Mar. 2000 | XAVIER PERRET,1 CHRISTIAN STAHELIN,2 AND WILLIAM J. BROUGHTON1*
The article explores the molecular basis of symbiotic promiscuity in legumes and rhizobia. It begins with a historical overview of the study of nitrogen-fixing symbioses, highlighting early observations and experiments that demonstrated the role of bacteria in nodule formation. The focus then shifts to the molecular mechanisms governing broad host range (promiscuity) in rhizobia, which are considered a paradigm for other microbial symbioses.
The introduction of flavonoids and Nod factors is discussed as key signals in the initial phases of nodulation, guiding the reorientation of root hair cell wall growth and the formation of infection threads. NodD proteins play a central role in this process by binding to specific DNA motifs (nod boxes) and regulating the expression of nodulation genes.
The article delves into the detailed structures and functions of Nod factors, including their synthesis, modifications, and interactions with host plants. These modifications, such as fatty acid addition, 6-O glycosylation, sulfation, acetylation, and methylation, are crucial for host specificity and can be influenced by the host plant's flavonoids.
The role of Nod factors in opening the "outer door" for bacterial entry into legume root hairs is emphasized, followed by the exploration of additional bacterial signals required for the continued development of infection threads. The article also discusses the potential existence of an "inner door" within infection threads and other host range keys that are necessary for the formation of symbiotically proficient nodules.
Overall, the article provides a comprehensive analysis of the molecular mechanisms underlying the broad host range of rhizobia, highlighting the complex interplay between bacterial and plant molecules in establishing successful symbioses.The article explores the molecular basis of symbiotic promiscuity in legumes and rhizobia. It begins with a historical overview of the study of nitrogen-fixing symbioses, highlighting early observations and experiments that demonstrated the role of bacteria in nodule formation. The focus then shifts to the molecular mechanisms governing broad host range (promiscuity) in rhizobia, which are considered a paradigm for other microbial symbioses.
The introduction of flavonoids and Nod factors is discussed as key signals in the initial phases of nodulation, guiding the reorientation of root hair cell wall growth and the formation of infection threads. NodD proteins play a central role in this process by binding to specific DNA motifs (nod boxes) and regulating the expression of nodulation genes.
The article delves into the detailed structures and functions of Nod factors, including their synthesis, modifications, and interactions with host plants. These modifications, such as fatty acid addition, 6-O glycosylation, sulfation, acetylation, and methylation, are crucial for host specificity and can be influenced by the host plant's flavonoids.
The role of Nod factors in opening the "outer door" for bacterial entry into legume root hairs is emphasized, followed by the exploration of additional bacterial signals required for the continued development of infection threads. The article also discusses the potential existence of an "inner door" within infection threads and other host range keys that are necessary for the formation of symbiotically proficient nodules.
Overall, the article provides a comprehensive analysis of the molecular mechanisms underlying the broad host range of rhizobia, highlighting the complex interplay between bacterial and plant molecules in establishing successful symbioses.