacteria. These data indicate that not only the nodABC gene products but also the hsnD product are involved in the synthesis of Had factors. Subclones expressing only the nodA, nodB, or nodC genes or the same genes in pairs (nodAB, nodBC, nodAC) did not provide a compound with activity comparable to the NodABC factor, suggesting that all three genes are required for the production of the Had factor which is active on clover. Coinoculation of alfalfa plants with two strains of E. coli, one carrying the nodABC genes and the other expressing only hsnD, or combining exudates or cytosolic fractions from these strains did not result in root hair deformation on alfalfa. These data indicate that the HsnD protein itself or its product is not an additional alfalfa-specific extracellular signal but more likely is enzymatically involved in the modification of the basic compound determined by the nodABC genes.