Through transcriptome profiling, Dr. Andy Breakspear (John Innes Centre, UK) and a large team of researchers have identified multiple hormonal gene candidates associated with the rhizobial infection that is critical for nitrogen fixation in legumes. From these studies, they conclude that cell cycle reactivation is involved in infection.
Legumes are second only to grasses in importance to humans as a source of food, feed for livestock, and raw materials for industry. They are central to sustainable agriculture because they minimize the need for fertilizers by supplying their own nitrogen to the soil. This nitrogen fixation is enabled in part by a symbiotic association with rhizobia bacteria. Medicago truncatula, a member of the legume family, exhibits these symbiotic relationships. Rhizobia colonize legume roots via plant-made root hairs. Many genes are involved in the symbiosis signaling pathway, and several transcription factors have been identified. However, the molecular processes of root hair development are poorly understood. To better understand these mechanisms, Dr. Breakspears team used GeneChip Medicago Genome Array for transcriptome profiling in isolated pure populations of root hairs.
The study identified 230 newly identified genes that were specifically related to infection in root hairs. These genes also showed sharp upregulation during root hair development. Among the upregulated genes were multiple types of hormones, which had not previously been associated with Medicago root hair infection. The auxins represented the largest percentage of the hormone groups. The team selected these for functional studies, which revealed that auxin-responsive genes were upregulated and expressed in root hairs. Dr. Breakspear''s team then used a beta-glucuronidase (GUS) promoter gene assay, confirming the presence of auxin-related genes directly within root hairs, and specifically at the site of root hair development.
To understand the role of auxin in the infection pathway, the team profiled transcription factors of the genes they had identified to be involved during infection. This profiling revealed that the induction of auxin-signaling genes coincided with increased expression of genes involved in the regulation of rhizobial nodule growth, revealing potential targets for key symbiosis transcription factors. Dr. Breakspears team concluded that regulation of auxin signaling is necessary for initiation of rhizobial infection threads, and the onset of infection may involve reactivation of the cell cycle.
These studies provide a transcriptional framework for legume root hair infection, and they suggest a unique role for root hairs in root nodule symbiosis signaling pathways that are critical for the nitrogen fixation that supports legume growth and soil fertilization.